Terahertz broadband on-off keying modulator with stable input impedance

Abstract

The invention discloses a terahertz broadband on-off keying modulator with stable input impedance. According to the invention, a parallel common-base common-emitter bottom differential pair is adopted, and when the parallel common-base common-emitter bottom differential pair works in a differential mode state, a signal path formed by transistors Q6, Q5, Q7 and Q8 forms an amplification path, so that high current gain is ensured; when working in a common-mode state, signals from the common emitter transistor pairs Q5 and Q8 and signals from the common base transistor pairs Q6 and Q7 are offset, so that common-mode leakage signals are inhibited, and the'on-off 'isolation degree is further improved. The working frequency band is regulated and controlled by regulating and controlling the direct-current bias and the size of the common-base transistor to the Q6 and the Q7. According to the invention, a parallel common-base common-emitter differential pair is adopted as a double-balance differential structure OOK modulator of a bottom tail tube, so that low on / off input impedance change is realized while ultra-wideband input matching and high switch isolation are realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of radio frequency integrated circuits, and relates to a terahertz on-off keying (OOK) modulation circuit, and in particular to a terahertz broadband on-off keying modulator with stable input impedance. Background Art

[0002] On-off keying (OOK) modulation is a non-coherent modulation technology that converts digital signals into radio frequency signals with two different amplitudes. When the digital signal is 1, the modulator outputs a high-amplitude carrier signal; when the digital signal is 0, the modulator outputs a low-amplitude carrier signal. Due to the non-coherent characteristics of OOK modulation, there is no need to consider phase locking and recovery of the local oscillator signal in system design. At the same time, the sampling rate requirements for the analog-to-digital conversion interface circuit are low, so it has the advantages of simple structure, small area, and low power consumption. Thanks to the rich spectrum resources in the terahertz frequency band, communication systems based on OOK modulation have great application potential in short-distance, ultra-high-speed terahertz wireless communication scenarios.

[0003] In the OOK modulation circuit, the input end of the modulator is connected to the output end of the oscillator. If the input impedance of the OOK modulator changes too much when it is turned on and off, it will cause the instability of the amplitude and frequency of the oscillator output signal, resulting in reduced reliability and stability of the OOK modulation system. The traditional OOK modulator adopts a switch-type cascode structure to achieve the modulation function by controlling the gate (base) bias of the common gate (base) level. As the frequency increases to the terahertz band, the sharply deteriorating parasitic effects will cause the local oscillator to leak, causing the operating bandwidth and "on / off" isolation of the traditional modulator to deteriorate sharply, and the input impedance changes too much in the "on / off" state. In theory, the Gilbert structure can improve the "on / off" isolation of the OOK modulator, but this structure still has the problems of narrow bandwidth and large "on / off" input impedance difference. Therefore, it is necessary to invent an OOK modulator with ultra-wideband, low "on / off" input impedance change and high "on / off" isolation in the terahertz band, so as to be applied to the ultra-high-speed OOK transceiver system in the terahertz band, thereby solving the potential reliability and stability problems of the system.

[0004] In view of the above technical background and existing problems, the present invention provides a terahertz broadband on-off keying modulator with stable input impedance, which is based on the design of common-base common-emitter parallel differential pairs, replacing the common-emitter differential pairs of the traditional Gilbert structure, and realizing low "on / off" input impedance variation and ultra-wide operating bandwidth in the terahertz frequency band, while maintaining very high switch isolation to meet the requirements of high-speed communication systems. Summary of the invention

[0005] The purpose of the present invention is to provide a high-isolation terahertz OOK modulator with low input impedance variation in view of the deficiencies in the existing technology. The present invention adopts a double-balanced differential structure OOK modulator with a parallel common-base common-emitter differential pair as a bottom tail pipe, which realizes low "on / off" input impedance variation while achieving ultra-wideband input matching and high switch isolation.

[0006] The technical solution of the present invention is as follows: A high-isolation terahertz OOK modulator with low input impedance variation includes: a double-balanced top differential pair, a parallel cascode bottom differential pair, a bias circuit, an input balun, and an output balun. The power supply terminal of the output balun is connected to a power supply Vc, the ground terminal is grounded, the output terminal serves as the output terminal of the OOK modulator, and the input terminal is connected to the output terminal of the double-balanced top differential pair; the input terminal of the double-balanced top differential pair is connected to the output terminal of the parallel cascode differential pair; the input terminal of the parallel cascode differential pair is connected to the differential terminal of the input balun; the ground terminal of the input balun is grounded, and the input terminal is connected to a single-ended local oscillator input signal LO in .

[0007] in: The double-balanced top differential pair includes a first transistor Q1, a second transistor Q2, a third transistor Q3, and a fourth transistor Q4; the collector of the first transistor Q1 is connected to an input terminal of the output balun and the collector of the third transistor Q3, the base is grounded, the emitter is connected to the emitter of the second transistor Q2 and then connected to an output terminal of the parallel common-base common-emitter bottom differential pair; the collector of the second transistor Q2 is connected to the collector of the fourth transistor Q4 and then connected to another input terminal of the output balun, the base is connected to the base of the third transistor Q3 and then connected to the baseband input signal BB in The emitter of the third transistor Q3 is connected to the emitter of the fourth transistor Q4 and then connected to the other output end of the parallel cascode bottom differential pair; the base of the fourth transistor Q4 is grounded.

[0008] The parallel type common base common emitter bottom differential pair includes a fifth transistor Q5, a sixth transistor Q6, a seventh transistor Q7, and an eighth transistor Q8; the fifth transistor Q5 and the eighth transistor Q8 are a common emitter transistor pair; the sixth transistor Q6 and the seventh transistor Q7 are a common base transistor pair; the base of the fifth transistor Q5 is connected to the emitter of the seventh transistor Q7, serving as the in-phase input port of the parallel type common base common emitter bottom differential pair, and is connected to a differential end of the input balun; the emitter of the sixth transistor Q6 is connected to the base of the eighth transistor Q8, As the inverting input port of the parallel cascode bottom differential pair, it is connected to the other differential end of the input balun; the collector of the fifth transistor Q5 is connected to the collector of the sixth transistor Q6, serving as an output end of the parallel cascode bottom differential pair; the collector of the seventh transistor Q7 is connected to the collector of the eighth transistor Q8, serving as another output end of the parallel cascode bottom differential pair; the emitter of the fifth transistor Q5 and the emitter of the eighth transistor Q8 are grounded; the base of the sixth transistor Q6 is connected to the base of the seventh transistor Q7.

[0009] Preferably, a bias circuit is also included; the bias circuit includes a first resistor R1, a second resistor R2, a third resistor R3, a fourth resistor R4, a fifth resistor R5, a sixth resistor R6 and a seventh resistor R7; one end of the first resistor R1 is connected to one end of the second resistor R2 and then connected to a power supply V C , and the other end is connected to an output end of the parallel common base common emitter bottom differential pair; the other end of the second resistor R2 is connected to the other output end of the parallel common base common emitter bottom differential pair; one end of the third resistor R3 is connected to one end of the fourth resistor R4 and then connected to the base of the sixth transistor Q6, and the other end is connected to an output end of the parallel common base common emitter bottom differential pair; the other end of the fourth resistor R4 is connected to the other output end of the parallel common base common emitter bottom differential pair; one end of the fifth resistor R5 is connected to the emitter of the fifth transistor Q5, and the other end is grounded; one end of the sixth resistor R6 is connected to the emitter of the eighth transistor Q8, and the other end is grounded; one end of the seventh resistor R7 is connected to the ground end of the input balun, and the other end is grounded.

[0010] When the parallel common-base common-emitter bottom layer differential pair described in the present invention operates in a differential mode state, the signal path formed by the sixth transistor Q6 for in-phase amplification and the fifth transistor Q5 for inverting amplification and the signal path formed by the seventh transistor Q7 for in-phase amplification and the eighth transistor Q8 for inverting amplification constitute an amplification path, ensuring high current gain; when the parallel common-base common-emitter bottom layer differential pair operates in a common mode state, the signals from the common-emitter transistor pair Q5, Q8 and the common-base transistor pair Q6, Q7 are offset, thereby suppressing the common-mode leakage signal and further improving the "on-off" isolation.

[0011] For the common-emitter transistor pair Q5 and Q8, the "collector-emitter" parasitic capacitance of the common-base transistor pair Q6 and Q7 can be regarded as its own neutralization capacitance, thereby improving the isolation of the common-base common-emitter differential transistor pair and achieving impedance stability of the local oscillator input port; since this group of "neutralization capacitance" is actually the transistor base diffusion capacitance C1=τ F ×g1,τ F is the time constant, g m is the transistor transconductance; if the DC bias and size of the transistor are precisely designed, this group of "neutralizing capacitors" can offset the input parasitic capacitance in the parallel common-base and common-emitter bottom differential pair within an ultra-wide frequency band, and realize a purely resistive input impedance that hardly changes with frequency, thereby achieving an ultra-wide operating frequency band; in summary, the operating frequency band can be regulated by adjusting the DC bias and size of the common-base transistor pair Q6 and Q7.

[0012] The present invention has the following advantages and beneficial effects: (1) By designing a novel parallel cascode bottom differential pair, the present invention can significantly reduce the difference between the impedance of the local oscillator signal input port in the "on" and "off" states compared to the traditional double-balanced Gilbert unit, thereby improving the working stability of the oscillator.

[0013] (2) Based on the novel parallel common-base common-emitter bottom differential pair, the present invention reduces the matching difficulty and greatly improves the working bandwidth compared to the traditional double-balanced Gilbert unit.

[0014] (3) The novel parallel common-base common-emitter bottom differential pair can further improve the common-mode rejection performance based on the traditional Gilbert cell, thereby improving the circuit stability and isolation. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 is a circuit schematic diagram of the present invention; Figure 2 This is the schematic diagram of the traditional Gilbert structure OOK modulator; Figure 3 The input impedance Smith chart simulation results of the present invention and the traditional Gilbert structure in the "on / off" state are compared; Figure 4 The difference between the present invention and the traditional Gilbert structure in the "on / off" state 11 |Simulation results comparison; Figure 5 It is the S parameter simulation result of the present invention in the "on / off" state; Figure 6 This is the transient simulation result of the present invention. DETAILED DESCRIPTION

[0016] The present invention is further described below by way of examples in combination with the accompanying drawings, but the present invention is not limited to the scope described in the examples.

[0017] like Figure 1 As shown, this example provides a high-isolation terahertz band OOK modulator with low input impedance variation, which as a whole includes a double-balanced top differential pair, a parallel common-base common-emitter bottom differential pair, a bias circuit, an input balun, and an output balun. Among them, the double-balanced top differential pair is composed of transistors Q1, Q2, Q3, and Q4. The parallel common-base common-emitter bottom differential pair is composed of transistors Q5, Q6, Q7, and Q8, which is the core innovative part of the present invention. The bias circuit includes a first resistor R1, a second resistor R2, a third resistor R3, a fourth resistor R4, a fifth resistor R5, a sixth resistor R6, and a seventh resistor R7; the first resistor R1 and the second resistor R2 are connected to the power supply V C and the connection of the parallel common base common emitter bottom differential pair and the double balanced top differential pair; the third resistor R3 and the fourth resistor R4 are connected to the connection of the parallel common base common emitter bottom differential pair and the double balanced top differential pair and the base of the transistor Q6 and the transistor Q7; the fifth resistor R5 and the sixth resistor R6 are connected to the emitter of the transistor Q5 and the transistor Q8 respectively; the seventh resistor R7 is connected to the ground through the center tap of the input balun secondary coil. The bias circuit can use a resistor network to provide a DC bias current for the transistor, or can use other methods such as a current source, a radio frequency choke, etc. to provide a suitable DC bias point for the transistor.

[0018] The single-ended input port of the input balun is connected to the local oscillator signal LO in , and convert it into a set of differential signals, which are input into the parallel common base and common emitter bottom differential pair; the output balun converts the differential RF signal output by the double balanced top differential pair into a single-ended RF signal RF out , used to connect to the subsequent circuit. Since the circuit is a fully differential architecture, the center tap of the secondary coil of the input balun and the center tap of the primary coil of the output balun are differential virtual ground points, which can be used for DC feeding. For example, the center tap of the primary coil of the output balun is connected to the main power supply V C ; The bias resistor R7 is connected to the center tap of the secondary coil.

[0019] The single-ended input port of the input balun is connected to the local oscillator signal LO in ; The differential port is connected to the parallel type common base and common emitter bottom differential pair; The center tap of the output balun primary coil is connected to the power supply V C ; The differential port is connected to the double-balanced top differential pair; the single-ended port is the RF output port RF of the OOK modulated signal out ; The above description uses the bipolar junction transistor BJT as an example; the first to eighth transistors may also be metal oxide semiconductor field effect transistors MOSFET, or high electron mobility transistors HEMT; if the first to eighth transistors are metal oxide semiconductor field effect transistors MOSFET, or high electron mobility transistors HEMT, the naming method of the electrodes of the transistors is different, and the common base and common emitter structures should correspond to common gate and common source structures respectively.

[0020] Figure 2 It is a traditional Gilbert double-balanced structure, which has the advantage that the local oscillator signal leaking from the local oscillator input port to the RF port can be reversely offset, so it has good isolation. However, the bottom transistor of the traditional structure is a common-emitter structure, which has poor isolation. Therefore, the impedance change of the top differential pair during switching will cause the impedance change of the local oscillator signal input port, causing the instability of the oscillator.

[0021] The following are the layout simulation results of the design example: Figure 3 The simulation results of the Smith chart of this example and the traditional Gilbert double-balanced structure in the range of 200-280 GHz are shown. By comparison, it can be found that the input impedance curve of the present invention is closer to the origin on the Smith chart as a whole, whether it is in the on or off state, and the "on" state curve and the "off" state curve are close as a whole, indicating that the present invention has the advantages of ultra-wide working bandwidth and stable input matching; while the input impedance curve of the traditional Gilbert double-balanced structure is close to the origin only in a small range, and the "on" state curve and the "off" state curve are far apart, indicating that the traditional structure can only work in a narrow frequency band, and the input matching stability of the local oscillator port is poor, which easily causes unstable operation of the oscillator.

[0022] Figure 4 The |S in the range of 200 – 280 GHz for this example and the conventional Gilbert double-balanced structure 11 |Simulation results. Through the simulation results, it can be intuitively found that the present invention achieves a return loss of less than -10 dB in the range of 200-280 GHz; while the -10 dB bandwidth of the traditional Gilbert double-balanced structure is only 35 GHz. The overall simulation results show that the double-balanced structure of the present invention greatly improves the input matching performance in a wide frequency band. Compared with the traditional Gilbert double-balanced structure, the overall improvement in the range of 200-280 GHz is nearly -10 dB. The comparison results show that the improved Gilbert double-balanced structure has greatly improved the bandwidth performance compared to the traditional structure.

[0023] Figure 5The S-parameter simulation results of this example are shown. It can be seen from the figure that the insertion loss of the new OOK modulator in the on state at 240 GHz is less than 4 dB, and the on / off isolation reaches -51 dB, which can fully guarantee the reliability of the OOK communication system.

[0024] Figure 6 The transient response simulation result of this example under 24 Gbps baseband signal modulation. It can be seen that when the input signal is "0", the amplitude of the output voltage is close to 0 V, indicating that the OOK modulator proposed in the present invention has the advantage of high isolation and can ensure the reliability of the OOK communication system. At the same time, when the control signal changes, the switching time of the RF signal amplitude of the chip in this example increases from 10% to more than 90% in less than 0.02 ns, which can theoretically support a modulation rate of up to 50 Gbps, providing support for ultra-high-speed OOK communication systems.

[0025] This example is implemented based on 130nm SiGe BiCMOS process.

[0026] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A high-isolation terahertz OOK modulator with low input impedance variation, characterized in that It includes a double-balanced top-layer differential pair, a parallel common-base common-emitter bottom-layer differential pair, a bias circuit, an input balun, and an output balun; the power supply end of the output balun is connected to a power supply Vc, the ground end is grounded, the output end serves as the output end of the OOK modulator, and the input end is connected to the output end of the double-balanced top-layer differential pair; the input end of the double-balanced top-layer differential pair is connected to the output end of the parallel common-base common-emitter differential pair; the input end of the parallel common-base common-emitter differential pair is connected to the differential end of the input balun; the ground end of the input balun is grounded, and the input end is connected to a local oscillator input signal LO in .

2. A high-isolation terahertz OOK modulator with low input impedance variation according to claim 1, characterized in that: The double-balanced top differential pair includes a first transistor Q1, a second transistor Q2, a third transistor Q3, and a fourth transistor Q4; the collector of the first transistor Q1 is connected to an input terminal of the output balun and the collector of the third transistor Q3, the base is grounded, the emitter is connected to the emitter of the second transistor Q2 and then connected to an output terminal of the parallel common-base common-emitter bottom differential pair; the collector of the second transistor Q2 is connected to the collector of the fourth transistor Q4 and then connected to another input terminal of the output balun, the base is connected to the base of the third transistor Q3 and then connected to the baseband input signal BB in The emitter of the third transistor Q3 is connected to the emitter of the fourth transistor Q4 and then connected to the other output end of the parallel cascode bottom differential pair; the base of the fourth transistor Q4 is grounded.

3. The high-isolation terahertz OOK modulator with low input impedance variation according to claim 2, characterized in that: The parallel type common base common emitter bottom differential pair includes a fifth transistor Q5, a sixth transistor Q6, a seventh transistor Q7, and an eighth transistor Q8; the fifth transistor Q5 and the eighth transistor Q8 are a common emitter transistor pair; the sixth transistor Q6 and the seventh transistor Q7 are a common base transistor pair; the base of the fifth transistor Q5 is connected to the emitter of the seventh transistor Q7, serving as the in-phase input port of the parallel type common base common emitter bottom differential pair, and is connected to a differential end of the input balun; the emitter of the sixth transistor Q6 is connected to the base of the eighth transistor Q8, As the inverting input port of the parallel cascode bottom differential pair, it is connected to the other differential end of the input balun; the collector of the fifth transistor Q5 is connected to the collector of the sixth transistor Q6, serving as an output end of the parallel cascode bottom differential pair; the collector of the seventh transistor Q7 is connected to the collector of the eighth transistor Q8, serving as another output end of the parallel cascode bottom differential pair; the emitter of the fifth transistor Q5 and the emitter of the eighth transistor Q8 are grounded; the base of the sixth transistor Q6 is connected to the base of the seventh transistor Q7.

4. The high-isolation terahertz OOK modulator with low input impedance variation according to claim 3, characterized in that: The device also includes a bias circuit; the bias circuit includes a first resistor R1, a second resistor R2, a third resistor R3, a fourth resistor R4, a fifth resistor R5, a sixth resistor R6 and a seventh resistor R7; one end of the first resistor R1 is connected to one end of the second resistor R2 and then connected to a power source V C , and the other end is connected to an output end of the parallel common base common emitter bottom differential pair; the other end of the second resistor R2 is connected to the other output end of the parallel common base common emitter bottom differential pair; one end of the third resistor R3 is connected to one end of the fourth resistor R4 and then connected to the base of the sixth transistor Q6, and the other end is connected to an output end of the parallel common base common emitter bottom differential pair; the other end of the fourth resistor R4 is connected to the other output end of the parallel common base common emitter bottom differential pair; one end of the fifth resistor R5 is connected to the emitter of the fifth transistor Q5, and the other end is grounded; one end of the sixth resistor R6 is connected to the emitter of the eighth transistor Q8, and the other end is grounded; one end of the seventh resistor R7 is connected to the ground end of the input balun, and the other end is grounded.

5. A high-isolation terahertz OOK modulator with low input impedance variation according to any one of claims 1 to 4, characterized in that: When the parallel-type common-base common-emitter bottom layer differential pair operates in a differential mode state, a signal path formed by the sixth transistor Q6 for in-phase amplification and the fifth transistor Q5 for inverting amplification and a signal path formed by the seventh transistor Q7 for in-phase amplification and the eighth transistor Q8 for inverting amplification constitute an amplification path; when the parallel-type common-base common-emitter bottom layer differential pair operates in a common mode state, signals from the common-emitter transistor pair Q5, Q8 and the common-base transistor pair Q6, Q7 are offset, thereby suppressing common-mode leakage signals.

6. A high-isolation terahertz OOK modulator with low input impedance variation according to any one of claims 1 to 4, characterized in that: The operating frequency band can be regulated by adjusting the DC bias and size of the common base transistor pair Q6 and Q7.

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