CN101833083A - Radio-frequency front-end circuit of L-band radar receiver - Google Patents

Abstract

The invention relates to the technical field of radar receivers, in particular to a radio frequency front-end circuit of an L-band radar receiver. The invention comprises a diode limiter (100), a transceiver switch (101), a low noise amplifier (102), a radio frequency LC bandpass filter (103), a down-conversion mixer (104), a surface acoustic wave filter (105 ), a variable gain amplifier (106), an intermediate frequency amplifier (107); the L-band radio frequency signal received by the antenna passes through a diode limiter (100), a transceiver switch (101), a low noise amplifier (102) and a radio frequency LC filter (103) is sent to the down-conversion mixer (104) to be converted into an intermediate frequency signal, then through a narrow-band surface acoustic wave filter (105), and finally output to A through a variable gain amplifier (106) and an intermediate frequency amplifier (107) /D device. The invention improves the system noise performance and sensitivity, and has the advantages of high performance, few external components, small volume and low power consumption.

Description

L-band radar receiver RF front-end circuit

technical field

The invention relates to the technical field of radar receivers, in particular to a radio frequency front-end circuit of an L-band radar receiver.

Background technique

As an important part of the radar system, the main function of the radar receiver is to amplify, convert, filter and digitize the weak signal received by the radar antenna, and at the same time suppress interference from the outside, clutter and noise inside the machine, so that the signal Keep as much target information as possible to facilitate the next step of signal processing and data processing. Frequency bandwidth, low noise, large dynamic and high stability are the requirements of modern receivers.

Since the concept of software radio was put forward in 1992, due to its outstanding versatility and openness, software radio receivers have gained sufficient attention and development. The modern software radio receiver structure generally includes RF low-pass sampling digital structure, RF band-pass sampling digital structure, broadband intermediate frequency band-pass sampling digital structure and zero-IF baseband digital structure. Strong anti-interference ability, stable and reliable, flexible design, has been widely used in modern radar systems.

The L-band (1GHz-2GHz) radar receiver has a long range, low external noise, small antenna size, and good angular resolution. Many microwave monolithic integrated circuits (MMICs) can work in this frequency band, with good performance and price ratio, and the rapidly developing filter miniaturization technology also provides a good means for the realization of radar receivers in this band.

The general IF band-pass sampling digital receiver completes the functions of filtering, amplification, gain control and frequency conversion before the A/D conversion, and converts the higher carrier frequency to the lower IF. If the data rate is not high after A/D conversion, it can be directly sent to DSP; if the data rate is high after A/D conversion, it needs to be digitally down-converted and converted to baseband digital signal, and then sent to DSP. DSP implements various digital signal processing with relatively low data rates, and completes various functions such as demodulation, decoding, and error correction. With the rapid development of digital technology, the digital processing of the receiver is more and more stable and flexible, and the difficulty in the development of the receiver is more and more reflected in the RF and A/D modules before digitization. This requires the RF front-end frequency bandwidth, large dynamic range, high sensitivity, low noise figure, high image rejection, high linearity, small size, low power consumption, high stability, and the circuit should be as simple as possible. These indicators are somewhat contradictory to each other and need to be considered comprehensively.

Contents of the invention

For the above-mentioned technical problems, the purpose of this invention is to provide a radio frequency front-end circuit of an L-band radar receiver, to achieve high sensitivity, large dynamic range, low noise figure, high linearity, strong anti-interference ability, convenient debugging and Small size requirements.

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

Diode limiter (100), transceiver switch (101), low noise amplifier (102), radio frequency LC bandpass filter (103), down-conversion mixer (104), surface acoustic wave filter (105), Variable gain amplifier (106), intermediate frequency amplifier (107);

The L-band radio frequency signal received by the antenna is sent to the down-conversion mixer (104) after passing through the diode limiter (100), the transceiver switch (101), the low-noise amplifier (102) and the radio frequency LC filter (103) to be converted into The intermediate frequency signal passes through a narrow-band surface acoustic wave filter (105), and finally is output to the A/D device through a variable gain amplifier (106) and an intermediate frequency amplifier (107).

The diode limiter uses two Schottky diodes with opposite directions to form a bidirectional limiter circuit.

The transceiver switch uses a single-chip suction RF switch chip ADG901BRM, and the on-off of the transceiver switch is controlled by an external TB signal.

PI-type networks are arranged before and after the low-noise amplifier, and PI-type networks are arranged before and after the intermediate frequency amplifier, and the PI-type network is composed of three resistors.

The low noise amplifier adopts RF2320 chip to form.

The down conversion mixer adopts LT5522 chip to form.

The variable gain amplifier is composed of HMC626LP5 chip.

The intermediate frequency amplifier is composed of RF2317 chip.

The present invention has the following advantages and positive effects:

1) Putting the low-noise amplifier in front can make the antenna, transceiver switch and low-noise amplifier compact together, which is beneficial to reduce the adverse effects caused by the feeder loss; the pre-selection filtering and image suppression functions are integrated into one radio frequency section Realization of high-performance LC band-pass filter simplifies the system structure without weakening the system index. At the same time, the filter is placed after the low-noise amplifier, which improves the system noise performance and sensitivity, and the use of the limiter can effectively protect the low-noise amplifier. noise amplifier;

2) The double-balanced mixer used integrates an RF input transformer and a high-speed differential LO buffer amplifier, and the RF and LO inputs are internally matched to a broadband single-ended input, which has the advantages of high performance, few external components, small size and low power consumption ;

3) The surface acoustic wave filter in the intermediate frequency band has narrow bandwidth and small insertion loss, and at the same time has high out-of-band rejection and superior performance; the in-band fluctuation of the radio frequency receiving module is extremely small, and the intermediate frequency module adopts a high-performance 6bit digitally controlled gain amplifier, stepping Small but with a large controllable range; the input 1dB power compression point of all devices exceeds 10dBm, the amplifier chip even exceeds 20dBm, the output third-order intercept point almost reaches above 30dBm, the linearity is very high, and the system dynamic range is relatively large. big;

4) A small three-terminal EMI filter is used between the public power supply and each load, which is beneficial to reduce interface interference and improve the circuit's ability to resist external electromagnetic interference; PI type resistance attenuation is set between chips with mismatched impedances Network, within a wide frequency range, can not only achieve impedance matching, but also achieve isolation between the front and rear stages. By properly selecting the design value, the attenuation can be reduced to less than 1dB; the higher intermediate frequency simplifies the design of the RF filter and is commercialized There are many products, and the intermediate frequency filter is easy to realize; the narrowband higher intermediate frequency signal reduces the design difficulty of A/D sampling, which is beneficial to the design of the digital processing part.

Description of drawings

Fig. 1 is a structural block diagram of the radio frequency front-end circuit of the L-band radar receiver provided by the present invention.

Fig. 2 is a circuit diagram of the low noise amplifier and its front and rear PI network in the present invention.

Fig. 3 is a circuit diagram of a frequency mixing circuit in the present invention.

Fig. 4 is a circuit diagram of a digitally controlled variable gain amplifier circuit in the present invention.

Fig. 5 is an electrical schematic diagram of the fixed gain amplifier and its front and rear PI-type networks in the present invention.

Detailed ways

Below in conjunction with accompanying drawing, the present invention will be further described with specific embodiment:

The radio frequency front-end circuit of the L-band radar receiver provided by the present invention specifically adopts the following technical solutions, referring to Fig. 1, including:

Diode limiter 100, transceiver switch 101, low noise amplifier 102, radio frequency LC bandpass filter 103, down conversion mixer 104, surface acoustic wave filter 105, variable gain amplifier 106, intermediate frequency amplifier 107;

The L-band radio frequency signal received by the antenna is sent to the down-conversion mixer 104 to be converted into an intermediate frequency signal through a diode limiter 100, a transceiver switch 101, a low-noise amplifier 102 and a radio frequency LC filter 103, and then passed through a narrow-band surface acoustic wave The filter 105 is finally output to the A/D device through the variable gain amplifier 106 and the intermediate frequency amplifier 107.

The signal sent from the antenna first passes through the limiter 100, and the limiter 100 uses two Schottky diodes in opposite directions to form a bidirectional limiter circuit to prevent excessive signal power from damaging the receiver;

The transceiver switch 101 uses a single-chip sink-type RF switch chip ADG901BRM. Its operating frequency can reach up to 2.5GHz. At 1GHz, the input 1dB power compression point can reach 17dBm, the isolation can reach 37dB, and the insertion loss is only 0.8dB. The on-off of this switch is controlled by the external TB signal;

After passing through the transceiver switch 101, the radio frequency signal enters the low noise amplifier 102 and its front and rear PI network; the PI network is composed of three resistors, which is an attenuation network with impedance matching and front and rear stage isolation, and has a very wide operating frequency band. An appropriate resistance value can reduce the attenuation to below 1dB, and when a greater attenuation is required, the corresponding resistance value can be changed; since the low-noise amplifier 102 and the fixed-gain amplifier are both standard 75-ohm input and output impedances, the PI The type network is set accordingly before and after the low noise amplifier and the fixed gain amplifier;

After the low-noise amplifier 102 and its front and rear PI-type networks, the radio frequency signal enters the radio frequency LC bandpass filter 103. Since the radio frequency signal has a wide bandwidth, surface acoustic wave filters and crystal filters with better square coefficients are not suitable, so Using LC filter;

The RF LC bandpass filter should not only suppress the signal and noise outside the receiving frequency band of the receiver, but also suppress the image frequency signal caused by frequency mixing; considering that the larger the number of filter sections, the greater the insertion loss and the smaller the circuit size Larger, but at the same time the image frequency suppression is higher; so here can only be a compromise design: the insertion loss of the filter should not be too large, the size of the filter circuit should not be too large, and at the same time, it must ensure that the image frequency has enough Inhibition. This circuit uses a 4th-order filter, the insertion loss is less than 2dB, and the image frequency rejection is at least 45dB, which meets the design requirements. The signal from the RF filter is sent to the active double-balanced mixer to realize the conversion from RF to IF. The intermediate frequency signal passes through the intermediate frequency filter first, and the surface acoustic wave filter is used due to good square coefficient, small insertion loss, and narrow bandwidth. The typical insertion loss of this filter is 3.5dB, the 3dB bandwidth is only 75KHz, the in-band fluctuation is 0.5dB, and the stopband rejection beyond the center frequency ±0.5MHz exceeds 40dB; the crystal filter can achieve a narrower bandwidth, but its cost is high and the work less frequent. The LC filter cannot achieve such a narrow bandwidth and such a good square factor. Therefore, SAW filters are the best choice for IF filters.

After the intermediate frequency filter, the signal is sent to the variable gain amplifier 106. This is to ensure that the system has a large dynamic range, which can suppress strong signals and amplify weak signals; finally, after passing through a fixed gain amplifier, the intermediate frequency signal can be sent to the A/D Perform intermediate frequency sampling and subsequent digital processing.

系统噪声系数主要由前面几级决定，如果越靠前的电路噪声系数越低，增益越大，那么系统的噪声系数就越低。因此这里需要使用低噪声放大器。考虑到滤波器置于低噪声放大器之前相当于加大了低噪声放大器损耗，恶化了噪声系数，而L波段外部噪声较低，所以将滤波器设置在低噪声放大器之后，把预选滤波和镜频滤波的功能集中在一个射频滤波器。我们使用的低噪声放大电路工作频率最高可达2.5GHz，噪声系数低于2dB，在1GHz时增益达到17dB，输出1dB功率压缩点达到25dBm，输出三阶截点达到34dBm。Figure 2 shows the circuit diagram of the low-noise amplifier RF2320 and its front and rear PI-type networks. It can be seen from the figure that the public power supply passes through the three-terminal EMI filter, which is beneficial to improve the circuit's ability to resist external electromagnetic interference. In fact, other chips related to the public power supply have done this kind of treatment. According to the calculation formula of cascade circuit noise figure,

The noise figure of the system is mainly determined by the previous stages. If the noise figure of the front circuit is lower and the gain is larger, the noise figure of the system will be lower. So here need to use low noise amplifier. Considering that placing the filter before the low noise amplifier is equivalent to increasing the loss of the low noise amplifier and deteriorating the noise figure, and the external noise of the L-band is low, so the filter is set after the low noise amplifier, and the preselection filter and image frequency The filtering function is concentrated in one RF filter. The working frequency of the low-noise amplifier circuit we use can reach up to 2.5GHz, the noise figure is lower than 2dB, the gain reaches 17dB at 1GHz, the output 1dB power compression point reaches 25dBm, and the output third-order intercept point reaches 34dBm.

Figure 3 shows the circuit diagram of the mixer circuit based on LT5522. The RF input frequency range is 1.2GHz to 2.3GHz without external matching (intermediate frequency band). We connect a 2.2pF capacitor in parallel to the input pin to lower the minimum RF input frequency to 600MHz, thus completely covering the L-band. If a 3.9nH inductor is connected in parallel to the input pin, the maximum RF input frequency can be extended to 2.7GHz. The LO input frequency is between 400MHz and 2700MHz without external matching. After necessary matching measures, the intermediate frequency output frequency is between 0.1MHz and 1000MHz. Compared with passive mixers, this circuit still achieves small volume and low cost even though it integrates RF input transformer and limiting LO buffer amplifier, and due to the integration of LO buffer, LO drive The level reduction can be from -10 to 0dBm. Between 50MHz and 2700MHz, the RF to LO isolation exceeds 45dB. When the LO is between 400MHz and 2700MHz, the RF input power is -7dBm, the LO power is -5dBm, and the intermediate frequency is 140MHz, the leakage from LO to RF is lower than -50dBm, and the leakage from LO to IF is lower than -49dBm, which has a high LO-RF isolation and LO-IF isolation. At 900MHz, the input 1dB compression point of the mixer reaches 10.8dBm, and the input third-order intercept point reaches 25dBm, which has high linearity. The IF output is in differential form with an output impedance of 400 ohms. First, the low-pass matching of 400 ohms to 200 ohms is achieved through the three-element network shown in the figure. The network is tuned to an IF frequency of 140MHz. A 4:1 transformer is then used to transform the 200-ohm differential output into a 50-ohm single-ended output. As an impedance transformation and double-ended to single-ended transformation, the transformer has wider frequency adaptability than the lumped element network composed of capacitors and inductors, and the isolation between LO and IF is better, and two intermediate frequencies are provided through the center tap of the transformer. Required collector bias voltage for output pins.

Figure 4 shows the circuit diagram of the digitally controlled variable gain amplifier HMC626LP5. The current variable gain amplifiers are generally divided into three categories, one is an analog-controlled voltage-controlled gain amplifier, which uses a changing analog voltage to control the gain; one uses the SPI interface for gain control; the other uses a parallel digital interface for gain control. The latter two types belong to digital control, which is simpler and easier than analog control, but the SPI interface requires an external microcontroller or DSP to provide control signals, while the parallel control interface can be as simple as a DIP switch and a resistance. At the same time, it can still receive external parallel control signals when needed, and the control by the single-chip microcomputer or DSP will not be affected at all, which greatly facilitates the circuit debugging work, and the design is more flexible and reliable. The numerically controlled variable gain amplifier we use is controlled by 6 bits, with a step of 0.5dB, and the gain variation range is between 8.5dB and 40dB, which meets the design requirements. Its noise figure is only 2.8dB, and its output 1dB compression point reaches 20dBm. The output third-order intercept point reaches 36dBm, which has extremely low noise and high linearity.

系统输入三阶截点主要由后几级电路决定，因此作为最后一级的放大器应当有较高的线性度。我们使用的固定增益放大器增益达到14.3dB，在100MHz时输出1dB功率压缩点高达25.5dBm，输出三阶截点高达47dBm，符合设计对动态范围的要求。从固定增益放大器输出的中频信号，将被送到A/D采样模块进行中频数字化处理。Figure 5 shows the circuit diagram of the fixed-gain amplifier RF2317 and its front and rear PI-type networks. To simplify the power supply design, it uses the same power supply as the LNA. The bias resistance of the output pin can be further increased to reduce the bias voltage and current, thereby reducing power consumption. Since the input and output impedance is 75 ohms, like the low noise amplifier, a PI type network is also used here for impedance matching and front and rear stage isolation. According to the calculation formula of the input third-order intercept point of the cascade circuit,

The third-order intercept point of the system input is mainly determined by the latter stages of circuits, so the amplifier as the last stage should have a higher linearity. The fixed-gain amplifier we use has a gain of 14.3dB, an output 1dB power compression point of up to 25.5dBm at 100MHz, and an output third-order intercept point of up to 47dBm, which meets the design requirements for dynamic range. The intermediate frequency signal output from the fixed gain amplifier will be sent to the A/D sampling module for intermediate frequency digital processing.

The main technical parameter of the present invention is: system sensitivity is-107.9dBm (S+N/N=12dB); The gain variation range of system is 29.5dB～61dB; System noise figure is 5.3dB; System input third-order intercept point is- 4dBm, input 1dB power compression point is -25.4dBm; system spurious free dynamic range is 65.4dB. The system intermediate frequency frequency is 140MHz, and the intermediate frequency 3dB signal bandwidth is 75KHz; the image rejection degree is greater than 45dB, and the intermediate frequency rejection degree is greater than 50dB; the system output third-order intercept point is greater than 20dBm. High linearity, good working performance, and certain application value.

Claims (8)

1. a radio frequency front-end circuit of L band radar receiver, it is characterized in that, comprising: Diode limiter (100), transceiver switch (101), low noise amplifier (102), radio frequency LC bandpass filter (103), down-conversion mixer (104), surface acoustic wave filter (105), Variable gain amplifier (106), intermediate frequency amplifier (107); The L-band radio frequency signal received by the antenna is sent to the down-conversion mixer (104) after passing through the diode limiter (100), the transceiver switch (101), the low-noise amplifier (102) and the radio frequency LC filter (103) to be converted into The intermediate frequency signal passes through a narrow-band surface acoustic wave filter (105), and finally is output to the A/D device through a variable gain amplifier (106) and an intermediate frequency amplifier (107). 2. the radio frequency front-end circuit of L band radar receiver according to claim 1, is characterized in that: The diode limiter uses two Schottky diodes with opposite directions to form a bidirectional limiter circuit. 3. the radio frequency front-end circuit of L band radar receiver according to claim 1, is characterized in that: The transceiver switch uses a single-chip suction RF switch chip ADG901BRM, and the on-off of the transceiver switch is controlled by an external TB signal. 4. according to the radio frequency front-end circuit of the L band radar receiver described in any one in claim 1,2,3, it is characterized in that: PI-type networks are arranged before and after the low-noise amplifier, and PI-type networks are arranged before and after the intermediate frequency amplifier, and the PI-type network is composed of three resistors. 5. the radio frequency front-end circuit of L band radar receiver according to claim 4, is characterized in that: The low noise amplifier adopts RF2320 chip to form. 6. the radio frequency front-end circuit of L band radar receiver according to claim 5, is characterized in that: The down conversion mixer adopts LT5522 chip to form. 7. according to the radio frequency front-end circuit of the described L band radar receiver of claim 5 or 6, it is characterized in that: The variable gain amplifier is composed of HMC626LP5 chip. 8. the radio frequency front-end circuit of L band radar receiver according to claim 7, is characterized in that: The intermediate frequency amplifier is composed of RF2317 chip.

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