CN101207403A - Broadband RF front-end with high dynamic range in the medium and short wave bands - Google Patents

Abstract

The invention relates to a wideband radio frequency front end with a high dynamic range in the middle and short wave bands, and the invention relates to a wide band radio frequency front end of a spread spectrum signal receiving system in the middle and short wave bands. It overcomes the disadvantages of high noise figure, small dynamic range and low image suppression of the existing radio frequency front end. The RF front-end adopts a two-stage frequency conversion and low intermediate frequency output structure. The RF signal enters the intermediate frequency baseband signal processing part after passing through the preselected amplifier module, frequency conversion module, variable gain amplifier module and intermediate frequency amplifier module. The intermediate frequency baseband signal processing part is in the clock signal Driven to do digital demodulation. Among them, the variable gain amplifier module is implemented in a digital way, and the dynamic requirements of the system are guaranteed by the three-way digital variable gain amplifier module. The two-way local oscillator signal of the frequency conversion module and the clock signal provided to the digital baseband processing part are generated by DDS+PLL. The PLL multiplies the frequency of the output signal of the crystal oscillator. After being buffered and driven, it is input to the three-way DDS to realize the signal output.

Description

Broadband RF front-end with high dynamic range in the medium and short wave bands

technical field

The invention relates to a wideband radio frequency front end of a medium and short wave band spread spectrum signal receiving system.

Background technique

The medium and short wave frequency band generally refers to the frequency band of 0.5MHz-30MHz. The medium and short wave communication uses the ionosphere as the transmission medium, and the transmission distance can reach thousands of kilometers. Because medium and short wave communication has the characteristics of strong invulnerability and long transmission distance, it is widely used. Currently, medium and short wave frequency bands are not used by high dynamic range communication systems (such as mobile communication). The spread spectrum communication system refers to the communication system in which the spectrum of the information to be transmitted is expanded by a specific spread spectrum function to become a broadband signal, sent to the channel for transmission, and then compressed by corresponding means to obtain the transmitted information. Since the spectrum of the spread spectrum signal is much larger than the bandwidth of the original information, the signal after the spread spectrum is often submerged under the channel noise, making it difficult to be monitored. Moreover, the spread spectrum function or spread spectrum sequence generally has pseudo-random characteristics, so the spread spectrum communication system has extremely strong confidentiality. The radio frequency front-end is an important part of the medium and short wave spread spectrum system, which directly affects the operating distance, anti-interference ability and system bandwidth of the spread spectrum system.

Modern receiver architecture generally includes: image suppression receiver structure, zero-IF receiver structure, low-IF receiver structure and digital IF receiver structure based on software radio. The above structures have their own advantages and disadvantages: the image suppression receiver structure includes the Hartley image suppression structure and the Weaver image suppression structure, which can completely eliminate the image response and image noise in theory, but because the Hartley image suppression structure requires a fixed phase shift device, and a broadband fixed phase shifter is difficult to implement, so it is not suitable for wideband receivers. The Weaver image suppression structure is relatively complex, and the gain and phase mismatch of the two channels is relatively large. In actual use, the image suppression will decrease as the mismatch increases. The zero-IF receiver structure is another effective structure for suppressing image interference, which directly converts radio frequency signals into baseband signals, so image interference can be completely eliminated. But it is precisely because of the characteristics of this structure that the influence of DC drift and low-frequency noise is inevitable, and the leakage of the local oscillator or the radiation and leakage of the low-noise amplifier are just down-converted to the useful signal, thus affecting the overall system performance. In order to eliminate various adverse effects of the zero-IF receiver structure, the IF can be selected at a lower but non-zero frequency, which is the low-IF receiver structure. However, this structure will bring image interference, so the general low-IF receiver structure uses an orthogonal image rejection mixer and polyphase filter, both of which exist after mixing the signal and image interference. The phase difference is used to distinguish the signal from the interference, but this kind of structure is relatively complicated and difficult to realize.

With the continuous development of digital signal processing technology, the structure of digital IF receiver based on software radio came into being. The structure of digital IF receiver of general software radio completes the functions of filtering, amplification, gain control and frequency conversion before A/D conversion. , to convert the RF signal to an intermediate frequency. After A/D conversion, digital down-conversion processing is performed by a dedicated digital signal processing device to reduce the data flow rate, convert the IF (intermediate frequency) digital signal into a baseband digital signal, and then send it to a general-purpose DSP for processing, so as to achieve relative data rates for various data rates. Low digital baseband signal processing, complete the realization of various anti-interference, anti-multipath, adaptive equalization algorithms, etc.; and functions such as error correction FEC, anti-interleaving, and decryption. Although this structure has good versatility and openness, it has high requirements for the RF front-end: low noise figure, high dynamic range, high image rejection and high linearity, etc. The RF front end of the current structure is difficult to meet the above requirements.

Contents of the invention

The purpose of the present invention is to provide a wideband radio frequency front-end with high dynamic range in medium and short wave bands, so as to overcome the shortcomings of high noise figure, small dynamic range and low image rejection in the radio frequency front-end of the existing structure. It consists of a pre-selected amplifier module 3, a first-stage digital variable gain amplifier module 4, a first-stage frequency conversion module 5, a second-stage digital variable gain amplifier module 6, a second-stage frequency conversion module 7, and a third-stage digital variable gain The amplifying module 8 and the intermediate frequency amplifying module 9 are composed, and the signal output end of the pre-selected amplifying module 3 is connected to the signal input end of the first-stage digital variable gain amplifying module 4, and the signal output end of the first-stage digital variable gain amplifying module 4 is connected to the first-stage digital variable gain amplifying module 4. The signal input end of the first-stage frequency conversion module 5, the signal output end of the first-stage frequency conversion module 5 is connected to the signal input end of the second-stage digital variable gain amplification module 6, and the signal output end of the second-stage digital variable gain amplification module 6 Connect the signal input end of the second-stage frequency conversion module 7, the signal output end of the second-stage frequency conversion module 7 is connected to the signal input end of the third-stage digital variable gain amplification module 8, and the signal of the third-stage digital variable gain amplification module 8 The output end is connected to the signal input end of the intermediate frequency amplification module 9 .

When working, the input terminal of the preselection amplifier module 3 is connected to the antenna 1 through its internal low noise amplifier 3-1 to receive radio frequency signals. The output end of the intermediate frequency amplification module 9 is connected to the input end of the baseband signal processing module 10 . The radio frequency signal enters the radio frequency front end from the antenna 1, passes through the pre-selection amplifier module 3, the first-stage variable gain amplifier module 4 and the first-stage frequency conversion module 5 in sequence, and then outputs the high-frequency signal, and the high-frequency signal passes through the second-stage variable gain The amplifying module 6 and the second-stage frequency conversion module 7 output the low-intermediate frequency signal, and then enter the baseband processing part after passing through the third-stage variable gain amplifying module 8 and the intermediate frequency amplifying module 9 . The RF front-end down-converts the RF signal to an intermediate frequency signal through two-stage frequency conversion. It can achieve a dynamic range of more than 100dB; the pre-selection amplifier module’s pre-low noise amplifier has high gain and low noise figure, which can effectively reduce the noise figure of the front end; two-stage frequency conversion can achieve high image rejection, and can meet weak signals Input requirements for gain. The invention overcomes the disadvantages of high noise coefficient, small dynamic range and low image suppression of the radio frequency front end of the existing structure, and has great popularization value.

Description of drawings

Fig. 1 is a structural schematic diagram of the present invention, Fig. 2 is a structural schematic diagram of Embodiment 2, Fig. 3 is a structural schematic diagram of a first stage digital variable gain amplification module 4, Fig. 4 is a local oscillator signal generator based on DDS+PLL mode And the schematic diagram of the structure of the baseband digital clock signal generating device.

Detailed ways

Specific Embodiment 1: The present embodiment will be specifically described below with reference to FIG. 1 . This embodiment consists of a preselected amplification module 3, a first-stage digital variable gain amplification module 4, a first-stage frequency conversion module 5, a second-stage digital variable gain amplification module 6, a second-stage frequency conversion module 7, and a third-stage digital variable gain amplifier module. The variable gain amplification module 8 and the intermediate frequency amplification module 9 are composed, the signal output end of the preselection amplification module 3 is connected to the signal input end of the first-stage digital variable gain amplification module 4, and the signal output end of the first-stage digital variable gain amplification module 4 Connect the signal input end of the first stage frequency conversion module 5, the signal output end of the first stage frequency conversion module 5 connects the signal input end of the second stage digital variable gain amplification module 6, the signal of the second stage digital variable gain amplification module 6 The output end is connected to the signal input end of the second-stage frequency conversion module 7, and the signal output end of the second-stage frequency conversion module 7 is connected to the signal input end of the third-stage digital variable gain amplification module 8, and the third-stage digital variable gain amplification module 8 The signal output terminal of the signal is connected to the signal input terminal of the intermediate frequency amplification module 9.

Specific Embodiment 2: The present embodiment will be specifically described below in conjunction with FIGS. 1 , 2 , 3 and 4 . The difference between this embodiment and Embodiment 1 is that the preselection amplification module 3 includes a low noise amplifier 3-1, a preselection filter 3-2 and a radio frequency amplifier 3-3, and the output end of the low noise amplifier 3-1 is connected with the preselection filter The input end of the preselection filter 3-2 and the output end of the preselection filter 3-2 are connected to the input end of the radio frequency amplifier 3-3. The preselection amplifier module 3 is the first stage where the radio frequency signal enters the front end, and plays the role of filtering out interference, reducing system noise figure, and amplifying weak signals. The gain of the preselected amplification module 3 is about 18dB. According to the formula for calculating the noise figure of the cascaded network, let the amplification factor of the nth stage of the cascaded network be G _n , the noise figure be F _n , and the noise figure of the cascaded network be F _Σ . The magnification and noise figure are the factors that affect the system noise system

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The key factor of the number, that is, the larger the preamplification factor and the smaller the noise figure, the smaller the noise figure of the system. Therefore, the parameter design of the preselection amplifier module directly affects the performance of the whole system. Low noise amplifier 3-1 has a lower noise figure and a higher amplification factor, which is in line with the design principles of cascaded networks. The insertion loss of the preselection filter is also a key factor affecting the noise figure of the RF front-end, so the bandwidth can be appropriately widened to reduce the insertion loss of the filter. However, the pre-selection filter plays a role in suppressing the image frequency, so its bandwidth cannot be too wide, and a moderate method needs to be adopted to meet the requirements of both. This embodiment adopts a 5th-order Chebyshev LC bandpass filter, and the bandwidth is widened to 1.5 times of the signal bandwidth, so that the insertion loss can be less than 1.5dB, and the image suppression can reach 80dB.

The first stage digital variable gain amplification module 4 comprises digital attenuator 4-1, fixed gain amplifier 4-2, data sampling module 4-3, control module 4-4 and π attenuator 4-5 realized by FPGA, digital The input end of the attenuator 4-1 is connected to the output end of the preselection amplifier module 3, the output end of the digital attenuator 4-1 is connected to the input end of the π attenuator 4-5, and the output end of the π attenuator 4-5 is connected to the fixed gain amplifier The input terminal of 4-2, the output terminal of the fixed gain amplifier 4-2 is connected to the input terminal of the first stage frequency conversion module 5 and the input terminal of the data sampling module 4-3, and the output terminal connection of the data sampling module 4-3 is realized by FPGA The input end of the control module 4-4, the output end of the control module 4-4 implemented by FPGA is connected to the feedback signal input end of the digital attenuator 4-1. The fixed gain amplifier 4-2 ensures the full gain of the signal in the channel, and the digital attenuator 4-1 adjusts the attenuation according to the signal power to realize digital automatic gain control. The power of the signal is determined by the FPGA control data sampling module 4-3 sampling the intermediate frequency output signal. The signal sampled by the data sampling module 4-3 is buffered and stored inside the FPGA and modulo calculation is performed. The modulo output data is detected by the peak value The maximum value of the sampled signal is obtained by the judgment, and then the power of the signal is calculated. The power value of the signal is compared with the preset threshold value to obtain a control signal for increasing or decreasing the attenuation, thereby controlling the sliding of the attenuation. This embodiment adopts the method of successive comparison, and utilizes the periodicity of the carrier signal to realize the peak value comparison with a certain accuracy in a small number of comparison times. Since the initial phase of data sampling module 4-3 sampling is random, when sampling at a fixed frequency, different initial phases cannot all be sampled to the peak value, but under certain accuracy requirements, the accuracy can be obtained in a limited number of samples. the maximum value. The actual sampling rate of the system is 6.1 times the signal frequency. When the number of comparisons is 54, the accuracy error is 0.037dBm, which can fully meet the requirements of the system. In actual work, when the power is in the critical state of adjusting attenuation, due to the fluctuation and hysteresis of peak detection, its output value may fluctuate at a certain point, so that the amplification factor of the first-stage digital variable gain amplifier module 4 Continuous adjustment cannot guarantee stable operation; at the same time, in order to avoid the saturation of the amplifier, this embodiment adopts a multi-threshold judgment method: different thresholds correspond to different attenuation slips, and at the same time, it is necessary to set the attenuation when the attenuation needs to be increased. The threshold value is low, and the threshold value is high when attenuation needs to be reduced. In this way, the overshoot can be effectively controlled and fluctuations can be reduced. In the actual application, the dynamic range of the received spread spectrum signal is very large, so the system adopts three-stage automatic gain control, which are respectively located in the radio frequency band, the first intermediate frequency band and the second intermediate frequency band of the system. It can guarantee a gain of about 40dB, and distribute the dynamic gain to different frequency bands. The gain of the first-stage digital variable gain amplifying module 4 is about 0 to 30 dB. The second-stage digital variable gain amplifying module 6 and the third-stage digital variable gain amplifying module 8 are the same in structure, gain multiple and working principle as the first-stage digital variable gain amplifying module 4 .

The first stage frequency conversion module 5 comprises a mixer 5-1, a local oscillator signal generator 5-2, a low-pass filter 5-3 and an amplifier 5-4, and the two input ends of the mixer 5-1 are respectively connected to the first The output end of the first-stage digital variable gain amplification module 4 and the output end of the local oscillator signal generator 5-2, the output end of the mixer 5-1 is connected to the input end of the low-pass filter 5-3, and the low-pass filter The output terminal of 5-3 is connected to the input terminal of amplifier 5-4. The low-pass filter 5-3 is a crystal filter. The gain of the amplifier 5-4 is about 22dB, and the output frequency of the first stage frequency conversion module 5 is about 10.7MHz.

The local oscillator signal generator 5-2 is composed of a crystal oscillator 5-2-1, a phase-locked loop 5-2-2, a buffer driver 5-2-3, a direct digital frequency synthesizer 5-2-4 and a low pass The filter 5-2-5 is composed, the output end of the crystal oscillator 5-2-1 is connected to the input end of the phase-locked loop 5-2-2, and the output end of the phase-locked loop 5-2-2 is connected to the buffer driver 5-2 -3 input, an output of the buffer driver 5-2-3 is connected to the input of a direct digital frequency synthesizer 5-2-4, and an output of the direct digital frequency synthesizer 5-2-4 is connected to a low Input to pass filter 5-2-5. The frequency of the crystal oscillator 5-2-1 is 10MHz, and the model of the phase-locked loop 5-2-2 is ADF4001. The model of one-way direct digital frequency synthesizer 5-2-4 is AD9850, both of which are products of American AD Company. The output frequency of the local oscillator signal generator 5-2 is about 13.5MHZ.

The local oscillator signal used for frequency mixing in the second-stage frequency conversion module 7 can be obtained from the second output end of the buffer driver 5-2-3 of the local oscillator signal generator 5-2, that is, through two direct digital frequency synthesizers 7-16 and the two-way low-pass filter 7-17 obtain the local oscillator signal used for frequency mixing in the second-stage frequency conversion module 7, and the frequency of the local oscillator signal is about 11.5MHZ. The clock signal of the subsequent baseband signal can also be obtained from the third output end of the buffer driver 5-2-3 through the three-way direct digital frequency synthesizer 16, the three-way low-pass filter 17 and the level drive circuit 18.

Since the system requires high frequency accuracy and stability, the local oscillator signal generator requires the selected crystal oscillator to have the same or higher standard as the system. The higher the accuracy and stability of the crystal oscillator output frequency, the lower its output frequency. In order to ensure the accuracy and stability requirements, the crystal oscillator selected in this embodiment needs frequency multiplication to reach the frequency of the local oscillator signal, so The local oscillator signal generator adopts DDS+PLL (that is, direct digital frequency synthesis + phase-locked loop) mode. Since the local oscillator signal does not need to have a fast frequency settling time and a very wide frequency output range, it is different from the traditional DDS+PLL mode that uses DDS signal excitation PLL mode and DDS signal interpolation PLL mode. The output signal of the DDS is used as the reference clock of the DDS system, and the DDS outputs the local oscillator signal according to the frequency control word. Although this method cannot improve the phase noise brought by the PLL and the spurs brought by the DDS, its structure is simple and can meet the frequency and frequency resolution requirements of the local oscillator signal without deteriorating the frequency accuracy. and stability. The radio frequency front end of this embodiment adopts two-stage frequency conversion, the first-stage frequency conversion module implements up-conversion to output high-medium frequency signals, and the second-stage frequency conversion module down-converts signals to low-intermediate frequency output. This structure is simpler than the Hartley structure and Weaver structure that suppress the image interference, and the image interference can fully meet the system requirements. The low-pass filter 5-3 in the first-stage frequency conversion module 5 adopts a crystal filter. Since the pre-selection amplification module 3 appropriately increases the bandwidth, the bandwidth of the signal after frequency conversion must be strictly guaranteed, otherwise the spread spectrum gain will be affected. Compared with the ordinary LC filter, although the crystal filter has a larger insertion loss, it has a better square coefficient and out-of-band suppression, which can fully meet the requirements of bandwidth and interference suppression. In order to provide a stable and accurate clock for the digital baseband processing part, like the local oscillator signal, the clock output requires high frequency accuracy and stability, so the output of the local oscillator signal source PLL can be used to drive the DDS of the clock module , the same accuracy and stability as the local oscillator signal can be obtained. Since the baseband signal processing part requires the clock to have a CMOS level, the output of the clock needs to pass through the driving circuit to meet the level requirement.

The intermediate frequency amplification module 9 includes an intermediate frequency amplifier 9-1 and an anti-aliasing filter 9-2, the input end of the intermediate frequency amplifier 9-1 is connected to the output end of the third stage digital variable gain amplification module 8, and the output of the intermediate frequency amplifier 9-1 The terminal is connected to the input terminal of the anti-aliasing filter 9-2. Since the filter after the first-stage frequency mixing uses a crystal filter with a better rectangular coefficient, the signal bandwidth has been well guaranteed, so the parameter setting of the final anti-aliasing filter can be appropriately relaxed, and also can meet the requirements. The gain of the intermediate frequency amplifying module 9 is about 22dB.

The high dynamic range broadband radio frequency front end of the medium and short wave frequency band of the present invention adopts a broadband intermediate frequency sampling software radio structure. The baseband signal processing part performs digital demodulation driven by the clock module. The advantages of this structure are: (1) the low noise amplifier of the pre-selected amplifier module has high gain and low noise figure, which can effectively reduce the noise figure of the front end; (2) two-stage frequency conversion can achieve higher image rejection, and can simultaneously Satisfy the gain requirements for weak signal input; (3) The local oscillator signal and the clock provided to the digital signal processing part adopt DDS (direct digital frequency synthesis) + PLL (phase-locked loop) mode to obtain higher frequency resolution and Stability; (4) The crystal filter has a better square coefficient than the ordinary LC filter, which can better meet the bandwidth requirements; (5) The three-stage automatic gain control can effectively distribute the dynamic range, and the digital automatic Compared with the analog method, the gain control has the characteristics of flexible and accurate control.

Claims (6)

1. intermediate waves frequency range high dynamic range broadband rf front end, it is characterized in that it is by preliminary election amplification module (3), first order digital variable gain amplification module (4), first order frequency-variable module (5), second level digital variable gain amplification module (6), second level frequency-variable module (7), third level digital variable gain amplification module (8) and intermediate frequency amplification module (9) are formed, the signal output part of preliminary election amplification module (3) connects the signal input part of first order digital variable gain amplification module (4), the signal output part of first order digital variable gain amplification module (4) connects the signal input part of first order frequency-variable module (5), the signal output part of first order frequency-variable module (5) connects the signal input part of second level digital variable gain amplification module (6), the signal output part of second level digital variable gain amplification module (6) connects the signal input part of second level frequency-variable module (7), the signal output part of second level frequency-variable module (7) connects the signal input part of third level digital variable gain amplification module (8), and the signal output part of third level digital variable gain amplification module (8) connects the signal input part of intermediate frequency amplification module (9).

2. intermediate waves frequency range high dynamic range broadband rf front end according to claim 1, it is characterized in that preliminary election amplification module (3) comprises low noise amplifier (3-1), preselection filter (3-2) and radio frequency amplifier (3-3), the output of low noise amplifier (3-1) connects the input of preselection filter (3-2), and the output of preselection filter (3-2) connects the input of radio frequency amplifier (3-3).

3. intermediate waves frequency range high dynamic range broadband rf front end according to claim 1, it is characterized in that first order digital variable gain amplification module (4) comprises digital pad (4-1), fixed gain amplifier (4-2), data sampling module (4-3), control module (4-4) and π attenuator (4-5) by the FPGA realization, the input of digital pad (4-1) connects the output of preliminary election amplification module (3), the output of digital pad (4-1) connects the input of π attenuator (4-5), the output of π attenuator (4-5) is connected and fixed the input of gain amplifier (4-2), the output of fixed gain amplifier (4-2) connects the input of first order frequency-variable module (5) and the input of data sampling module (4-3), the output of data sampling module (4-3) connects the input of the control module (4-4) that is realized by FPGA, and the output of the control module (4-4) that is realized by FPGA connects the feedback signal input of digital pad (4-1).

4. intermediate waves frequency range high dynamic range broadband rf front end according to claim 1, it is characterized in that first order frequency-variable module (5) comprises frequency mixer (5-1), local oscillation signal generator (5-2), low pass filter (5-3) and amplifier (5-4), two inputs of frequency mixer (5-1) connect the output of first order digital variable gain amplification module (4) and the output of local oscillation signal generator (5-2) respectively, the output of frequency mixer (5-1) connects the input of low pass filter (5-3), and the output of low pass filter (5-3) connects the input of amplifier (5-4).

5. intermediate waves frequency range high dynamic range broadband rf front end according to claim 4, it is characterized in that local oscillation signal generator (5-2) is by crystal oscillator (5-2-1), phase-locked loop (5-2-2), buffering driver (5-2-3), one tunnel Direct Digital Frequency Synthesizers (5-2-4) and No. one low pass filter (5-2-5) are formed, the output of crystal oscillator (5-2-1) connects the input of phase-locked loop (5-2-2), the output of phase-locked loop (5-2-2) connects the input of buffering driver (5-2-3), an output of buffering driver (5-2-3) connects the input of one tunnel Direct Digital Frequency Synthesizers (5-2-4), and the output of one tunnel Direct Digital Frequency Synthesizers (5-2-4) connects the input of No. one low pass filter (5-2-5).

6. intermediate waves frequency range high dynamic range broadband rf front end according to claim 1, it is characterized in that intermediate frequency amplification module (9) comprises intermediate frequency amplifier (9-1) and frequency overlapped-resistable filter (9-2), the input of intermediate frequency amplifier (9-1) connects the output of third level digital variable gain amplification module (8), and the output of intermediate frequency amplifier (9-1) connects the input of frequency overlapped-resistable filter (9-2).

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