A C-band frequency synthesis method

Abstract

The application provides a C-band frequency synthesis method. A double DDS scheme is adopted, the relationship between a DDS reference clock signal and an output frequency is optimally selected, a mixing circuit is mainly used to reduce the deterioration of the spurious and phase noise indexes caused by frequency multiplication, and a low-spurious, low-phase-noise multi-frequency point continuous wave is synthesized. The time-sharing function of one DDS is multiplexed, and a large-bandwidth linear frequency modulation continuous wave local oscillator signal can be synthesized through filter switching without the need of frequency multiplication to expand the bandwidth. The application multiplexes the functions of the DDS circuit through the optimization of the frequency synthesis frequency, adopts the least circuit to realize the fusion of two functions, retains the advantages of the low-spurious and low-phase-noise multi-frequency point continuous wave local oscillator, and can realize the large-bandwidth linear frequency modulation continuous wave local oscillator signal without the need of an additional harmonic generator to generate a microwave frequency marker. The system scheme is simple, has high integration, small equipment quantity, high reliability and strong versatility.

Description

Technical Field

[0001] This invention belongs to the technical field of radar frequency synthesizer subsystems, specifically relating to a C-band frequency synthesis method that takes into account both the generation of large-bandwidth linear frequency modulated continuous wave local oscillators and low-spurious, low-phase-noise, multi-frequency continuous wave local oscillator signals. Background Technology

[0002] One commonly used system for high-resolution range radar is the frequency-modulated (FM) system. In this system, both the transmitted and received local oscillators are wide-bandwidth linear frequency modulated (LFM) signals. The requirements for spurious signals and phase noise in wide-bandwidth LFM local oscillators are not high, and they are generally achieved using DDS (Distributed Frequency Diode) frequency multiplication. However, when this type of radar operates in narrowband mode, the local oscillator needs to be a point-frequency continuous wave, and the requirements for spurious signals and phase noise are much higher. Directly using the frequency-multiplied DDS signal cannot meet the system requirements in terms of spurious signals and phase noise. The traditional solution is to use a harmonic generator with a switching filter circuit to generate multi-frequency continuous waves, and to use a DDS frequency multiplier circuit when operating with wide-bandwidth LFM signals. Both circuits coexist and are switched via a switch. This method is redundant, involves a large number of devices, and has low integration, which is no longer sufficient to meet the increasingly high integration requirements of radar applications. Summary of the Invention

[0003] To overcome the shortcomings of existing technologies, this invention provides a C-band frequency synthesis method. It employs a dual-DDS scheme, optimizing the relationship between the DDS reference clock signal and the output frequency. The main component is a mixer circuit to reduce spurious emissions and phase noise degradation caused by frequency doubling, synthesizing a low-spurious, low-phase-noise multi-frequency continuous wave. One of the DDS functions is time-division multiplexed, and switched via a filter, eliminating the need for frequency doubling to expand the bandwidth, thus synthesizing a large-bandwidth linear frequency modulated continuous wave local oscillator signal. This invention optimizes the frequency synthesis frequency by multiplexing the DDS circuit functions, achieving the fusion of two functions with minimal circuitry. It retains the advantages of low spurious emissions and low phase noise from multi-frequency continuous wave local oscillators while achieving a large-bandwidth linear frequency modulated continuous wave local oscillator signal. It eliminates the need for an additional harmonic generator to produce microwave frequency standards, resulting in a simple system with high integration, small equipment requirements, high reliability, and strong versatility.

[0004] A C-band frequency synthesis method is characterized by the following: a reference signal generated by a 100MHz low-phase-noise isothermal crystal oscillator is input, and after 35 frequency multiplications, a 3.5GHz frequency signal is generated for use as the reference clock for DDS1 and DDS2. Simultaneously, one power splitter is used to mix the signal with the intermediate frequency standard signal output from DDS1 to generate a 100MHz bandwidth S-band point-frequency continuous wave signal with a 5MHz interval. This signal is then bandpass filtered and amplified, and used as a mixing local oscillator to mix with the output signal of DDS2 to generate a 600MHz bandwidth linear frequency modulated continuous wave or a point-frequency continuous wave signal with a 5MHz interval. After bandpass filtering and amplification, the signal is output.

[0005] Specifically, the intermediate frequency standard signal output by DDS1 is a 250MHz to 350MHz signal with a bandwidth of 100MHz and a frequency not higher than 1 / 10 of the DDS reference clock, with a frequency interval of not less than 5MHz.

[0006] Specifically, the DDS2 output signal is a microwave frequency standard signal of 800MHz to 1400MHz with a frequency not higher than 1 / 3 of the DDS reference clock frequency below the L band and a frequency interval of 100MHz.

[0007] Specifically, when the DDS2 operates on a multi-frequency continuous wave local oscillator signal, it outputs microwave frequency standard signals with an interval of 100MHz from 800MHz to 1400MHz through no more than 7 filters in the switching filter board; when the DDS2 operates on a 600MHz bandwidth linear frequency modulated continuous wave local oscillator signal, it outputs linear frequency modulated continuous wave signals from 800MHz to 1400MHz through a broadband bandpass filter in the switching filter board.

[0008] The beneficial effects of this invention are as follows: By employing a dual-DDS scheme and optimizing the relationship between the DDS reference clock signal and the output frequency, low-spurious, low-phase-noise multi-frequency continuous wave can be synthesized, improving spurious emissions to below -70dBc and controlling phase noise degradation to no more than 3dB. By time-division multiplexing the function of one of the DDS components and switching it via a filter, a large-bandwidth linear frequency modulated continuous wave local oscillator signal can be synthesized without frequency doubling to expand the bandwidth. This invention achieves the fusion of two functions with minimal circuitry, retaining the advantages of low spurious emissions and low phase noise of multi-frequency continuous wave local oscillators while also realizing a large-bandwidth linear frequency modulated continuous wave local oscillator signal. It eliminates the need for an additional harmonic generator to produce microwave frequency standards, resulting in a simple system design with high integration, small equipment requirements, high reliability, and strong versatility. Attached Figure Description

[0009] Figure 1 This is a schematic diagram of a C-band frequency synthesis method according to the present invention;

[0010] Figure 2 This is a schematic diagram illustrating a design example of a C-band frequency synthesis method according to the present invention;

[0011] Figure 3 This is the schematic diagram of a switching filter board. Detailed Implementation

[0012] The present invention will be further described below with reference to the accompanying drawings and embodiments. The present invention includes, but is not limited to, the following embodiments.

[0013] One of the main functions of a frequency synthesizer is to provide a local oscillator signal within the operating bandwidth of the radar, typically multiple low-spurious, low-phase-noise point-frequency continuous waves. Providing a large-bandwidth linear frequency-modulated continuous wave signal is an important extended function of frequency synthesizers, providing an instantaneous large-bandwidth local oscillator signal for high-resolution range radars employing frequency-modulated systems. Typically, C-band radars operate with a bandwidth of 600MHz. This invention provides a C-band frequency synthesis method for generating multi-frequency point-frequency continuous wave local oscillator signals with a C-band bandwidth greater than 600MHz and a frequency interval ≥5MHz. Simultaneously, it can achieve a large-bandwidth linear frequency-modulated continuous wave local oscillator signal with a bandwidth of 600MHz through functional control, meeting the general requirements of C-band radar applications.

[0014] like Figure 1 As shown, the specific implementation process of the present invention is as follows: A reference signal generated by a 100MHz low phase noise isothermal crystal oscillator is input, and after 35 frequency multiplications, a 3.5GHz frequency signal is generated for the reference clock of DDS1 and DDS2. At the same time, one power splitter is used to mix with the intermediate frequency standard signal output by DDS1 to generate a 100MHz bandwidth S-band point frequency continuous wave signal with a 5MHz interval. After bandpass filtering and amplification, this signal is used as the mixing local oscillator to mix with the output signal of DDS2 to generate a 600MHz bandwidth linear frequency modulated continuous wave or a point frequency continuous wave signal with a 5MHz interval. After bandpass filtering and amplification, it is output.

[0015] Figure 2 A design example of this invention is given. Two DDS devices use the domestically produced GM4912, with a reference clock of 3.5 GHz. DDS1 outputs a signal from 250 to 350 MHz, with a frequency interval of 5 MHz; DDS2 outputs a signal from 800 MHz to 1400 MHz, with a frequency interval of 100 MHz. The linear frequency modulation (LFM) operating bandwidth is 600 MHz from 800 MHz to 1400 MHz. When DDS2 operates in LFM mode, the output frequency of DDS1 can be adjusted to change the starting frequency of the LFM signal, with an adjustment range of 100 MHz.

[0016] a) Implementation of low-phase-noise, low-spurious multi-frequency continuous-wave local oscillator signals

[0017] The DDS1 generates an intermediate frequency (IF) standard signal with a bandwidth of 100MHz and a frequency not exceeding 1 / 10 of the DDS reference clock. The frequency interval can be arbitrarily set. For radar applications, it is generally not lower than 5MHz. Here, 250MHz to 350MHz can be selected, with a frequency interval of 5MHz. The DDS1 output signal is filtered to suppress DDS output spurious signals. Since the DDS clock is selected as 3.5GHz, near-end spurious signals below 1 / 10 of the clock frequency are reduced to -70dBc. Far-end spurious signals are suppressed using a bandpass filter.

[0018] The DDS2 generates microwave frequency standard signals in the L-band, with a frequency not exceeding one-third of the DDS reference clock and a frequency interval of 100MHz. Since its output frequency is below one-third of the DDS clock frequency, it is necessary to suppress in-band image spurious signals at the DDS output using a switching filter bank. When the output is between 800MHz and 1400MHz, up to seven filters are required to improve spurious signals to below -70dBc.

[0019] The DDS1 and 3.5GHz point-frequency continuous wave mixers, using a single bandpass filter, can suppress intermodulation spurious emissions to below -70dBc. The final mixing stage, using a wideband bandpass filter, further suppresses intermodulation spurious emissions to below -70dBc. Of all the frequency synthesis stages, only the 3.5GHz frequency is generated using a frequency multiplier circuit; the other DDS circuits do not employ frequency multipliers, thus avoiding spurious emissions degradation. Therefore, the final output spurious emissions are on the same order of magnitude as the DDS output spurious emissions, at -70dBc.

[0020] The phase noise of the mixer output signal depends on the phase noise levels of the two signals being mixed. When the phase noise of one signal is significantly lower than that of the other, the output signal phase noise is close to that of the worse signal; otherwise, the output signal phase noise deteriorates by 3dB. Both DDSs use 3.5GHz as the reference clock. The highest output frequency of DDS1 is less than 1 / 10 of 3.5GHz, which does not affect the phase noise of the mixed signal. The highest output frequency of DDS2 is less than 1 / 3 of 3.5GHz, and considering the device noise floor, it is close to the 3.5GHz phase noise, which can be expected to deteriorate by 3dB. Therefore, the mixer output phase noise specification is 3dB more than the 3.5GHz phase noise output, resulting in the optimal final signal phase noise specification.

[0021] b) Implementation of a large-bandwidth linear frequency modulated continuous wave local oscillator signal

[0022] When the DDS2 operates with a multi-frequency continuous wave local oscillator signal, it is configured to generate microwave frequency standard signals with intervals of 100MHz, such as 800MHz to 1400MHz. Figure 3 As shown, the output frequency can be selected through a maximum of 7 filters in the switching filter board. When the DDS2 operates with a 600MHz bandwidth linear frequency modulated continuous wave local oscillator signal, it is configured to generate a linear frequency modulated continuous wave signal of 800MHz to 1400MHz, as shown below. Figure 3 As shown, the output is obtained through a broadband bandpass filter of 800MHz to 1400MHz in the switching filter board.

[0023] Considering the feasibility of broadband bandpass filters, the upper limit frequency of the microwave frequency standard should be less than 1 / 3 of the clock frequency, and the lower limit frequency should be greater than 1 / 2 of the upper limit frequency.

[0024] This invention utilizes dual DDSs, one for an intermediate frequency standard and the other for a microwave standard, providing instantaneous high bandwidth. The frequency relationship between the DDS output frequency and the clock is optimized, primarily employing a mixer circuit to reduce spurious emissions and phase noise degradation caused by frequency doubling, while maintaining superior performance.

[0025] If the traditional design method is used, compared to the present invention, an additional set of harmonic generator circuit and frequency multiplier circuit will be required for microwave frequency standard generation. Furthermore, due to frequency multiplication, spurious noise and phase noise are aggravated by 20 × log(4) = 12 dB.

Claims

1. A C-band frequency synthesis method, characterized in that... Specifically, the input is a reference signal generated by a 100MHz low-phase-noise temperature-controlled crystal oscillator. After being multiplied 35 times, a 3.5GHz frequency signal is generated for use as the reference clock for DDS1 and DDS2. Simultaneously, one power splitter is used to mix with the intermediate frequency standard signal output from DDS1 to generate a 100MHz bandwidth S-band continuous wave signal with a 5MHz interval. After bandpass filtering and amplification, this signal is used as the mixing local oscillator to mix with the output signal of DDS2 to generate a 600MHz bandwidth linear frequency modulated continuous wave or a 5MHz interval point frequency continuous wave signal in the C-band. After bandpass filtering and amplification, the signal is output.

2. The C-band frequency synthesis method as described in claim 1, characterized in that: The intermediate frequency standard signal output by DDS1 is a 250MHz to 350MHz signal with a bandwidth of 100MHz and a frequency not higher than 1 / 10 of the DDS reference clock, with a frequency interval of not less than 5MHz.

3. The C-band frequency synthesis method as described in claim 1, characterized in that: The DDS2 output signal is a microwave frequency standard signal of 800MHz to 1400MHz with a frequency not higher than 1 / 3 of the DDS reference clock and a frequency interval of 100MHz below the L band.

4. The C-band frequency synthesis method as described in claim 1, characterized in that: When the DDS2 operates with a multi-frequency continuous wave local oscillator signal, it outputs microwave frequency standard signals with an interval of 100MHz from 800MHz to 1400MHz through no more than 7 filters in the switching filter board. When the DDS2 operates with a 600MHz bandwidth linear frequency modulated continuous wave local oscillator signal, it outputs linear frequency modulated continuous wave signals from 800MHz to 1400MHz through a broadband bandpass filter in the switching filter board.

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