A hybrid frequency synthesizer based on quadrature modulation

Abstract

The invention belongs to the field of electronic technology and discloses a hybrid frequency synthesizer based on orthogonal modulation, comprising: an integrated phase-locked loop, a loop filter, a voltage-controlled oscillator, a power divider, an orthogonal modulator, a standard frequency generator, two direct digital frequency synthesizers, a radio frequency amplifier, a filter, and a controller. The hybrid frequency synthesizer is applied to a local oscillator or radio frequency excitation signal of an ultrashort wave superheterodyne communication device, has a wide frequency coverage range, low noise and spurious, short frequency switching time, and excellent noise and spurious performance.

Description

Technical Field

[0001] The invention belongs to the field of electronic technology, and in particular relates to a hybrid frequency synthesizer based on orthogonal modulation, which is applied to a local oscillator or radio frequency excitation signal of an ultrashort wave superheterodyne communication device. Background Art

[0002] Among the current various communication devices, frequency synthesizers are the core components of modern instruments and communication systems. PLL frequency synthesis technology has the advantages of high frequency and good spurious suppression capability, but its frequency switching speed is low, and the phase noise is poor when the step is small; while DDS has fast agility and high frequency and phase resolution capabilities, but its spurious suppression capability is poor. Therefore, the combination of DDS and PLL frequency synthesis technologies can improve the overall performance of the system and has become the current mainstream design solution. However, with the expansion of product demand and changes in the application environment, new requirements are put forward for equipment performance indicators. For example, in order to reduce the number of equipment types, the original multiple frequency band radio stations need to be synthesized into a wide frequency band radio station; in order to further improve the anti-interference ability of the radio station and avoid enemy tracking interference, the frequency hopping rate needs to be increased, which will put higher requirements on the channel frequency switching time of the equipment; in order to better meet the communication requirements of multiple devices co-located in the same vehicle, the bottom noise, spurious and other performance of the equipment need to be further improved. Summary of the invention

[0003] In view of the above problems, the object of the present invention is to provide a hybrid frequency synthesizer based on orthogonal modulation, which has a wide frequency coverage range, low noise and spurious emission, short frequency switching time, and excellent noise and spurious emission performance.

[0004] In order to achieve the above object, the present invention adopts the following technical solution to achieve it.

[0005] A hybrid frequency synthesizer based on orthogonal modulation, the hybrid frequency synthesizer comprising: an integrated phase-locked loop, a loop filter, a voltage-controlled oscillator, a power divider, an orthogonal modulator, a standard frequency generator, two direct digital frequency synthesizers, a radio frequency amplifier, a filter, and a controller;

[0006] Wherein, the control signal output terminal of the controller is respectively connected to the control signal input terminals of the integrated phase-locked loop and two direct digital frequency synthesizers;

[0007] The signal output terminal of the standard frequency generator is connected to the standard frequency input terminals of the integrated phase-locked loop and two direct digital frequency synthesizers respectively;

[0008] The signal output terminals of the two direct digital frequency synthesizers are respectively connected to the I-channel signal input terminal and the Q-channel signal input terminal of the orthogonal modulator;

[0009] The signal output end of the orthogonal modulator is connected to the signal input end of the integrated phase-locked loop, the signal output end of the integrated phase-locked loop is connected to the signal input end of the loop filter, the signal output end of the loop filter is connected to the signal input end of the voltage-controlled oscillator, the signal output end of the voltage-controlled oscillator is connected to the signal input end of the power divider, the signal output end of the power divider is connected to the signal input end of the radio frequency amplifier, the signal output end of the radio frequency amplifier is connected to the signal input end of the filter, and the signal output end of the filter outputs a local oscillator signal;

[0010] The radio frequency signal output end of the power divider is connected to the radio frequency signal input end of the orthogonal modulator.

[0011] The characteristics and further improvements of the technical solution of the present invention are:

[0012] (1) The standard frequency generator is used to generate a 28.8 MHz reference frequency signal and send the reference frequency signal to an integrated phase-locked loop and two direct digital frequency synthesizers;

[0013] The orthogonal modulator is used to receive two output signals of two direct digital frequency synthesizers and a radio frequency signal output by a power divider, and perform orthogonal mixing on the two output signals and the radio frequency signal respectively, thereby obtaining a synthesized signal;

[0014] The integrated phase-locked loop is used to receive the synthesized signal output by the orthogonal modulator and the reference frequency signal output by the standard frequency generator, and compare the phases of the synthesized signal and the reference frequency signal to obtain an output phase-discrimination voltage signal;

[0015] The loop filter is used to filter the phase-discrimination voltage signal to obtain a filtered voltage signal;

[0016] The voltage-controlled oscillator is used to set the free oscillation frequency to the locking frequency according to the filtered voltage signal and output a radio frequency signal;

[0017] The power divider is used to divide the radio frequency signal into two output paths, one path is output to the orthogonal modulator for orthogonal mixing, and the other path is output to the radio frequency amplifier for amplification;

[0018] The filter is used to filter out harmonics and spurious signals from the signal amplified by the radio frequency amplifier to obtain an output local oscillator signal.

[0019] (2) The controller is used to send control signals to the integrated phase-locked loop and two digital frequency synthesizers respectively; the control signal includes a control frequency word, a clock and an enable signal.

[0020] (3) The integrated phase-locked loop is composed of an integrated circuit chip ADF4153 and its peripheral circuits;

[0021] The loop filter is composed of an active low-noise operational amplifier MC33078D, a single-pole single analog switch MAX4648, a single-pole dual analog switch MAX4649, and a resistor and capacitor network;

[0022] The voltage controlled oscillator is composed of a chip RFMD-16 and its peripheral circuits;

[0023] The orthogonal modulator is composed of integrated circuit AD8346 and its peripheral circuits;

[0024] Two direct digital frequency synthesizers are composed of integrated circuit AD9834 and its peripheral circuits;

[0025] The power divider is composed of a transformer CP0603A1441 and its peripheral matching resistors;

[0026] The radio frequency amplifier is composed of a radio frequency integrated circuit MGA-61563 and its peripheral inductors and capacitors;

[0027] The filter is composed of two low-pass filters LFCN-1500 and LFCN-1800 and two radio frequency electronic switches AS211-334;

[0028] The controller is composed of a single chip microcomputer or a FPGA controller.

[0029] The frequency synthesizer has an output frequency of 940MHz to 1810MHz, a wide frequency range, low noise, and low spurious. The phase noise is less than -80dBc / Hz (2kHz away from the main spectrum), and the spurious level reaches -80dBc. The improvement of these indicators can play an important role in improving the overall indicators of the ultra-short wave communication system. At the same time, because the frequency combination locking time is less than 35μs and the frequency switching time is fast, it can meet the application of the ultra-short wave radio frequency hopping system and is very suitable for high-performance ultra-short wave radio application environment. The frequency synthesizer can be applied to any ultra-short wave frequency band frequency synthesizer, and has the advantages of simple circuit, high integration, short frequency conversion time, and high phase noise and spurious indicators. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0031] Figure 1A schematic diagram of the structure of a hybrid frequency synthesizer based on orthogonal modulation provided by an embodiment of the present invention;

[0032] Figure 2 A schematic diagram of the working principle of a hybrid frequency synthesizer based on orthogonal modulation provided by an embodiment of the present invention;

[0033] Figure 3 A schematic diagram of the circuit structure of a hybrid frequency synthesizer based on orthogonal modulation provided by an embodiment of the present invention;

[0034] Figure 1 Among them, label 1-integrated phase-locked loop PLL, label 2-loop filter, label 3-voltage controlled oscillator, label 4-orthogonal modulator, label 5-standard frequency, label 6-digital frequency synthesizer, label 7-digital frequency synthesizer, label 8-power divider, label 9-RF amplifier, label 10-filter, label 11-controller, label 12-power management. DETAILED DESCRIPTION

[0035] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0036] The present invention provides a hybrid frequency synthesizer based on orthogonal modulation, a design scheme using orthogonal modulation combined with DDS interpolation PLL, such as Figure 1 As shown, it at least includes an integrated phase-locked loop PLL, which is connected end to end with a loop filter, a voltage-controlled oscillator VCO, a power divider, and an orthogonal modulator to form a closed-loop system; the standard frequency is connected to the PLL and two direct digital frequency synthesizers DDS respectively; the two DDSs are connected to the orthogonal modulator; the RF amplifier is connected to the power divider and the filter respectively; the controller FPGA is connected to the integrated phase-locked loop PLL and the two direct digital frequency synthesizers DDS respectively.

[0037] The working principle of the device of the present invention is that the reference frequency 5 generates a reference frequency of 28.8MHz, which is respectively sent to the integrated phase-locked loop 1, the direct digital frequency synthesizer 6 and the direct digital frequency synthesizer 7. The two output signals of the direct digital frequency synthesizer 6 and the direct digital frequency synthesizer 7 and the radio frequency signal output by the voltage-controlled oscillator 3 are added to the orthogonal modulator 4 for orthogonal mixing (0° and 90° phase directions), and the obtained signals are synthesized. During the synthesis process, the sideband signals are superimposed on each other, and the useless sidebands are offset each other, thereby greatly reducing the complexity of the broadband and small-step frequency source, making a broadband mixing phase-locked loop device that originally could not use a filter to filter out the image signal The design was successfully implemented. The output signal of the orthogonal modulator 4 and the reference frequency generated by the standard frequency 5 are added to the integrated phase-locked loop 1 for phase comparison. The voltage output by the phase detector is filtered by the loop filter 2, and the filtered voltage is added to the voltage-controlled oscillator 3, bringing the voltage-controlled free oscillation frequency to the locking frequency. In order to achieve fast frequency locking and good spurious and phase noise, the loop filter 2 adopts the variable loop bandwidth method. The RF signal output by the voltage-controlled oscillator 3 is divided into two outputs through the power divider 8. One is sent to the orthogonal modulator 4 to participate in orthogonal mixing, and the other output is amplified by the RF amplifier 9, and then the harmonics and spurious are filtered out by the filter to output the local oscillator signal.

[0038] Loop filter 2 is widened during loop capture to increase the capture range and shorten the capture time. When the local oscillator signal frequency accuracy and stability are the same as the temperature compensated crystal oscillator in the frequency synthesizer.

[0039] The controller 11 inputs a control frequency word, a clock and an enable signal to the integrated phase-locked loop 1, the direct digital frequency synthesizer 6 and the direct digital frequency synthesizer 7 respectively according to the required local oscillator operating frequency.

[0040] Reference Figure 2 , the working principle of the device of the present invention is described as follows:

[0041] The I and Q orthogonal signals from the direct digital frequency synthesis DDS are respectively orthogonally mixed with the f0 signal generated by the voltage-controlled oscillator VCO. The generated frequency signal is used as the RFin (fN) of the integrated phase-locked loop PLL, and then the pump current generated by the reference frequency obtained by dividing the reference frequency enters the loop filter and is amplified by the operational amplifier as the tuning voltage of the voltage-controlled oscillator, thereby controlling the output frequency of the VCO.

[0042] The local oscillator output frequency f0 = 28.8*N / R + fdds; where F = 28.8MHz (standard frequency) / R (division ratio); N = INT[f0÷F]; As can be seen from the above formula, the frequency of f0 is obtained by adding the PLL output frequency and the DDS output frequency, and the PLL output N*F = 28.8*N / R, that is, integer division is adopted, the frequency mantissa is generated by the DDS, and then the PLL and DDS frequencies are mixed and added as the final frequency output. By selecting a suitable R value within a 28.8 / R cycle, the frequency mantissa is generated by the DDS. For points where the mantissa is close to 0, 14.4 and 28.8, better spurious is obtained by shifting the N value by minus 1. The frequency resolution is the resolution of the PLL: △f0 = N*28.8 / R.

[0043] The frequency synthesizer uses integer frequency division through PLL, the frequency mantissa is generated by DDS, and then the two components are added through mixed addition to obtain a high operating frequency and high-resolution output. Therefore, when outputting the same frequency, the parasitic output and phase noise of the loop can be reduced, and the frequency capture performance and locking time are also improved.

[0044] The device of the present invention provides the local oscillator signal required for frequency conversion for wide-band ultra-short wave radio stations based on the hybrid frequency synthesizer technology of orthogonal modulation. The design scheme of orthogonal modulation combined with DDS interpolation PLL is adopted, and the output local oscillator signal has the characteristics of wide frequency range, high resolution, short locking time, low phase noise, small size, low power consumption, etc.

[0045] The structure of the hybrid frequency synthesizer based on orthogonal modulation of the present invention is that it includes an integrated phase-locked loop 1, which is connected end to end with a loop filter 2, a voltage-controlled oscillator 3, and an orthogonal modulator 4 to form a closed-loop system; a standard frequency 5 is respectively connected to the integrated phase-locked loop 1, a direct digital frequency synthesizer 6, and a direct digital frequency synthesizer 7; the direct digital frequency synthesizer 6 and the direct digital frequency synthesizer 7 are connected to the orthogonal modulator 4; a radio frequency amplifier 9 is respectively connected to a power divider 8 and a filter 10; and a controller 11 is respectively connected to the integrated phase-locked loop 1, the direct digital frequency synthesizer 6, and the direct digital frequency synthesizer 7.

[0046] Further, see Figure 3 1 is a circuit structure block diagram of the device of the present invention.

[0047] The integrated phase-locked loop 1 is composed of the integrated circuit ADF4153 and peripheral circuits. ADF4153 is a high-performance fractional frequency phase-locked loop circuit, which is widely used in CATV equipment, mobile communication base stations, wireless portable devices (GSM, PCS, DCS, CDMA, WCDMA), and various communication test equipment. It includes a low-noise digital frequency detector, a high-precision charge pump, and a programmable divider. The RF working bandwidth is 500MHz~4GHz, and the frequency range is wide. It has a built-in programmable dual-mode pre-divider 4 / 5, 8 / 9; programmable built-in charge pump current. There is an external port to expand the tuning voltage. It has analog and digital lock detection capabilities, as well as the ability to adjust noise to spurious. The frequency detector and phase detector inside the ADF4153 are used as both a frequency detector and a phase detector. During the loop capture process, when the frequency difference is large, the frequency detection function takes effect in the feedback loop, and the frequency error voltage quickly drives the frequency of the VCO close to the set frequency. When the frequency difference drops to a small enough level, the phase detection function takes effect and eventually locks the signal.

[0048] Loop filter 2 is composed of active low-noise operational amplifier MC33078D, single-pole single analog switch MAX4648, single-pole dual analog switch MAX4649, and resistor and capacitor networks. The loop filter uses an active loop filter with variable bandwidth. The variable loop bandwidth method can dynamically change the relevant parameters that determine the loop bandwidth to achieve the purpose of reducing the frequency hopping time. At the same time, the reference spurious and phase noise in the locked state will not deteriorate.

[0049] The voltage controlled oscillator 3 is composed of the circuit RFMD-16 and peripheral circuits. The operating frequency range of RFMD-16 is 920MHz~1840MHz, and the tuning voltage is 0.5V~20V.

[0050] The quadrature modulator 4 is composed of the integrated circuit AD8346 and peripheral circuits. The internal circuit of AD8346 mainly includes the local oscillator signal interface circuit, mixer, differential voltage-current converter, differential-single-ended output converter and deviation circuit. The local oscillator signal interface circuit generates two orthogonal local oscillator signals, which are respectively provided to the mixers of the I and Q channels; the mixer is used to complete the mixing function; the differential voltage-current converter converts the baseband signal into a differential current form, and then provides it to the two mixers; the differential-single-ended output converter combines the two mixed signals and converts them into a single-ended form and outputs them from the VOUT terminal; the deviation circuit generates the balanced current reference value when the temperature of each part of the circuit changes and the current reference value required for constant temperature. When the current value of each part of the circuit exceeds the current reference value, the output is turned off. The local oscillator LOIP is orthogonally mixed with the DDS signal, and then the resulting signal is synthesized, so that the required signals are superimposed on each other, and the useless signals are offset. AD8346 is used for 0.8-2.5GHz RF quadrature modulation and can be widely used in digital spread spectrum communication systems, cellular transmission systems and other fields. The baseband signal bandwidth that AD8346 can modulate is 70MHz. With a single-ended 2.7-5.5V power supply, the current value is 45mA in the static working state and only 1uA in the sleep state.

[0051] Standard frequency 5 uses a temperature-compensated crystal oscillator circuit. Frequency 28.8MHz, power supply 3V±0.3V, current less than 20mA, output sine wave, output amplitude greater than 10dBm, output load 50Ω, temperature stability: ±0.5ppm@-40~85℃, single-sideband phase noise: ≤-120dBc / Hz@100Hz, ≤-140dBc / Hz@1kHz, ≤-150dBc / Hz@10kHz. The voltage control terminal Vc of the temperature-compensated crystal oscillator can control the accuracy of the output frequency by adjusting the voltage.

[0052] Direct digital synthesizer 6 and direct digital synthesizer 7 are composed of integrated circuit AD9834 and peripheral circuits. The internal circuit of AD9834 mainly includes numerical control oscillator (NCO), frequency and phase regulator, sine lookup table (SIN ROM), DAC converter and voltage modulator. It can output high-performance sine wave, triangle wave and square wave. The output frequency and initial phase can be set by software programming and are easy to adjust.

[0053] The power divider 8 is composed of a transformer CP0603A1441 and peripheral matching resistors.

[0054] The RF amplifier 9 is composed of the RF integrated circuit MGA-61563 and peripheral inductors and capacitors. MGA-61563 is a low-noise, high-linearity RF amplifier with an operating frequency range of 100MHz to 6GHz, a noise figure of 1.2dB, a gain of 16.6dB, an output 1dB compression point P115.8dBm, and a third-order intermodulation OIP328.5dBm.

[0055] The filter 10 is composed of two low-pass filters LFCN-1500 and LFCN-1800 and two radio frequency electronic switches AS211-334. The low-pass filter LFCN-1500 is used to filter out the harmonics of the frequency output signal of 920 to 1500 MHz, and the low-pass filter LFCN-1800 is used to filter out the harmonics of the frequency output signal of 1500 to 1820 MHz. The two electronic switches AS211-334 are used for frequency band switching, and the operating frequency is 100 MHz to 4 GHz, and the input power 1 dB compression point P1 is greater than 30 dBm.

[0056] The controller 11 can be composed of a single chip microcomputer or an FPGA. It receives external frequency information, calculates the configuration parameters according to the received frequency value, and then controls the DDS and PLL at the same time, outputs the clock, data, enable and other signals of the DDS and PLL chips, and also controls the fast and slow locking of the loop filter during the frequency locking process, and completes the frequency control status reporting, as well as the switching control of the two harmonic filters output by the local oscillator. The FPGA uses the SPI communication interface to communicate with the outside.

[0057] The power management 12 is composed of linear power chip TPS79230D, LT1761ES5 and peripheral circuits, which convert the external input +3.3V and +6.2V into +3V and +5V as the power supply for VCO, DDS and phase-locked loop chips.

[0058] A person of ordinary skill in the art can understand that all or part of the steps of implementing the above-mentioned method embodiment can be completed by hardware related to program instructions, and the aforementioned program can be stored in a computer-readable storage medium. When the program is executed, it executes the steps of the above-mentioned method embodiment; and the aforementioned storage medium includes: ROM, RAM, magnetic disk or optical disk, etc., various media that can store program codes.

[0059] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily think of changes or substitutions within the technical scope disclosed by the present invention, which should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A hybrid frequency synthesizer based on orthogonal modulation, characterized in that: The hybrid frequency synthesizer includes: an integrated phase-locked loop, a loop filter, a voltage-controlled oscillator, a power divider, an orthogonal modulator, a standard frequency generator, two direct digital frequency synthesizers, a radio frequency amplifier, a filter, and a controller; Wherein, the control signal output terminal of the controller is respectively connected to the control signal input terminals of the integrated phase-locked loop and two direct digital frequency synthesizers; The signal output terminal of the standard frequency generator is connected to the standard frequency input terminals of the integrated phase-locked loop and two direct digital frequency synthesizers respectively; The signal output terminals of the two direct digital frequency synthesizers are respectively connected to the I-channel signal input terminal and the Q-channel signal input terminal of the orthogonal modulator; The signal output end of the orthogonal modulator is connected to the signal input end of the integrated phase-locked loop, the signal output end of the integrated phase-locked loop is connected to the signal input end of the loop filter, the signal output end of the loop filter is connected to the signal input end of the voltage-controlled oscillator, the signal output end of the voltage-controlled oscillator is connected to the signal input end of the power divider, the signal output end of the power divider is connected to the signal input end of the radio frequency amplifier, the signal output end of the radio frequency amplifier is connected to the signal input end of the filter, and the signal output end of the filter outputs a local oscillator signal; The radio frequency signal output end of the power divider is connected to the radio frequency signal input end of the orthogonal modulator.

2. A hybrid frequency synthesizer based on orthogonal modulation according to claim 1, characterized in that: The standard frequency generator is used to generate a reference frequency signal of 28.8 MHz, and send the reference frequency signal to an integrated phase-locked loop and two direct digital frequency synthesizers; The orthogonal modulator is used to receive two output signals of two direct digital frequency synthesizers and a radio frequency signal output by a power divider, and perform orthogonal mixing on the two output signals and the radio frequency signal respectively, thereby obtaining a synthesized signal; The integrated phase-locked loop is used to receive the synthesized signal output by the orthogonal modulator and the reference frequency signal output by the standard frequency generator, and compare the phases of the synthesized signal and the reference frequency signal to obtain an output phase-discrimination voltage signal; The loop filter is used to filter the phase-discrimination voltage signal to obtain a filtered voltage signal; The voltage-controlled oscillator is used to set the free oscillation frequency to the locking frequency according to the filtered voltage signal and output a radio frequency signal; The power divider is used to divide the radio frequency signal into two output paths, one path is output to the orthogonal modulator for orthogonal mixing, and the other path is output to the radio frequency amplifier for amplification; The filter is used to filter out harmonics and spurious signals from the signal amplified by the radio frequency amplifier to obtain an output local oscillator signal.

3. A hybrid frequency synthesizer based on orthogonal modulation according to claim 2, characterized in that: The controller is used to send control signals to the integrated phase-locked loop and two digital frequency synthesizers respectively; the control signals include a control frequency word, a clock and an enable signal.

4. A hybrid frequency synthesizer based on orthogonal modulation according to claim 1, characterized in that: The integrated phase-locked loop is composed of an integrated circuit chip ADF4153 and its peripheral circuits; The loop filter is composed of an active low-noise operational amplifier MC33078D, a single-pole single analog switch MAX4648, a single-pole dual analog switch MAX4649, and a resistor and capacitor network; The voltage controlled oscillator is composed of a chip RFMD-16 and its peripheral circuits; The orthogonal modulator is composed of integrated circuit AD8346 and its peripheral circuits; Two direct digital frequency synthesizers are composed of integrated circuit AD9834 and its peripheral circuits; The power divider is composed of a transformer CP0603A1441 and its peripheral matching resistors; The radio frequency amplifier is composed of a radio frequency integrated circuit MGA-61563 and its peripheral inductors and capacitors; The filter is composed of two low-pass filters LFCN-1500 and LFCN-1800 and two radio frequency electronic switches AS211-334; The controller is composed of a single chip microcomputer or a FPGA controller.

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