Passive transmit / receive circuits for time division duplex communications systems

Abstract

Transmit / receive circuits include an input, an output, a first circulator that has a first circulator port, a second circulator port and a third circulator port, where the first circulator port is coupled to the input, a second circulator that has a fourth circulator port, a fifth circulator port and a sixth circulator port, where the fourth circulator port is coupled to the second circulator port and the fifth circulator port is coupled to the output, and a low noise amplifier and a first bandpass filter that are coupled in between the sixth circulator port and the third circulator port.

Description

PASSIVE TRANSMIT / RECEIVE CIRCUITS FOR TIME DIVISION DUPLEX COMMUNICATIONS SYSTEMSCROSS-REFERENCE TO RELATED APPLICATION

[0001] The present application claims priority to Italian Patent Application No. 102024000001290, filed January 24, 2024, the entire content of which is incorporated herein by reference.FIELD

[0002] The present invention relates to communications systems and, more particularly, to time division duplex communications systems.BACKGROUND

[0003] Cellular communications systems are well known in the art. In a typical cellular communications system, a geographic area is divided into a series of regions that are referred to as "cells," and each cell is served by a base station. The base station may include baseband equipment, radios and antennas that are configured to provide two-way radio frequency ("RF") communications with fixed and mobile subscribers that are positioned throughout the cell. The base station antennas generate radiation beams that are directed outwardly to serve the entire cell or a portion thereof. Typically, a base station antenna includes one or more phase-controlled arrays of radiating elements, which are commonly referred to as phased array antennas.

[0004] Conventionally, most cellular communications systems have operated as frequency division duplex systems, where two-way radio communications is supported using two distinct radio channels that are in different frequency bands. In a frequency division duplex system, a first channel in a first frequency band is used to transmit "downstream" communications from the base station radio to fixed and mobile subscribers throughout the cell,and a second channel that is in a second, different frequency band is used to transmit "upstream" communications from the subscribers to the base station radio. Because the downstream and upstream communications are transmitted in separate frequency bands, simultaneous transmission in both the upstream and downstream directions is possible without significant interference therebetween.

[0005] Time division duplex communications systems are also known in the art. A time division duplex system uses a single frequency band for both downstream and upstream communications by separating the downstream and upstream communications in time. In particular, a time interval called a "frame" may be defined that is sub-divided into a plurality of smaller "time slots." Downstream communications may be performed during some of the time slots within each frame while upstream communications may be performed in other of the time slots. Thus, a time division duplex communication system operates by toggling transmission directions (at high speed) over a time interval. The relative capacity of the downstream and upstream links can be altered in favor of one direction over the other direction. This is easily accomplished by providing a greater time allocation through additional time slots to downstream (or upstream) transmission intervals relative to upstream (or downstream) transmission intervals. This asymmetric capability is useful for communication processes characterized by unbalanced information flow, such as when a relatively short upstream message prompts a large information download as is typical with Internet access.

[0006] To support two-way communications using a single frequency channel, time division duplex systems may have a guard time interval between transmit and receive data streams, as shown in FIG. 1. This guard time interval enables a base station to switch from transmit mode to receive mode and subscribers to switch from receive mode to transmit mode, and vice versa. One or more transmit / receive circuits are provided that are used to switch the base station between the transmit and receive modes and / or to route the transmit and receive RF signals through the proper circuit elements. During the guard time interval, the base station and subscriber are not transmitting modulated data, as the guard time interval is provided to allow the base station transmitter (or receiver) section to ramp down, the transmit / receive circuit to switch modes (if a switching transmit / receive circuit is used), and the base station receiver (or transmitter) section to activate.

[0007] In cellular communications, time division duplexing is primarily used with so- called "active" beamforming arrays that operate in conjunction with active beamforming radios to dynamically generate high gain antenna beams that have adjustable sizes, shapes and pointing directions. These active beamforming arrays include multiple columns of radiating elements. Each column of radiating elements in an active beamforming array is typically coupled to a respective port of a beamforming radio (or two ports if dual-polarized radiating elements are used). The beamforming radio may dynamically adjust the amplitudes and phases of the subcomponents of an RF signal that are fed to each column of the beamforming array to generate antenna beams that have narrowed beamwidths in the azimuth plane (and hence higher antenna gain). These narrowed antenna beams can be electronically steered in the azimuth plane by proper selection of the amplitudes and phases of the sub -components of an RF signal

[0008] Active beamforming systems typically include high power amplifiers that are used to amplify the transmit RF signals that are output by the radios before the RF signals are fed to the respective columns of radiating elements in the beamforming array. Active beamforming systems also typically include low noise amplifiers that amplify the signals received at the antenna before they are passed to the radio. As discussed above, time division duplex beamforming systems also include transmi t / receive circuits that are interposed along each connection between the beamforming radio and the antenna. The low noise amplifiers are typically embedded in the respective transmit / receive circuits. The high power amplifiers may also be embedded in the transmit / receive circuits or may be located elsewhere (e.g., integrated into the beamforming radio).

[0009] A number of transmit / receive circuits are known in the art. For example, FIG. 2 is a high-level circuit diagram of a conventional transmit / receive circuit 1 for a time division duplex communications system that is interposed between a radio 90 and an antenna 92. The transmit / receive circuit 1 may, for example, be interposed between a first port of a beamforming radio and one of the columns of radiating elements of a multi-column beamforming array included in antenna 92. Referring to FIG. 2, the transmit / receive circuit 1 includes an input 2, an output 4, a first bidirectional path 10, a transmit path 30, a second bidirectional path 50, and a receive path 60. The first bidirectional path 10 is coupled between the input 2 and a first 1x2 switch 20 that selectively connects the first bidirectional path 10 to one of the transmit path 30 and the receive path 60. The first switch 20 includes a first port 22 that is coupled to the firstbi directional path 10, a second port 24 that is coupled to the transmit path 30, and a third port 26 that is coupled to the receive path 60. The transmit path 30 is interposed between the first switch 20 and a second 1x2 switch 40 that selectively connects the second bidirectional path 50 to one of the transmit path 30 and the receive path 60. The second switch 40 includes a fourth port 42 that is coupled to the transmit path 30, a firth port 44 that is coupled to the second bidirectional path 50, and a third port 46 that is coupled to the receive path 60. The second bidirectional path 50 is interposed between the second switch 40 and the output 4. A first bandpass filter 12 is provided along the first bidirectional path 10 and a second bandpass filter 52 is provided along the second bidirectional path 50. A low noise amplifier 62 is provided along the receive path 60. A power detector 70 is provided that is configured to sense a power level (or another parameter such as a current or a voltage) at the first port 22 of switch 20 and to provide a control signal to switches 20 and 40 based on the sensed power level.

[0010] In operation, if the power detector 70 detects a power level above a preselected threshold, then the power detector 70 outputs a control signal that controls the first switch 20 to connect the first port 22 to the second port 24 thereof, and that controls switch 40 to connect the fourth port 42 to the fifth port 44 thereof. Alternatively, if the power detector 70 detects a power level that is below the preselected threshold, then the power detector 70 outputs a control signal that controls the first switch 20 to connect the first port 22 to the third port 26 thereof, and that controls the second switch 40 to connect the fourth port 42 to the sixth port 46 thereof. The power detector 70 may be used to distinguish between the transmit and receive time slots since the minimum power level of the transmit signals may be at least an order of magnitude greater than a maximum power of the received RF signals. Thus, when the base station is in transmit mode, RF signals entering the transmit / receive circuit 1 at input 2 pass from the first switch 20 to the second switch 40 using the transmit path 30, thereby bypassing the low noise amplifier 62. Conversely, when the base station is in receive mode, RF signals entering the transmit / receive circuit 1 at output 4 pass from the second switch 40 to the first switch 20 using the receive path 60 so that the received RF signals are amplified by the low noise amplifier 62.

[0011] Transmit / receive circuits are also known in the art that use circulators in place of the pair of switches 20, 40 included in transmit / receive circuit 10 of FIG. 2. FIG. 3 is a high- level circuit diagram of a conventional transmit / receive circuit 100 that uses such circulators. As shown in FIG. 3, the transmit / receive circuit 100 includes an input 102, an output 104, a firstbidirectional path 110, a transmit path 130, a second bidirectional path 150, and a receive path 160. A first bandpass filter 112 is provided along the first bidirectional path 110. The first bidirectional path 110 is coupled between the input 102 and a first circulator 120 that connects the first bidirectional path 110 to the transmit path 130 and that also connects the receive path 160 to the first bidirectional path 110. The transmit path 130 is interposed between the first circulator 120 and a second circulator 140 that connects the transmit path 130 to the second bidirectional path 150 and that also connects the second bidirectional path 150 to the receive path 160. A second bandpass filter 152 is provided along the second bidirectional path 150. The second bidirectional path 150 is interposed between the second circulator 140 and the output 104. A low noise amplifier 162 is provided along the receive path 160.

[0012] The conventional transmi t / receive circuit 100 may be used to pass transmit RF signals between a radio 190 and an antenna 192 as follows. Transmit RF signals output by the radio 190 are provided to the input 102. Herein, "transmit RF signals" refer to RF signals that are output from a radio and passed to an antenna for transmission into free space. These transmit RF signals travel along the first bidirectional path 110 through the first bandpass filter 112 to a first port 122 of the first circulator 120. The transmit RF signals are output at a second port 124 of the first circulator 120 onto the transmit path 130. The transmit path 130 is configured (by operation of the first and second circulators 120, 140) to only carry transmit RF signals, with the exception of noise signals that unintentionally leak onto the transmit path 130 (e.g., unintended reflections, attenuated noise signals that pass through filters, etc.). The transmit RF signals pass from the transmit path 130 to the fourth port 142 of the second circulator 140 (note that herein the three ports of any second circulators are referred to as fourth, fifth and sixth ports to distinguish these ports from the first, second and third ports of an associated first circulator). The transmit RF signals are output at the fifth port 144 of the second circulator 140 onto the second bidirectional path 150, where they are passed through the second bandpass filter 152 to the output 104. The output 104 is connected to the antenna 192 (e.g., to an antenna port that feeds a column of radiating elements of a phased array antenna).

[0013] The conventional transmi t / receive circuit 100 may be used to pass received RF signals between the antenna 192 and the radio 190 as follows. Herein, "received RF signals" refer to RF signals that are received by an antenna and passed to a radio. Received RF signals that are received by the antenna are passed to the output 104. The received RF signals are inputto transmit / receive circuit 100 at output 104 and travel along the second bidirectional path 150 where they pass through the bandpass filter 152. The bandpass filter 152 filters out (i.e., heavily attenuates) any out-of-band RF energy that may be included in the received RF signal. The second bandpass filter 152 may be identical to the first bandpass filter 112. The filtered received RF signal passes to the fifth port 144 of the second circulator 140, and the second circulator 140 passes the filtered received RF signal through the sixth port 146 thereof onto the receive path 160. The receive path 160 is configured (by operation of the first and second circulators 120, 140) to only carry received RF signals, with the exception of noise signals that unintentionally leak onto the receive path 160 (e.g., unintended reflections, attenuated noise signals that pass through filters, etc.). A low noise amplifier 162 is provided along the receive path 160 that amplifies the received RF signals. The amplified received RF signals are passed to the third port 126 of the first circulator 120. The received RF signals are output at the first port 122 of the first circulator 120 onto the first bidirectional path 110 to the input 102, where they are passed to the radio 190.

[0014] Transmit / receive circuits are also known in the art that use a combination of a switch and a circulator. FIG. 4 is a high-level circuit diagram of a conventional receive channel transmit / receive circuit 100' that uses such an approach. As can be seen by comparing FIG. 4 to FIGS. 2-3, the transmit / receive circuit 100' is identical to the transmit / receive circuit 100, except that (1) in transmit / receive circuit 100' the first circulator 120 is replaced with a 1x2 switch 20 (which may the same as the first switch 20 of transmit / receive circuit 1) and (2) a power detector 170 is added that is used to control the first switch 20. As all of the components of transmit / receive circuit 100' have been discussed above, further description of transmit / receive circuit 100' will be omitted here.SUMMARY

[0015] Pursuant to embodiments of the present invention, transmit / receive circuits are provided that comprise an input, an output, a first circulator that has a first circulator port, a second circulator port and a third circulator port, where the first circulator port is coupled to the input, a second circulator that has a fourth circulator port, a fifth circulator port and a sixth circulator port, where the fourth circulator port is coupled to the second circulator port and the fifth circulator port is coupled to the output, and a low noise amplifier and a first bandpass filter that are coupled in between the sixth circulator port and the third circulator port.

[0016] In some embodiments, an input of the low noise amplifier is coupled to the sixth circulator port and the first bandpass filter is coupled in between an output of the low noise amplifier and the third circulator port.

[0017] In some embodiments, the transmit / receive circuit may further comprise a second bandpass filter that is coupled in between the input and the first circulator port and / or a third bandpass filter that is coupled in between the fifth circulator port and the output.

[0018] In some embodiments, a width of a first passband of the first bandpass filter is narrower than a width of a second passband of the second bandpass filter and narrower than a width of a third passband of the third bandpass filter. For example, in some embodiments, the width of the first passband is between 95% and 99% the width of the third passband.

[0019] In some embodiments, the transmit / receive circuit is provided in conjunction with a beamforming radio that includes a fourth bandpass filter that has a fourth passband, where the width of the first passband is closer to a width of the fourth passband than it is to the width of the third passband.

[0020] In some embodiments, a first frequency band that is between the lower edge of the second passband and a lower edge of an operating frequency band for the transmit / receive circuit defines a first guard band, and a second frequency band that is between the upper edge of the third passband and an upper edge of the operating frequency band for the transmit / receive circuit defines a second guard band. In such embodiments, the first bandpass filter may be configured to attenuate radio frequency energy that is within the first and second guard bands sufficiently so that it does not have a positive gain loop. In other embodiments, the first bandpass filter may be configured to attenuate radio frequency energy that is within the first and second guard bands more than a maximum allowable gain of the low noise amplifier.

[0021] Pursuant to further embodiments of the present invention, transmit / receive circuits are provided that comprise an input, an output, a transmit path that is configured to act as an RF transmission path for transmit signals only, a receive path that is configured to act as an RF transmission path for receive signals only, the receive path including a first bandpass filter, a first bidirectional path that has a first end that is coupled to the input and a second end that is coupled to both the transmit path and the receive path, the first bidirectional path configured to act as an RF transmission path for both transmit and receive signals, the first bidirectional path including a second bandpass filter, and a second bidirectional path that has a first end that iscoupled to both the transmit path and the receive path and a second end that is coupled to the output, the second bidirectional path configured to act as an RF transmission path for both transmit and receive signals, the second bidirectional path including a third bandpass filter. A first passband of the first bandpass filter is narrower than a second passband of the second bandpass filter.

[0022] In some embodiments, the first passband is narrower than a third passband of the third bandpass filter.

[0023] In some embodiments, the receive path further includes a low noise amplifier. In some embodiments, the first bandpass filter is coupled between the low noise amplifier and the first bidirectional path.

[0024] In some embodiments, the transmi t / receive circuit further comprises a first circulator that has a first circulator port that is coupled to the first bidirectional path, a second circulator port that is coupled to the transmit path, and a third circulator port that is coupled to the receive path.

[0025] In some embodiments, the transmi t / receive circuit further comprises a second circulator that has a fourth circulator port that is coupled to the transmit path, a fifth circulator port that is coupled to the second bidirectional path, and a sixth circulator port that is coupled to the receive path.

[0026] In some embodiments, a width of the first passband is between 95% and 99% a width of the second passband.

[0027] In some embodiments, the transmi t / receive circuit is provided in conjunction with a beamforming radio that includes a fourth bandpass filter that has a fourth passband, where the width of the first passband is closer to a width of the fourth passband than it is to the width of the second passband.

[0028] In some embodiments, a first frequency band that is between the lower edge of the second passband and a lower edge of an operating frequency band for the transmit / receive circuit defines a first guard band, and a second frequency band that is between the upper edge of the third passband and an upper edge of the operating frequency band for the transmit / receive circuit defines a second guard band. In such embodiments, the first bandpass filter may be configured to attenuate radio frequency energy that is within the first and second guard bands sufficiently so that it does not have a positive gain loop. In other embodiments, the firstbandpass filter may be configured to attenuate radio frequency energy that is within the first and second guard bands more than a maximum allowable gain of the low noise amplifier.

[0029] Pursuant to still further embodiments of the present invention, transmit / receive circuits are provided that comprise an input, an output, a low noise amplifier coupled between the output and the input, a first bandpass filter that has a first passband that is coupled to an output of the low noise amplifier, a second bandpass filter that has a second passband that is coupled to the input, and a third bandpass filter that has a third passband that is coupled to the output. A first frequency band that is between the lower edge of the second passband and a lower edge of an operating frequency band for the transmit / receive circuit defines a first guard band, and a second frequency band that is between the upper edge of the third passband and an upper edge of the operating frequency band for the transmit / receive circuit defines a second guard band, and wherein the first bandpass filter is configured to attenuate radio frequency energy that is within the first and second guard bands more than a maximum allowable gain of the low noise amplifier.

[0030] In some embodiments, the transmit / receive circuit further comprises a first circulator that has a first circulator port that is coupled to the input via a first bidirectional path and that is configured to pass both transmit and receive signals and a second circulator that has a fifth circulator port that is coupled to the output via a second bidirectional path that is configured to pass both transmit and receive signals.

[0031] In some embodiments, the transmit / receive circuit further comprises a transmit path that extends between a second circulator port of the first circulator and a fourth circulator port of the second circulator.

[0032] In some embodiments, the transmit / receive circuit further comprises a receive path that extends between a sixth circulator port of the second circulator and a third circulator port of the first circulator.

[0033] In some embodiments, the low noise amplifier and the first bandpass filter are part of the receive path.

[0034] In some embodiments, the second bandpass filter is part of the first bidirectional path and the third bandpass filter is part of the second bidirectional path.

[0035] In some embodiments, a width of a first passband of the first bandpass filter is narrower than a width of a second passband of the second bandpass filter.

[0036] In some embodiments, the width of the first passband is narrower than a width of a third passband of the third bandpass filter.

[0037] In some embodiments, the width of the first passband is between 95% and 99% the width of the third passband.BRIEF DESCRIPTION OF THE DRAWINGS

[0038] FIG. 1 is a schematic diagram that illustrates how a single radio channel can be used to support two-way radio communication in a time division duplex communications system.

[0039] FIG. 2 is a high-level circuit diagram of a conventional transmit / receive circuit for a time division duplex communications system.

[0040] FIG. 3 is a high-level circuit diagram of another conventional transmit / receive circuit for a time division duplex communications system.

[0041] FIG. 4 is a high-level circuit diagram of yet another conventional transmit / receive circuit for a time division duplex communications system.

[0042] FIG. 5 is a high-level circuit diagram of a transmit / receive circuit for a time division duplex communications system according to embodiments of the present invention.

[0043] FIG. 6 is an annotated version of the high-level circuit diagram of an example implementation of the transmit / receive circuit of FIG. 5 that illustrates how the transmit / receive circuit avoids positive gain loop behavior.

[0044] FIG. 7 is a high-level circuit diagram of a transmit / receive circuit for a time division duplex communications system according to further embodiments of the present invention.

[0045] FIG. 8 is a high-level circuit diagram of a transmit / receive circuit for a time division duplex communications system according to an additional embodiment of the present invention.DETAILED DESCRIPTION

[0046] The use of time division duplex communications systems is increasing with the deployment of fifth generation ("5G") cellular networks. In 5G systems, much faster switching requirements are specified as the guard bands between the transmit and receive slots in a frame have been reduced considerably. Tradeoffs exist between the speed, power handling capabilities and cost of a transmit / receive circuit. For example, the switch-based transmit / receive circuit 1 ofFIG. 2 may have good power handling capabilities and fast switching times, but requires a power detector and a fast high power switches, both of which may be expensive. The circulatorbased transmi t / receive circuit 100 of FIG. 3 may be cheaper than the switch-based transmit / receive circuit 1 of FIG. 2, but may have reduced isolation and reliability. The hybrid transmi t / receive circuit 100' of FIG. 4 provides a compromise between the transmit / receive circuits 1, 100 of FIGS. 2 and 3, but may still be relatively expensive. Thus, it may be a challenge to provide transmit / receive circuits suitable for use in 5G cellular communications systems that operate at sufficiently high speeds and power handling capabilities while having reasonable cost.

[0047] Pursuant to embodiments of the present invention, transmit / receive circuits are provided that are suitable for use in 5G cellular communications systems. The transmit / receive circuits disclosed herein may be interposed between a time division duplex beamforming radio and a beamforming antenna, or may be integrated within the radio. The transmit / receive circuits according to embodiments of the present invention may be passive circuits (although they typically include an integrated low noise amplifier, which is an active device) that have high reliability, and may also have fast switching speeds and good power handling capabilities. The transmit / receive circuits according to embodiments of the present invention may be cheaper than conventional transmit / receive circuits, while at the same time avoiding positive gain loop behavior that may destroy the low noise amplifier in some conventional transmit / receive circuits.

[0048] Circulator-based transmit / receive circuits may have significant cost advantages over transmit / receive circuits that include one or more high power, high speed switches. A typical high speed, high power RF switch may cost on the order of $50, and a typical power detector may cost on the order of $ 10, whereas a suitable circulator may only cost about $5. Thus, the transmit / receive circuit 100 of FIG. 3 may be significantly cheaper than the transmit / receive circuits 1 and 100' of FIGS. 2 and 4. Unfortunately, the transmit / receive circuit 100 of FIG. 3 may exhibit positive gain loop behavior. Positive gain loop behavior refers to a condition where an out-of-band RF signal that is input to a transmit / receive circuit repeatedly loops through the transmit and receive paths and increases in magnitude on each loop. As noted above, such positive gain loop behavior may destroy the low noise amplifier.

[0049] Positive gain loop behavior may occur because RF filters do not exhibit ideal behavior, but instead attenuate signals at different frequencies differently. Typically, the amountthat RF signals within a "passband" of the filter are attenuated is relatively constant as a function of frequency, although larger variations occur near the edges of the passband. The amount of attenuation that occurs within a stopband of the filter often varies significantly, although filters are typically designed so that the attenuation at all relevant frequencies within the stopband is below a certain level. The amount of attenuation in the transition bands that are provided between the passband and the two stopbands of a bandpass filter varies occurs significantly as a function of frequency. Note that herein, the "passband" of an RF filter refers to the frequency range where RF signals input to the filter experience attenuation that are within 2 dB of the minimum attenuation applied by the filter.

[0050] A simple example illustrates how positive gain loop behavior may occur in the transmit / receive circuit 100 of FIG. 3. Each of the bandpass filters 112, 152 that are included in the transmit / receive circuit 100 will typically have a passband that is a little larger than the operating frequency band of the radio 190. For example, if the radio 190 is designed to operate in the 3.4-3.8 GHz frequency band (and hence the transmit / receive circuit 100 is also designed to operate in the 3.4-3.8 GHz frequency band), then the two bandpass filters 112, 152 that are included in the transmit / receive circuit 100 will have a passband of perhaps 3.39-3.81 GHz (i.e., the passband of the filters extends 10 MHz below and 10 MHz above the operating frequency band). The provision of a 10 MHz "guard band" on each side of the passband may ensure that the bandpass filters 112, 152 exhibit only a small amount of attenuation at the lower and upper edges of the operating frequency band with a relatively inexpensive filter design (i.e., the bandpass filters 112, 152 need not have as sharp a roll-off at the edges of the operating frequency band).

[0051] Typically, the radio 190 will also include a bandpass filter (not shown) that is designed to filter out RF energy that is not within the operating frequency band. The bandpass filters 112, 152 in the transmit / receive circuit 100 are provided to attenuate out-of-band noise signals that may be generated in the transmit / receive circuit 100 by the low noise amplifier 162 and / or the circulators 120, 140. The noise signals generated by the low noise amplifier 162 and the circulators 120, 140 tends to have lower signal levels than spurious noise generated by the radio (e.g., by a high power amplifier in the radio), which noise is attenuated by the bandpass filter in the radio. The bandpass filter in the radio 190 typically has a narrower passband than the two bandpass filters 112, 152 that are included in the transmit / receive circuit 100 of FIG. 3 tobetter attenuate the potentially higher signal level spurious noise generated in the radio. With respect to the above example, the bandpass filter in the radio 190 may have, for example, a passband of 3.395-3.805 GHz or a passband of 3.397-3.803 GHz. If the RF energy that is received at the antenna 192 and passed to the transmit / receive circuit 100 includes slightly out- of-band RF energy (referred to herein as a "noise signal") such as, for example, RF energy at 3.392 GHz, then this noise signal will be passed to the second bandpass filter 152. Since RF energy at 3.392 GHz is near the edge of the passband of the second bandpass filter 152 it will be partly attenuated / reflected by the second bandpass filter 152 and partly passed along the second bidirectional path 150 to the second circulator 140, which passes this RF noise signal onto the receive path 160. The RF noise signal is amplified by the low noise amplifier 162 and passed through the first circulator 120 to the first bandpass filter 112, where since the noise signal is near the edge of the passband of the first bandpass filter 112, it will again be partly attenuated / reflected by the first bandpass filter 112. The portion of the noise signal that passes through the first bandpass filter 112 is then passed to the radio 190. As discussed above, the radio 190 includes a bandpass filter (not shown) with a narrower passband (e.g., a 3.395- 3.805 GHz passband). Consequently, the noise signal will not pass to the radio 190, but instead will partly be attenuated and partly be reflected by the bandpass filter in the radio 190. The portion of the noise signal that is reflected by the bandpass filter in the radio 190 then (partly) passes back through the first bandpass filter 112 to the first circulator 120, from the first circulator 120 to the transit path 130, from the transmit path 130 to the second circulator 140, and from the second circulator 140 to the second bidirectional path 150. When the reflected noise signal reaches the second bandpass filter 152 it again partly passes through the second bandpass filter 152 and is partly attenuated / reflected by the second bandpass filter 152. The portion of the reflected noise signal that is further reflected by the second bandpass filter 152 then passes back through the second circulator 140 to the low noise amplifier 162, and this noise signal again passes to the radio 190 in the same manner as the original noise signal. As is readily apparent, a loop is created where the slightly out-of-band noise signal loops through the transmit / receive circuit 100 and is amplified by the low noise amplifier 162 during each loop. The noise signal can thus grow in magnitude to a point where it eventually damages or even destroys the low noise amplifier 162.

[0052] The transmit / receive circuits according to embodiments of the present invention may include an additional bandpass filter that is provided on the receive path that has a passband that is smaller than the passbands of the bandpass filters included on the first and / or second bidirectional paths of the transmit / receive circuits. This additional bandpass filter may, for example, have a passband that is about the same as, or even narrower than, the passband of the bandpass filter included in the radio. The additional bandpass filter may be designed to attenuate slightly out-of-band noise signals input thereto sufficiently such that portions of any slightly out- of-band noise signal output by the additional bandpass filter that loop back to the low noise amplifier due to reflections (and thus are repeatedly amplified by the low noise amplifier) will have a magnitude that is less than the noise signal that was originally fed to the additional bandpass filter. In this manner, even though loop behavior may still occur, the magnitude of the noise signal that is fed to the low noise amplifier during each loop will continually get smaller, protecting the low noise amplifier from potentially destructive behavior. As a result, simple, relatively inexpensive transmit / receive circuits may be provided according to embodiments of the present invention that are not susceptible to destructive loop behavior.

[0053] Embodiments of the present invention will now be discussed in further detail with reference to the attached drawings.

[0054] FIG. 5 is a high-level circuit diagram of a transmit / receive circuit 200 for a time division duplex communications system according to embodiments of the present invention.

[0055] As shown in FIG. 5, the transmit / receive circuit 200 includes an input 202 and an output 204. The input 202 and the output 204 may each comprise, for example, any suitable element that may pass RF signals into and out of the transmit / receive circuit 200. By way of non-limiting examples, the input 202 and / or the output 204 may comprise an RF transmission line segment such as a microstrip RF transmission line, a stripline RF transmission line, or a coaxial cable, or may comprise an RF transmission line transition such as, for example, an airstrip to microstrip transition or a coaxial cable to microstrip transition.

[0056] The input 202 may be connected to a radio 290 and the output 204 may be connected to one or more radiating elements of an antenna 292. A first bidirectional path 210 connects the input 202 to a first port 222 of a first circulator 220. RF signals input at the first port 222 of the first circulator 220 are output at a second port 224 thereof onto a transmit path 230 which passes the RF signals to the fourth port 242 of a second circulator 240. RF signalsinput at the fourth port 242 of the second circulator 240 are output at a fifth port 244 thereof onto a second bidirectional path 250, which passes the RF signals to the output 204. RF signals received at the antenna 292 are passed along the second bidirectional path 250 to the fifth port 244 of the second circulator 240, which outputs these RF signals at a sixth port 246 thereof onto a receive path 260. The received RF signals pass from the receive path 260 to a third port 226 of the first circulator 220 and are output at the first port 222 of the first circulator 220 onto the first bidirectional path 210, where they pass to the input 202 and the radio 290.

[0057] As shown in FIG. 5, a first bandpass filter 264 and a low noise amplifier 262 are provided along the receive path 260. The low noise amplifier 262 may amplify RF signals input thereto to a higher signal level. The function and operation of the first bandpass filter 264 will be discussed in detail below. In addition, a second bandpass filter 212 is provided along the first bidirectional path 210, and a third bandpass filter 252 is provided along the second bidirectional path 250. The first through third bandpass filters 264, 212, 252 may be designed to generally pass RF signals that are within the operating frequency band of the radio 290 and to block RF signals that are outside of the operating frequency band. The second and third bandpass filters 212 and 252 may remove out-of-band noise that may be generated by non-linear elements within transmit / receive circuit 200 such as the circulators 220, 240 and may also remove out-of-band noise introduced external to transmit / receive circuit 200. For example, the second bandpass filter 212 may remove RF noise generated by non-linearities in a high power amplifier in the radio 290.

[0058] The first bandpass filter 264 has a passband that is smaller than the passband of the second and / or third bandpass filters 212, 252. For example, if the transmit / receive circuit 200 has an operating frequency range of 3.4-3.8 GHz, the second and third bandpass filters 212, 252 may each have a passband of, for example, 3.39-3.81 GHz so that each of these filters has a 10 MHz guard band on either side of the operating frequency range. Thus, the "width" of the passband of each of the second and third bandpass filters 212, 252 is 420 MHz. In contrast, the first bandpass filter 264 may have a narrower passband than the second and / or third bandpass filters 212, 252. In some embodiments, the bandpass filter 264 may have a passband is similar or identical to the passband of the bandpass filter in the radio 290, for example, a passband of 3.395- 3.805 GHz (i.e., a passband having a width of 410 MHz) or a passband of 3.397-3.803 GHz (i.e., a passband having a width of 406 MHz). In some embodiments, the first bandpassfilter 264 may have a passband that is narrower than the passband of the bandpass filter in the radio 290. The first bandpass filter 264 may be designed so that any slightly out-of-band noise (e.g., RF energy in the guard bands of the second and third bandpass filters 212, 252) that is input to the low noise amplifier 262 is sufficiently attenuated by bandpass filter 264 such that any portion of the out-of-band noise that proceeds to loop through the transmit / receive circuit 200 back to the low noise amplifier 262 will have a magnitude that is less than the magnitude of the out-of-band noise signal that was originally input to the low noise amplifier 262.Consequently, even if the transmit / receive circuit 200 exhibits loop behavior with respect to out- of-band noise, the out-of-band noise becomes more attenuated each time it loops through the transmit / receive circuit 200 so that it is eventually eliminated.

[0059] A simple example that helps to explain how the transmit / receive circuit 200 may prevent positive loop gain behavior is illustrated in FIG. 6. FIG. 6 depicts one embodiment of the transmit / receive circuit of FIG. 5 with the passbands for the three bandpass filters 264, 212, 252 added along with the passband of the bandpass filter in the radio 290. FIG. 6 also includes and adds arrows that illustrate the path of a received slightly out-of-band noise signal. As shown in FIG. 6, for purposes of this example, the second and third bandpass filters 212, 252 each have passbands of 3.39-3.81 GHz, the bandpass filter that is included in radio 290 has a passband of 3.395-3.805 GHz, and the first bandpass filter 264 has a passband of 3.397-3.803 GHz.

[0060] Referring to FIG. 6, if a slightly out-of-band noise signal (e.g., RF energy that is in the 3.39-3.999 GHz or 3.801-3.810 GHz frequency ranges is received at antenna 292 during a receive time slot, then this noise signal will be passed to the third bandpass filter 252, where it will be attenuated / reflected by the third bandpass filter 252, but since the noise signal is so close to the passband of the third bandpass filter 252, a portion of the noise signal will pass through the third bandpass filter 252 and travel through the second circulator 240 to the low noise amplifier 262. The noise signal is amplified by the low noise amplifier 262, but then is attenuated by the first bandpass filter 264 before passing through the first circulator 220 to the second bandpass filter 212. The degree to which the first bandpass filter 264 attenuates the noise signal will depend upon the frequency range of the noise signal, with the noise signal being attenuated more the farther it is from the passband of the first bandpass filter 264. If the out-of-band noise signal is very close to an edge of the operating frequency band (e.g., 3.399 GHz), then the noise signal will experience very little attenuation by the first bandpass filter 264 since the noise signal willbe within the passband of bandpass filter 264. In contrast, if the out-of-band noise signal is very close to an edge of the passbands of the second and third bandpass filters (e.g., 3.391 GHz), then the noise signal will experience some but not full attenuation by the first bandpass filter 264 since the noise signal is only slightly outside the passband of the first bandpass filter 264.

[0061] The first circulator 220 passes the portion of the attenuated noise signal that is output by the first bandpass filter 264 to the second bandpass filter 212. The second bandpass filter 212 will mostly pass the attenuated noise signal to the radio 290 since the second bandpass filter 212 has a wider guard band such that slightly out-of-band noise is typically within the passband of the second bandpass filter 212. The bandpass filter in the radio 290, however, has a narrower passband and hence will typically attenuate / reflect at least some of the attenuated noise signal, thereby generating a first reflected noise signal that is reflected back through the second bandpass filter 212 (which mostly passes the first reflected noise signal) and onto the transmit path 230. This first reflected noise signal passes through the second circulator 240 to the third bandpass filter 252. The third bandpass filter 252 will typically mostly pass the first reflected noise signal (depending on the frequency of the noise signal) but will reflect a small portion of the first reflected noise signal (which is referred to herein as a second reflected noise signal) back through the second circulator 240 to the low noise amplifier 262. The low noise amplifier 262 amplifies the second reflected noise signal and passes it to the first bandpass filter 264. Thus, as can be seen from FIG. 6, out-of-band noise may loop through the transmit / receive circuit 200.

[0062] As described above, a serious problem occurs if the out-of-band noise signal loops through a transmit / receive circuit with positive gain, since the noise signal then grows in magnitude as it loops through the circuit until it eventually has a magnitude such that it damages the low noise amplifier 262 (which is typically the circuit element that is most susceptible to damage by high power signals). Since the first bandpass filter 264 has a narrower passband than the second and third bandpass filters 212 and 252, it will attenuate more of any slightly out-of- band noise signal than will the second and third bandpass filters 212 and 252, thereby reducing the magnitude of any slightly out-of-band noise signal that loops through transmit / receive circuit 200.

[0063] In some embodiments, the passband of the first bandpass filter 264 may be designed so that any slightly out-of-band noise (which herein refers to any RF energy that is outside the operating frequency band of the transmit / receive circuit that is within 20 MHz of theoperating frequency band) that is received by the antenna 292 and passed to the low noise amplifier 262 is sufficiently attenuated by the first bandpass filter 264 such that any portion of the out-of-band noise that loops through the transmit / receive circuit 200 back to the low noise amplifier 262 will have a magnitude that is less than the magnitude of the out-of-band noise signal that was originally input to the low noise amplifier 262. This ensures that the out-of-band noise signal will not grow in magnitude as it loops through transmit / receive circuit 200.

[0064] In other embodiments, the passband of the first bandpass filter 264 may be designed so that any slightly out-of-band noise that is received by the antenna 292 that is reflected back along the transmit path 230 may have a magnitude that is less than the magnitude of the out-of-band noise signal that was originally input to the low noise amplifier 262. This also ensures that the out-of-band noise signal will not grow in magnitude as it loops through transmit / receive circuit 200.

[0065] The transmit / receive circuit 200 may operate similarly to the conventional transmit / receive circuits discussed with reference to FIGS. 2-4, but may be implemented using low cost circulators while also reducing or preventing the possibility of positive loop gain behavior that could destroy the low noise amplifier. Moreover, the extra bandpass filter 264 included in transmit / receive circuit 200 may be relatively inexpensive (e.g., $10) and thus the transmit / receive circuit 200 may be significantly less expensive than conventional transmit / receive circuits that employ one or more high power, high speed switches (e.g., transmit / receive circuits 1 and 100' above).

[0066] As described above, pursuant to some embodiments of the present invention, transmit / receive circuits such as transmit / receive circuit 200 are provided that comprise an input 202, an output 204, a first circulator 220 that has a first circulator port 222, a second circulator port 224 and a third circulator port 226, where the first circulator port 222 is coupled to the input 202. The transmit / receive circuit 200 further comprises a second circulator 240 that has a fourth circulator port 242, a fifth circulator port 244 and a sixth circulator port 246, where the fourth circulator port 242 is coupled to the second circulator port 224 and the fifth circulator port 244 is coupled to the output 204. The transmit / receive circuit 200 also includes a low noise amplifier 262 and a first bandpass filter 264 that are coupled in between the sixth circulator port 246 and the third circulator port 226.

[0067] In some embodiments, an input of the low noise amplifier 262 is coupled to the sixth circulator port 246 and the first bandpass filter 264 is coupled in between an output of the low noise amplifier 262 and the third circulator port 226. It will be appreciated, however, that embodiments of the present invention are not limited thereto. For example, in other embodiments, the first bandpass filter 264 may be coupled in between the sixth circulator port 246 and the low noise amplifier 262.

[0068] In some embodiments, the transmi t / receive circuit 200 may further comprise a second bandpass filter 212 that is coupled in between the input 202 and the first circulator port 222 and / or a third bandpass filter 252 that is coupled in between the fifth circulator port 244 and the output 204. In such embodiments, a first passband of the first bandpass filter 264 may be narrower than a second passband of the second bandpass filter 212 and narrower than a third passband of the third bandpass filter 252. For example, a width of the first passband of the first bandpass filter 264 may be between 95% and 99% a width of a second passband of the second bandpass filter 212 and of a width of a third passband of the third and passband filter 252. For example, in the example above, the second and third bandpass filters 212, 252 each have passbands having widths of 420 MHz and the first bandpass filter 264 has a passband that has a width of 406 MHz, which is 96.66% of the width of the passbands of the second and third bandpass filters 212, 252. The transmi t / receive circuit 200 may be used in conjunction with a beamforming radio 290 that includes a fourth bandpass filter 291. In some embodiments, the passband of the first bandpass filter 264 may be less than or equal to the passband of the fourth bandpass filter 291.

[0069] Still referring to FIG. 5, it can be seen that pursuant to further embodiments of the present invention transmit / receive circuits such as transmi t / receive circuit 200 are provided that comprise an input 202, an output 204, a transmit path 230 that is configured to act as an RF transmission path for transmit signals only, a receive path 260 that is configured to act as an RF transmission path for receive signals only, the receive path including a first bandpass filter 264, a first bidirectional path 210 that has a first end that is coupled to the input 202 and a second end that is coupled to both the transmit path 230 and the receive path 260, the first bidirectional path 210 including a second bandpass filter 212 and configured to act as an RF transmission path for both transmit and receive signals, and a second bidirectional path 250 that has a first end that is coupled to both the transmit path 230 and the receive path 260 and a second end that is coupledto the output 204, the second bidirectional path 250 including a third bandpass filter 252 and configured to act as an RF transmission path for both transmit and receive signals. A first passband of the first bandpass filter 264 is narrower than a second passband of the second bandpass filter 212 and / or is narrower than a third passband of the third bandpass filter 252.

[0070] In some embodiments, the receive path 260 may further include a low noise amplifier 262. The first bandpass filter 264 may be coupled between the low noise amplifier 262 and the first bidirectional path 210. The transmit / receive circuit 200 may further comprise a first circulator 220 that has a first circulator port 222 that is coupled to the first bidirectional path 210 and / or a second circulator 240 that has a fifth circulator port 244 is coupled to the second bidirectional path 250.

[0071] Continuing to refer to FIG. 5, pursuant to still further embodiments of the present invention transmit / receive circuits such as transmit / receive circuit 200 are provided that comprise an input 202, an output 204, a low noise amplifier 262 that is coupled between the output 204 and the input 202, a first bandpass filter 264 that has a first passband that is coupled to an output of the low noise amplifier 262, a second bandpass filter 212 that has a second passband that is coupled to the input 202, and a third bandpass filter 252 that has a third passband that is coupled to the output 204. A difference between the lower edge of the second passband and a lower edge of an operating frequency band for the transmit / receive circuit 200 defines a first guard band, a difference between the upper edge of the third passband and an upper edge of the operating frequency band defines a second guard band, and the first bandpass filter 264 is configured to attenuate radio frequency energy that is within the first and second guard bands more than a maximum allowable gain of the low noise amplifier 262.

[0072] Some beamforming radios include an "antenna monitoring" feature that is used to detect if a base station antenna that is coupled to the radio is degraded or broken. The antenna monitoring feature triggers an alarm when a signal reflected by the antenna back to the radio is abnormal (e.g., too high), indicating a poor voltage standing wave ratio. Under normal operating conditions, a magnitude of the reflected signal may be much lower than the magnitude of the injected signal (e.g., 18-20 dB lower). When the antenna is broken, the magnitude of the reflected signal is typically much higher (e.g., only 5-6 dB below the magnitude of the injected signal). Thus, when the antenna monitor detects that the magnitude of the reflected signal (as compared to the magnitude of the injected signal) is above a certain threshold (e.g., 10 dB), theantenna monitor circuit may switch off the radio and raise an alarm. Since the transmit / receive circuit 200 uses a pair of circulators to route the transmit and receive signals, the injected RF energy that is reflected by the antenna will pass along the receive path 260. If the gain of the low noise amplifier 262 is sufficiently high, the amplified reflected signal may be sufficient to trigger the alarm in a radio that includes an antenna monitoring circuit. The alarm is triggered, however, not because the antenna is broken and hence presenting a true impedance mismatch, but because of the gain of the low noise amplifier. If the base station does not have an antenna monitoring circuit, then the gain of the low noise amplifier may be raised without consequence. However, if there is an antenna monitoring circuit in the radio, it may trigger an alarm, and for this reason it may be necessary to limit the gain of the low noise amplifier (e.g., to 6-8 dB).

[0073] FIG. 7 is a high-level circuit diagram of a transmit / receive circuit 200' for a time division duplex communications system according to further embodiments of the present invention. The transmit / receive circuit 200' is similar to the transmit / receive circuit 200 so the description below will focus on the differences between the two transmit / receive circuits 200, 200'.

[0074] As shown in FIG. 7, a third circulator 266 is provided along the receive path 260. The third circulator 266 is positioned in between the low noise amplifier 262 and the first bandpass filter 264. A first port 266A of the third circulator 266 is coupled to the output of the low noise amplifier 262, a second port 266B is coupled to the input of the first bandpass filter 264, and a third port 266C is coupled to ground through a termination resistor 269. The resistor 269 may form a matched termination that is impedance matched to the RF transmission lines of the receive path 260. The third circulator 266 is configured to route RF energy that may be flowing in the reverse direction along the receive path 260 to ground (i.e., RF energy incident to the third circulator 266 at the second port 266B thereof is routed to electrical ground through the third port 266C and the termination resistor 269). In other words, the third circulator 266 provides additional isolation at the output of the low noise amplifier 262 to protect the low noise amplifier 262 from signals travelling in the reverse or "wrong" direction along the receive path 260. RF energy may flow in this "wrong" direction because the first bandpass filter 264 and / or the first circulator 220 may reflect some of the energy incident thereto. The third circulator 266 may also increase the isolation between the transmit and receive paths 230, 260. Low cost circulators may only provide a limited level of isolation which may be insufficient in someapplications, particularly given the disparity in the RF signals levels that are passed along the transmit path 230 and the receive path 260. The third circulator 266 provides extra isolation by passing energy that leaks through the first circulator 220 to ground. All three circulators 220, 240, 266 may have the same design, although the third circulator 266 may have lower power handling capabilities as only lower power signals are passed along the receive path 260.

[0075] The transmi t / receive circuit 200 further includes a variable attenuator 268 that is provided along the receive path 260 between the first bandpass filter 264 and the first circulator 220. The variable attenuator 268 is used to adjust the gain to a desired level.

[0076] The transmi t / receive circuit 200 further includes a plurality of limiters 270. Each limiter 270 may be implemented, for example, as a PIN diode to ground. The limiters 270 are configured so that if an RF signal having too high of a power level (e.g., a power level that might damage components of the transmit / receive circuit 200') then the PIN diode turns on and shunts the RF signal to ground (e.g., through a termination resistor, not shown).

[0077] The transmit / receive circuit 200 further includes a low pass filter 254 that is coupled between the output 204 and the third bandpass filter 252. The low pass filter 254 may further attenuate lower frequency noise signals that are well below the operating frequency band of the transmit / receive circuit 200' to ensure that any such RF signals that are input to the low noise amplifier 262 will have very low magnitudes. The low pass filter 254 may also attenuate higher frequency noise signals that may be generated by the low noise amplifier 262 and / or the circulators 220, 240, 266 to meet systems requirements regarding spurious emission levels.

[0078] The transmit / receive circuit 200' may also include a fourth circulator 272 along the receive path 260. The fourth circulator 272 may be positioned, for example, between the attenuator 268 and the first circulator 220. The fourth circulator 272 passes received RF signals from the attenuator 268 to the third port 226 of the first circulator 220. If reflected RF energy is received at the second port 272B of the fourth circulator, it is passed to the limiter 270 or shunted to ground through a resistive termination.

[0079] It will be appreciated that any combination of the additional components that are included in the transmit / receive circuit 200' (as compared to the transmit / receive circuit 200 of FIG. 5) may be added to the transmit / receive circuit 200 of FIG. 5. For example, just the third circulator 266 might be added or just the fourth circulator 272 might be added along with their terminations to ground, or both might be added.

[0080] FIG. 8 is a high-level circuit diagram of a transmi t / receive circuit 300 for a time division duplex communications system according to an additional embodiment of the present invention. The transmit / receive circuit 300 of FIG. 8 is similar to the transmit / receive circuit 200 of FIG. 5, with the only difference being that the locations of the transmit path 330 and the receive path 360 are reversed as compared to the locations of the transmit path 230 and the receive path 260 in the transmit / receive circuit 200 of FIG. 5 and the direction of each of the circulators 220, 240 is reversed. Thus, further description of transmit / receive circuit 300 will be omitted.

[0081] The transmit / receive circuits according to embodiments of the present invention may have a number of advantages over conventional transmit / receive circuits. Since the transmit / receive circuits use circulators as opposed to switches, there is no delay in switching time between transmit and receive operations. This also means that no synchronization is needed, and that there is no data loss due to the need for a guard band between transmit and switching operations. There also are no spurious emissions which can occur with switch-based transmit / receive circuits. The transmit / receive circuits also may not include any active components other than the low noise amplifier, and do not need a power detector. The transmit / receive circuit also has low current requirements since it does not require a DC bias current, which is not available in some deployments. The transmit / receive circuits may also be significantly less expensive than a conventional transmit / receive circuit having similar capabilities and reliability.

[0082] The transmit / receive circuits according to embodiments of the present invention may be implemented in a tower mounted amplifier, within a radio, within a base station antenna or any other suitable location.

[0083] While the example embodiments of the present invention discussed above illustrate transmit / receive circuits that include a low noise amplifier along the receive path, it will be appreciated that a high power amplifier may be added along the transmit path of any of the above embodiments. The transmit / receive circuits according to embodiments of the present invention may be passive circuits that are relatively low cost while providing the switching speed and performance and power handling capabilities necessary for 5G applications.

[0084] The present invention has been described above with reference to the accompanying drawings. The invention is not limited to the illustrated embodiments; rather,these embodiments are intended to fully and completely disclose the invention to those skilled in this art. In the drawings, like numbers refer to like elements throughout.

[0085] It will be understood that when an element is referred to as being "connected" or "coupled" to another element, it can be directly connected or coupled to the other element, or intervening elements may be present. In contrast, when an element is referred to as being "directly connected" or "directly coupled" to another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a like fashion (i.e., "between" versus "directly between", "adjacent" versus "directly adjacent", etc.)

[0086] Spatially relative terms, such as "under", "below", "lower", "over", "upper", "top", "bottom" and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as "under" or "beneath" other elements or features would then be oriented "over" the other elements or features. Thus, the exemplary term "under" can encompass both an orientation of over and under. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.

[0087] Well-known functions or constructions may not be described in detail for brevity and / or clarity. As used herein the expression "and / or" includes any and all combinations of one or more of the associated listed items.

[0088] It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present invention.

[0089] It will be understood that the above embodiments may be combined in any way to provide a plurality of additional embodiments.

Claims

CLAIMS1. A transmi t / receive circuit, comprising: an input; an output; a first circulator that has a first circulator port, a second circulator port and a third circulator port, where the first circulator port is coupled to the input; a second circulator that has a fourth circulator port, a fifth circulator port and a sixth circulator port, where the fourth circulator port is coupled to the second circulator port and the fifth circulator port is coupled to the output; and a low noise amplifier and a first bandpass filter that are coupled in between the sixth circulator port and the third circulator port.

2. The transmit / receive circuit of Claim 1, wherein an input of the low noise amplifier is coupled to the sixth circulator port and the first bandpass filter is coupled in between an output of the low noise amplifier and the third circulator port.

3. The transmit / receive circuit of Claim 2, further comprising a second bandpass filter that is coupled in between the input and the first circulator port.

4. The transmit / receive circuit of Claim 3, further comprising a third bandpass filter that is coupled in between the fifth circulator port and the output.

5. The transmit / receive circuit of Claim 4, wherein a width of a first passband of the first bandpass filter is narrower than a width of a second passband of the second bandpass filter and narrower than a width of a third passband of the third bandpass filter.

6. The transmit / receive circuit of any of Claims 1-5, wherein the width of the first passband is between 95% and 99% the width of the third passband.

7. The transmit / receive circuit of any of Claims 1-5 in conjunction with a beamforming radio that includes a fourth bandpass filter that has a fourth passband, wherein the width of the first passband is closer to a width of the fourth passband than it is to the width of the third passband.

8. The transmit / receive circuit of any of Claims 1-5, wherein a first frequency band that is between the lower edge of the second passband and a lower edge of an operating frequency band for the transmit / receive circuit defines a first guard band, and a second frequency band that is between the upper edge of the third passband and an upper edge of the operating frequency band for the transmit / receive circuit defines a second guard band, and the first bandpass filter is configured to attenuate radio frequency energy that is within the first and second guard bands sufficiently so that it does not have a positive gain loop.

9. The transmit / receive circuit of any of Claims 1-5, wherein a first frequency band that is between the lower edge of the second passband and a lower edge of an operating frequency band for the transmit / receive circuit defines a first guard band, and a second frequency band that is between the upper edge of the third passband and an upper edge of the operating frequency band for the transmit / receive circuit defines a second guard band, and the first bandpass filter is configured to attenuate radio frequency energy that is within the first and second guard bands more than a maximum allowable gain of the low noise amplifier.

10. A transmit / receive circuit, comprising: an input; an output; a transmit path that is configured to act as a radio frequency ("RF") transmission path for transmit signals only; a receive path that is configured to act as an RF transmission path for receive signals only, the receive path including a first bandpass filter; a first bidirectional path that has a first end that is coupled to the input and a second end that is coupled to both the transmit path and the receive path, the first bidirectional path configured to act as an RF transmission path for both transmit and receive signals, the first bidirectional path including a second bandpass filter; and a second bidirectional path that has a first end that is coupled to both the transmit path and the receive path and a second end that is coupled to the output, the second bidirectional path configured to act as an RF transmission path for both transmit and receive signals, the second bidirectional path including a third bandpass filter,wherein a first passband of the first bandpass filter is narrower than a second passband of the second bandpass filter.

11. The transmit / receive circuit of Claim 10, wherein the first passband is narrower than a third passband of the third bandpass filter.

12. The transmit / receive circuit of Claim 10 or Claim 11, wherein the receive path further includes a low noise amplifier.

13. The transmit / receive circuit of Claim 12, wherein the first bandpass filter is coupled between the low noise amplifier and the first bidirectional path.

14. The transmit / receive circuit of Claim 13, further comprising a first circulator that has a first circulator port that is coupled to the first bidirectional path, a second circulator port that is coupled to the transmit path, and a third circulator port that is coupled to the receive path.

15. The transmit / receive circuit of Claim 14, further comprising a second circulator that has a fourth circulator port that is coupled to the transmit path, a fifth circulator port that is coupled to the second bidirectional path, and a sixth circulator port that is coupled to the receive path.

16. The transmit / receive circuit of any of Claims 10-15, wherein a width of the first passband is between 95% and 99% a width of the second passband.

17. The transmit / receive circuit of any of Claims 10-15 in conjunction with a beamforming radio that includes a fourth bandpass filter that has a fourth passband, wherein the width of the first passband is closer to a width of the fourth passband than it is to the width of the second passband.

18. The transmit / receive circuit of any of Claims 10-15, wherein a first frequency band that is between the lower edge of the second passband and a lower edge of an operating frequency band for the transmit / receive circuit defines a first guard band, and a second frequency band that is between the upper edge of the third passband and an upper edge of the operating frequency band for the transmit / receive circuit defines a second guard band, and wherein the firstbandpass filter is configured to attenuate radio frequency energy that is within the first and second guard bands more than a maximum allowable gain of the low noise amplifier.

19. The transmit / receive circuit of any of Claims 10-15, wherein a first frequency band that is between the lower edge of the second passband and a lower edge of an operating frequency band for the transmit / receive circuit defines a first guard band, and a second frequency band that is between the upper edge of the third passband and an upper edge of the operating frequency band for the transmit / receive circuit defines a second guard band, and wherein the first bandpass filter is configured to attenuate radio frequency energy that is within the first and second guard bands sufficiently so that it does not have a positive gain loop.

20. A transmit / receive circuit, comprising: an input; an output; a low noise amplifier coupled between the output and the input; a first bandpass filter that has a first passband that is coupled to an output of the low noise amplifier; a second bandpass filter that has a second passband that is coupled to the input; and a third bandpass filter that has a third passband that is coupled to the output; wherein a first frequency band that is between the lower edge of the second passband and a lower edge of an operating frequency band for the transmit / receive circuit defines a first guard band, and a second frequency band that is between the upper edge of the third passband and an upper edge of the operating frequency band for the transmit / receive circuit defines a second guard band, and wherein the first bandpass filter is configured to attenuate radio frequency energy that is within the first and second guard bands more than a maximum allowable gain of the low noise amplifier.

21. The transmit / receive circuit of Claim 20, further comprising a first circulator that has a first circulator port that is coupled to the input via a first bidirectional path and that is configured to pass both transmit and receive signals and a second circulator that has a fifth circulator port that is coupled to the output via a second bidirectional path that is configured to pass both transmit and receive signals.

22. The transmit / receive circuit of Claim 21, further comprising a transmit path that extends between a second circulator port of the first circulator and a fourth circulator port of the second circulator.

23. The transmit / receive circuit of Claim 22, further comprising a receive path that extends between a sixth circulator port of the second circulator and a third circulator port of the first circulator.

24. The transmit / receive circuit of Claim 23, wherein the low noise amplifier and the first bandpass filter are part of the receive path.

25. The transmit / receive circuit of Claim 24, wherein the second bandpass filter is part of the first bidirectional path and the third bandpass filter is part of the second bidirectional path.

26. The transmit / receive circuit of any of Claims 20-25, wherein a width of a first passband of the first bandpass filter is narrower than a width of a second passband of the second bandpass filter.

27. The transmit / receive circuit of Claim 26, wherein the width of the first passband is narrower than a width of a third passband of the third bandpass filter.

28. The transmit / receive circuit of Claim 26, wherein the width of the first passband is between 95% and 99% the width of the third passband.

29. The transmit / receive circuit of Claim 27 in conjunction with a beamforming radio that includes a fourth bandpass filter that has a fourth passband, wherein the width of the first passband is closer to a width of the fourth passband than it is to the width of the third passband.

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