Method for enhancing spurious frequency suppression of short-wave multi-path receiving

Abstract

The invention relates to the technical field of communication, in particular to a method for enhancing spurious frequency suppression in short-wave multi-channel reception, which comprises the following steps of: after converting a collected short-wave radio-frequency signal into a digital signal, respectively processing the digital signal through a first channel and a second channel, the first channel carries out filtering preprocessing on the digital signal to obtain a preprocessed signal, and then carries out down-conversion processing on the preprocessed signal to obtain a first baseband signal; the second channel performs down-conversion processing on the digital signal to obtain a second baseband signal; multichannel processing is carried out on digital signals at the same time, the application scene of short-wave broadband high-data-rate multi-channel down-conversion processing can be supported, the requirement for simultaneous receiving of short-wave multi-channel frequency points is met, each far-end interference source can be filtered through multi-channel digital resonance processing, and the receiving efficiency is improved. And the spurious suppression capability during digital down-conversion processing is enhanced.

Description

Technical Field

[0001] The present invention relates to the field of communication technologies, and particularly relates to a method for enhancing the suppression of spurious frequencies in multi-channel short-wave reception. Background Art

[0002] In long-distance communication, short-wave radio stations transmit information by means of the ionosphere, and the technology of single radio station and single reception is already very mature. With the continuous enrichment of the hardware resources of semiconductor chips and the continuous improvement of software radio digital signal processing technology, short-wave radio frequency digital receivers can achieve simultaneous reception of multiple frequency points by a single radio station, such as realizing short-wave frequency diversity reception, multi-frequency point monitoring, etc. Since traditional superheterodyne short-wave receivers are limited by fixed intermediate frequency points in hardware, multiple superheterodyne hardware circuits are required to achieve reception when multiple frequency points are received.

[0003] The short-wave electromagnetic environment is harsh, and the level of spurious frequency suppression in a short-wave receiver directly affects the quality of voice in sky-wave communication. A receiver with a superheterodyne architecture can suppress remote interference sources to a certain extent through an intermediate frequency analog narrow-band filter. For a receiver based on a radio frequency direct sampling architecture, remote interference is also sampled by an analog-to-digital converter. Therefore, during the digital down-conversion process, it is necessary to filter and suppress the collected interference sources to reduce the influence of spurious frequencies on normal reception.

[0004] For a multi-frequency point short-wave receiver, considering that the analog circuits of a superheterodyne architecture short-wave receiver increase with the number of short-wave reception channels, a miniaturized short-wave radio frequency digital receiver is selected to achieve short-wave multi-frequency point reception. After a short-wave radio frequency digital receiver directly samples a short-wave analog radio frequency signal (HF signal) through an analog-to-digital converter (ADC), digital down-conversion processing is performed: the radio frequency digital signal undergoes digital mixing, high-speed digital filtering and decimation, and low-speed digital filtering and decimation, and finally is converted into a low-speed baseband digital signal. The processing process is as Figure 1 shown. After the short-wave radio frequency signal collects data through a high-speed analog-to-digital converter (ADC), when performing down-conversion digital signal processing, first, the original high-speed radio frequency digital signal (f HF ) is digitally mixed with multiple digital carriers (f ci represents the carrier frequency of the nth path) to generate a digitally mixed signal (f HF ±f c_i), and then through multi - stage digital filtering and signal processing of extraction, the RF sampling data is transformed into a low - speed baseband digital signal. During high - speed filtering and extraction, when the stop - band rejection of the filter is large, the required chip hardware resources will increase accordingly; when the stop - band rejection of the high - speed filter is small, the required hardware resources will also be small. Therefore, in the chip hardware implementation, a trade - off needs to be made between the stop - band rejection ability of the digital filter and the chip hardware resources occupied (such as the number of required multipliers).

[0005] During the high - speed digital filtering / extraction process, a CIC filter that occupies less chip hardware resources is usually used. For a multi - stage CIC filter (the first - side - lobe rejection ability of a 5 - stage CIC is about 67 dB), compared with the acquisition data with a spurious - free dynamic range of about 100 dB after ADC sampling, there is still a 30 - dB gap, making the anti - interference ability of the multi - frequency short - wave receiver not ideal. Summary of the Invention

[0006] Aiming at the deficiencies of the prior art, the present invention proposes a method for enhancing the suppression of spurious frequencies in short - wave multi - path reception, which can not only meet the requirements of simultaneously receiving multiple short - wave frequency points, but also filter and suppress the interference sources at each remote end through an independent digital resonant filter, enhancing the anti - interference ability of the short - wave independent multi - path receiver.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] The present invention proposes a method for enhancing the suppression of spurious frequencies in short - wave multi - path reception, including: after converting the collected short - wave RF signal into a digital signal, the digital signal is processed through a first channel and a second channel respectively. The first channel first performs filter pre - processing on the digital signal to obtain a pre - processed signal, and then performs down - conversion processing on the pre - processed signal to obtain a first baseband signal; the second channel performs down - conversion processing on the digital signal to obtain a second baseband signal.

[0009] Specifically, the first channel and the second channel can simultaneously and independently implement multi - path down - conversion processing, and the spurious - suppression processing channel can be flexibly selected according to different reception modes.

[0010] Specifically, the first channel performs filter pre - processing on the digital signal through a second - order multi - path digital resonant filter.

[0011] Specifically, an analog - to - digital converter is used to collect the short - wave RF signal.

[0012] Specifically, the process of the down - conversion processing is: sequentially performing digital mixing, high - speed digital filtering / extraction, and low - speed digital filtering / extraction on the pre - processed signal or the digital signal.

[0013] Specifically, the process of digital mixing is as follows: Mix multiple digital carriers with the preprocessed signal or digital signal.

[0014] Specifically, during the down-conversion process, a class-D CIC filter is used for high-speed digital filtering / decimation, where D ≤ 5.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0016] (1) The present invention relies on the existing hardware platform of the short-wave radio frequency direct sampling architecture to perform multi-channel processing on the digital signal after analog-to-digital conversion. It can not only enhance the spurious suppression ability during digital down-conversion through the multi-channel digital resonance processing module, but also support the application scenarios of multi-channel down-conversion processing with short-wave broadband high data rates at the same time.

[0017] (2) The present invention adopts a radio frequency digital receiving architecture. In the case of requiring simultaneous reception of multiple short-wave frequency points, in the case of different short-wave working modes, such as modes with low phase requirements like MFSK modulation and analog voice, by performing multi-channel digital resonance filtering processing on the radio frequency digital sampling signal in parallel, the digital spurious suppression ability of each independent frequency point of the short-wave receiver is enhanced; and without adding hardware circuits, according to the actual effective signal bandwidth, the resonance filtering parameters can be flexibly modified, thereby realizing digital resonance filtering processing.

[0018] (3) Through the method provided by the present invention, during short-wave multi-channel digital down-conversion processing, only by software upgrade, it can automatically select the down-conversion channel in different application scenarios, and can also enhance the receiving spurious filtering suppression ability for the far end during baseband low-speed data reception. Description of the Drawings

[0019] Figure 1 is the principle block diagram of traditional short-wave radio frequency digital reception;

[0020] Figure 2 is the principle diagram of enhancing the spurious frequency suppression of short-wave multi-channel reception of the present invention; Detailed Embodiments

[0021] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0022] Embodiment

[0023] Reference Figure 2, this embodiment proposes a method for enhancing the suppression of spurious frequencies in shortwave multi-channel reception. After converting the collected shortwave radio frequency signal into a digital signal through an analog-to-digital converter (ADC), the digital signal is processed through a first channel and a second channel respectively. The first channel first performs filtering preprocessing on the digital signal to obtain a preprocessed signal, and then performs down-conversion processing on the preprocessed signal to obtain a first baseband signal to enhance the ability to suppress received spurs; the second channel performs down-conversion processing on the digital signal to obtain a second baseband signal to meet the requirement of high linearity in the process of processing the signal. In this embodiment, the first channel and the second channel can simultaneously and independently implement multi-channel down-conversion processing, and the spurious suppression processing channel can be flexibly selected according to different reception modes.

[0024] When the sampling rate f of the analog-to-digital converter s is 100 MSPS, when the interference source suddenly changes from small to large, after digital resonance preprocessing, it needs to pass through N 谐振 points of radio frequency data before it can stably output. Therefore, the effective filtering suppression time is T 谐振 (ms) (T 谐振 =(1 / 100000)*N 谐振 ); through actual measurement, when the range of the interference signal changing from small to large is 40 dB, after stable output after digital resonance filtering processing, the size of N 谐振 is about 3000, and T 谐振 ≈0.03 ms. In this embodiment, for the reception mode of analog voice or low-speed data transmission with a working bandwidth of 3 kHz (if the sampling rate is 8 kHz, the corresponding sampling time interval is about 0.125 ms), the digital signal after ADC conversion can be first filtered and preprocessed through the first channel, and then down-converted to suppress the digital interference signal; for the case where the data rate of the baseband digital signal is relatively large (if the bandwidth is 24 kHz, the corresponding sampling rate is 80 kHz, and its sampling time interval is about 0.0125 ms), the sampling time interval is smaller than the unstable time caused by filtering preprocessing. Therefore, when performing down-conversion processing with high phase requirements, the digital signal after ADC conversion is directly down-converted through the second channel without filtering preprocessing.

[0025] In this embodiment, sampling is performed by an ADC. For a short-wave receiver designed based on a software-defined radio architecture, the distance between the antenna and the analog-to-digital converter is minimized as much as possible. The 16-bit ADC has a maximum sampling data rate of up to 250 MSPS, and the received spurious-free dynamic range (SFDR) is not less than 100 dB. For short-wave radio frequency analog signals in the range of 1.6 MHz to 30 MHz, direct sampling can be performed by a high-speed ADC. Compared with traditional short-wave analog superheterodyne architecture receivers, data acquisition can be completed without relevant mixing analog circuits (such as analog mixers, analog frequency synthesis circuits, intermediate-frequency filters, etc.).

[0026] Among them, the process of the down-conversion processing is as follows: the preprocessed signal or digital signal is successively subjected to digital mixing, high-speed digital filtering / decimation, and low-speed digital filtering / decimation.

[0027] The process of the digital mixing is as follows: multiple digital carriers are mixed with the preprocessed signal or digital signal. The multiple digital carriers are generated by using a direct digital synthesizer (DDS) in the FPGA according to different multiple carrier frequencies, and the orthogonal digital carrier sine and cosine signals are generated. The frequency accuracy can be adjusted according to the quantization bits. For example, the carrier frequency of the i-th path is f c_i , with the unit of MHz, and the corresponding 32-bit carrier frequency word Freq word_i can be calculated according to Equation (1), where f s is the sampling data rate of the ADC, which is taken as 100 MHz here.

[0028]

[0029] After being sampled by the ADC, the f rf radio frequency digital signal includes the useful signal frequency (F sig ) and the interference signal frequency (F dist ). After being mixed with the multiple digital carriers, the frequencies of the generated digital mixing signals are respectively: (F dist ±f c_i ), where the useful signal after carrier removal is (F sig -f c_i ), and the others are interference signals; the multiple high-speed digital signals after mixing are transmitted to the digital down-conversion module.

[0030] f rf The i-th digital signal of the radio frequency digital signal after being preprocessed by the multiple digital resonators is f rf -resonance_i. After filtering and suppressing the interference signal F dist , multiple digital mixing processing is performed again.

[0031] During the down-conversion process, a class-D CIC filter is used for high-speed digital filtering / decimation, where D ≤ 5, relative to the radio frequency digital sampling rate f s , the i-th useful signal (F sig -f c_i ) after digital mixing is a narrowband signal, and its bandwidth ratio factor (b) is the ratio of the actual signal bandwidth (B) to the output sampling rate (f s / D) after decimation (D is the decimation factor), as shown in Equation (2).

[0032]

[0033] To make the b value as small as possible to obtain sufficient stopband attenuation and reduce the aliasing effect caused by interference signals, a CIC decimation filter is generally used in the first stage of the decimation system (where the input data sampling rate is the highest). The impulse response of the cascaded integrator-comb (CIC) filter has the following form, as shown in Equation (3):

[0034]

[0035] In the formula, D is the order of the CIC filter and also the decimation factor. The Z-transform of the CIC filter is shown in Equation (4):

[0036]

[0037] Since the level difference between the first sidelobe and the main lobe of a single-stage CIC is approximately 13.46 dB, the frequency response of the Q-stage CIC filter is shown in Equation (5):

[0038]

[0039] The first sidelobe suppression of the Q-stage CIC filter is (Q * 13.46) dB. Increasing the number of stages of the CIC will enhance the stopband suppression, but the in-band difference of the Q-stage CIC filter is also Q times that of a single stage. Therefore, the number of cascaded stages of the CIC filter should not be too large.

[0040] For the low-speed digital filtering / decimation at the back end of the shortwave receiving channel, since it works at the low-speed end, the stopband suppression ability of the filter can be improved by increasing the order of the FIR filter. When performing multiply-accumulate convolution operations, due to the low data rate and the working clock not being lower than the radio frequency sampling data rate, filtering calculations can be multiplexed for multiple signals in a time-sharing manner to reduce the occupation of chip hardware resources.

[0041] In the first channel, the digital signal is pre-filtered by a second-order multi-channel digital resonator filter. The second-order multi-channel digital resonator filter belongs to a narrow-band band-pass filter, which is suitable for high-speed filtering processing. By matching the zeros and poles of the filter, a resonance effect can be generated at ω = ±ω0 to form a digital resonator, and the remote interference is filtered and suppressed.

[0042] When |H(e jω )| is zero at ω = 0 and ω = π, the system function H(z) of the resonance filter is given by Equation (6), where K is a constant.

[0043]

[0044] The corresponding system frequency response H(e jω ) is given by Equation (7):

[0045]

[0046] The magnitude of the frequency response is given by Equation (8):

[0047]

[0048] The 3dB passband bandwidth Δω of the resonator B is given by Equation (9):

[0049]

[0050] When the values of K and r are determined, and the digital sampling data rate is f s , and the resonant frequency of the i-th carrier is f 0_i , the filter coefficients varying with the carrier frequency can be obtained as Equation (10):

[0051] -2r*cos(ω 0_i ) = -2r*cos((f 0_i / fs)*360) Equation (10)

[0052] Then the denominator filter coefficients of the resonance filter are given by Equation (11):

[0053] [a0 a1 a2] = [1 (2*r*cos(ω 0_i ))(-r 2 )] Equation (11)

[0054] The numerator filter coefficients of the resonance filter are given by Equation (12):

[0055] [b0 b1 b2] = [K 0 (-K)] Equation (12)

[0056] The system issues multiple carrier frequency points. By leveraging the floating-point calculation advantage of the DSP chip, the filtering coefficients of each resonant frequency point can be obtained. The resonant filtering parameters are transmitted through the interface protocol between the DSP chip and the FPGA chip.

[0057] Given the characteristics of parallel, high-speed real-time processing of the FPGA chip, when implementing multi-channel digital resonance processing, it is mainly implemented in the FPGA chip. During multi-channel digital resonance filtering processing, x(n) is the input signal, and y i (n) is the output signal of the i-th channel, and its resonant filtering processing process is shown in Equation (13):

[0058] a i_0 *y i (n) = b i_0 *x(n) + b i_1 *x(n - 1) + b i_2 *x(n - 2) - a i_1 *y i (n - 1)

[0059] -a i_2 *y i (n - 2)

[0060] Equation (13)

[0061] For different 3dB passband bandwidths of the resonator, corresponding parameters can be configured through software to meet the different working bandwidth requirements of each frequency point of the receiver.

[0062] The specific implementation manners of the present invention enable those skilled in the art to understand or implement the present invention. Various modifications to these embodiments will be obvious to those skilled in the art. The general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention.

[0063] It should be understood that the present invention is not limited to the above-described content and can be modified and changed without departing from its scope. The scope of the present invention is only limited by the appended claims.

Claims

1. A method for enhancing the suppression of spurious frequencies in short-wave multiplex reception, characterized in that, Including: After converting the collected short-wave radio frequency signal into a digital signal, the digital signal is processed through a first channel and a second channel respectively. The first channel first performs filtering preprocessing on the digital signal to obtain a preprocessed signal, and then performs down-conversion processing on the preprocessed signal to obtain a first baseband signal; The second channel performs down-conversion processing on the digital signal to obtain a second baseband signal.

2. The method for enhancing the spurious frequency suppression of shortwave multi-channel reception according to claim 1, wherein The first channel and the second channel can simultaneously and independently implement multi-channel down-conversion processing, and the spurious suppression processing channel can be flexibly selected according to different receiving modes.

3. The method for enhancing the spurious frequency suppression of short-wave multi-channel reception according to claim 1, characterized in that, The first channel performs filtering preprocessing on the digital signal through a second-order multi-channel digital resonator filter.

4. The method for enhancing the suppression of spurious frequencies in shortwave multi-channel reception according to claim 1, wherein An analog-to-digital converter is used to collect short-wave radio frequency signals.

5. The method for enhancing the spurious frequency suppression of short-wave multi-channel reception according to claim 1, wherein The process of the down-conversion processing is as follows: the preprocessed signal or the digital signal is sequentially subjected to digital mixing, high-speed digital filtering / decimation, and low-speed digital filtering / decimation.

6. The method for enhancing the spurious frequency suppression of short-wave multi-channel reception according to claim 5, characterized in that, The process of the digital mixing is as follows: multiple digital carriers are mixed with the preprocessed signal or the digital signal.

7. The method for enhancing the spurious frequency suppression of shortwave multiplex reception according to claim 5, wherein, During the process of the down-conversion processing, a class-D CIC filter is used for high-speed digital filtering / decimation, where D ≤ 5.

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