A circuit for achieving high image frequency rejection through single-stage down-conversion

Abstract

The present invention provides a circuit for achieving high image frequency rejection through one-time down-conversion. The circuit includes an IQ mixer (2), a 90° hybrid (3), three single-pole double-throw switches (4), two loads (5), a low-pass filter (6), and a frequency source (7). The IQ mixer (2) is respectively connected to the 90° hybrid (3) and the frequency source (7). The 90° hybrid (3) is respectively connected to the two loads (5) and the low-pass filter (6) through the three single-pole double-throw switches (4). The circuit of the present invention has a simple structure, is easy to realize product miniaturization, can simplify the layout of the receiver, reduces the cost compared with two-time down-conversion, and improves the efficiency.

Description

Technical Field

[0001] The present invention relates to the field of high image frequency rejection circuits, and in particular to a circuit for achieving high image frequency rejection through single-stage down-conversion. Background Art

[0002] When a radar operates, in addition to receiving useful frequency components, the receiver also receives other interfering frequency components, and the image frequency component is one of them. After down-conversion, the image frequency component and the signal obtained by down-converting the local oscillator will be superimposed on the intermediate frequency signal, and the resulting interference cannot be filtered out by an ordinary intermediate frequency band-pass filter. Therefore, the suppression of the image frequency has always been a key technology for radar receivers.

[0003] In recent years, the radar technology in China has developed rapidly, the volume of radars has become smaller and smaller, and at the same time, the requirements for image frequency rejection have become higher and higher. At present, there are two solutions for obtaining image frequency rejection: one is to use double-stage down-conversion, which will increase the number of components and the volume of the receiver, and the rejection ratio of the image frequency is about 70 dB; the other solution is to use single-stage quadrature mixing, the rejection ratio of which is limited, about 25 dB, and it cannot meet the high index requirements for image frequency rejection. Therefore, it is very necessary to design a high image frequency rejection circuit that can achieve single-stage down-conversion. Summary of the Invention

[0004] The purpose of the present invention is to provide a circuit for achieving high image frequency rejection through single-stage down-conversion, so as to solve the problems of the volume of double-stage down-conversion and the low index of single-stage down-conversion, realize the miniaturization of the receiver, and ensure the index at the same time.

[0005] The object of the present invention is achieved by the following technical solutions:

[0006] A circuit for achieving high image frequency rejection through single-stage down-conversion, the circuit includes an IQ mixer 2, a 90° hybrid coupler 3, three single-pole double-throw switches 4, two loads 5, a low-pass filter 6, and a frequency source 7. The IQ mixer 2 is respectively connected to the 90° hybrid coupler 3 and the frequency source 7, and the 90° hybrid coupler 3 is respectively connected to the two loads 5 and the low-pass filter 6 through the three single-pole double-throw switches 4.

[0007] As a further technical solution, a stage of image frequency filter 1 is added before the IQ mixer 2.

[0008] As a further technical solution, the 1 dB bandwidth of the image frequency filter 1 is greater than or equal to BW and less than or equal to BW + IF.

[0009] As a further technical solution, the local oscillator signal is divided into two segments of low local oscillator and high local oscillator by the frequency source 7. The low local oscillator is applicable to the low end of the radio frequency signal, and the high local oscillator is applicable to the high end of the radio frequency signal.

[0010] As a further technical solution, the local oscillator signal is divided into two segments of low local oscillator and high local oscillator by the frequency source 7, which are respectively: LOL = (F0 - 0.5×BW - IF) ~ (F0 - IF) and LOH = (F0 + IF) ~ (F0 + 0.5×BW + IF), where: F0 is the center frequency, BW is the bandwidth of the RF signal, and IF is the intermediate frequency obtained through mixing; when the frequency of the RF signal RFL = (F0 - 0.5×BW) ~ F0, the low local oscillator is used for mixing; when the frequency of the RF signal is RFH = F0 ~ (F0 + 0.5×BW), the high local oscillator is used for mixing.

[0011] As a further technical solution, the RF signal first passes through a 90° hybrid coupler (3), and then two single-pole double-throw switches are used to select the phase shift channel according to the states of the low local oscillator and the high local oscillator, and finally a single-pole double-throw switch is used to select the main channel.

[0012] As a further technical solution, the high-frequency components at the far end obtained through mixing are filtered out by the low-pass filter (6) of the intermediate frequency.

[0013] As a further technical solution, the image frequency filter 1 is replaced by 1 low-pass filter and 1 high-pass filter.

[0014] As a further technical solution, the low-pass filter 6 is replaced by a band-pass filter.

[0015] As a further technical solution, the three single-pole double-throw switches 4 are replaced by 1 double-pole double-throw switch.

[0016] Compared with the prior art, the present invention can solve the increase in devices and volume caused by two-stage down-conversion, and is easy to realize the miniaturization of products; moreover, this invention can simplify the layout of the receiver, reduce the cost compared with two-stage down-conversion, and improve the efficiency. Description of the Drawings

[0017] Figure 1 is the circuit schematic diagram of the present invention;

[0018] Figure 2 is the working flow chart of the present invention. Detailed Embodiments

[0019] The present invention will be described in detail below with reference to the drawings and specific embodiments.

[0020] Embodiment

[0021] The present invention mainly aims at current radars with miniaturization and high-index requirements. It is a circuit that can achieve high image frequency rejection through only one-stage down-conversion and can be applied to the receiving channel of a radar system. The present invention relates to a circuit that can achieve high image frequency rejection through one-stage down-conversion, and is used to solve the problem of miniaturizing the receiver on a miniaturized radar and meeting the requirement of high image frequency rejection ratio.

[0022] The present invention is realized through the following technical solutions: As Figure 1 and Figure 2 shown, a radio frequency signal with a center frequency of F0 and a bandwidth of BW is mixed to obtain an intermediate frequency IF. The circuit composition includes an image frequency filter (1), an IQ mixer (quadrature mixer) (2), a 90° hybrid (3), a single-pole double-throw switch (4), a load (5), a low-pass filter (6), and a frequency source (7). The IQ mixer (2) is respectively connected to the 90° hybrid (3) and the frequency source (7), and the 90° hybrid (3) is respectively connected to two loads (5) and the low-pass filter (6) through three single-pole double-throw switches (4).

[0023] In the present invention, the local oscillator signal required for mixing is first divided into two segments of low local oscillator and high local oscillator by the frequency source (7), which are respectively: LOL = (F0 - 0.5×BW - IF)~(F0 - IF) and LOH = (F0 + IF)~(F0 + 0.5×BW + IF). When the radio frequency signal frequency is RFL = (F0 - 0.5×BW)~F0, low local oscillator is used for mixing; when the radio frequency signal frequency is RFH = F0~(F0 + 0.5×BW), high local oscillator is used for mixing.

[0024] The mixing processes of low local oscillator and high local oscillator are described separately below:

[0025] a) Low local oscillator

[0026] Assume that when the amplitude of the radio frequency signal RFL is A, the amplitude of the local oscillator signal LOL is 1, and the amplitude of the image frequency signal IML is B.

[0027] Then, before entering the IQ mixer (2), the signal is:

[0028] fL(t) = AcosωRFL×t + BcosωIML×t

[0029] After passing through the IQ mixer (2), the two signals of I (in-phase signal) and Q (quadrature signal) obtained by mixing with the low local oscillator are respectively:

[0030]

[0031]

[0032] After the I-channel signal is phase-shifted by 90° through the 90° bridge (3), the intermediate-frequency signal obtained is:

[0033]

[0034] Then the output intermediate frequency is:

[0035]

[0036] b) High local oscillator

[0037] Assume that when the amplitude of the RF signal RFH is A, the amplitude of the local oscillator signal LOH is 1, and the amplitude of the image frequency signal IMH is B.

[0038] Then, before entering the IQ mixer (2), the signal is:

[0039] fH(t) = AcosωRFH×t + BcosωIMH×t

[0040] After passing through the IQ mixer (2) and mixing with the low local oscillator, the two signals of I (in-phase signal) and Q (quadrature signal) obtained are respectively:

[0041]

[0042]

[0043] After the Q-channel signal is phase-shifted by 90° through the 90° bridge (3), the intermediate-frequency signal obtained is:

[0044]

[0045] Then the output intermediate frequency is:

[0046]

[0047] At this time, the signal components of Asin(ωRFH + ωLOH)×t and B sin(ωIMH + ωLOH)×t can be filtered out by the low-pass filter (6) of the intermediate frequency.

[0048] From the above calculations, it can be seen that both the low local oscillator and the high local oscillator can achieve image frequency suppression, and the suppression degree mainly depends on the quadrature errors such as the IQ mixer (2), the 90° bridge (3), and the circuit wiring. If only this method is used, the image frequency suppression degree is only about 25 dB.

[0049] In the present invention, a stage of image frequency filter (1) is added before the IQ mixer (2). According to the existing method, the radio frequency frequency is F0 ± 0.5×BW. When the local oscillator is low, the image frequency is F0 ± 0.5×BW - 2×IF, and when the local oscillator is high, the image frequencies are F0 ± 0.5×BW + 2×IF respectively. According to the current radar bandwidth and intermediate frequency analysis, the image frequency will fall within the radio frequency signal bandwidth, and at this time, the image frequency filter (1) cannot completely filter out the image frequency; in the present invention, the radio frequency frequency is divided into two segments, LOL and LOH. When the radio frequency signal frequency is RFL, low local oscillator is used for mixing, and at this time, the image frequency IML = (F0 - 0.5×BW - 2×IF) to (F0 - 2×IF); when the radio frequency signal frequency is RFH, high local oscillator is used for mixing, and at this time, the image frequency IMH = (F0 + 2×IF) to (F0 + 0.5×BW + 2×IF). It can be seen that IML and IMH do not fall within the radio frequency signal bandwidth. By selecting a filter with a 1dB bandwidth greater than or equal to BW and less than BW + IF, the image frequency can be suppressed by 45dB to 80dB through this image frequency filter (1).

[0050] Meanwhile, the present invention utilizes the symmetry of the 90° hybrid (3), and through two single-pole double-throw switches (4), it is used to select to perform 90° phase shift on the I-channel or Q-channel signal components obtained by mixing, and then through one single-pole double-throw switch (4), the main channel or the load (5) channel is switched, and the obtained intermediate frequency signal is sent to the low-pass filter (6).

[0051] Finally, the ωIMH + ωLOH component obtained by mixing is filtered out through the low-pass filter (6).

[0052] To sum up, the circuit of the present invention can achieve high image frequency suppression in one-stage down-conversion, and the suppression degree is ≥70dB.

[0053] The replaceable solutions of this embodiment are as follows:

[0054] a) The three single-pole double-throw switches (4) in the schematic diagram can be replaced by one double-pole double-throw switch;

[0055] b) The image frequency filter (1) in the schematic diagram can be replaced by one low-pass filter and one high-pass filter;

[0056] c) The low-pass filter (6) in the schematic diagram can be replaced by a band-pass filter.

[0057] The present invention is verified through modeling and simulation in AWR software. First, a link is built in AWR, and the parameters of each device are set to the actual working parameters for simulation. Among them, the image rejection ratio of the IQ mixer is 27 dB, and the image filter has a loss of 70 dB at the image frequency. After simulation comparison, in the Ku band, when the bandwidth is 2 GHz and the intermediate frequency is 1080 MHz: when an input RF signal is applied, the output is about 0 dB; when the low local oscillator mode is used for the frequency RFL, the image rejection ratio is ≥100 dB, when the high local oscillator mode is used, the image rejection ratio is ≥60 dB, and the optimization is ≥40 dB; when the low local oscillator mode is used for the frequency RFH, the image rejection ratio is ≥90 dB, when the high local oscillator mode is used, the image rejection ratio is ≥50 dB, and the optimization is ≥40 dB. It can be seen that the circuit of the present invention can meet the expected indicators, proving its feasibility.

[0058] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. It should be noted that any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A circuit for achieving high image frequency rejection through one-time down-conversion, characterized in that, The circuit includes an IQ mixer (2), a 90° hybrid (3), three single-pole double-throw switches (4), two loads (5), a low-pass filter (6), and a frequency source (7). The IQ mixer (2) is respectively connected to the 90° hybrid (3) and the frequency source (7). The 90° hybrid (3) is connected to the two loads (5) and the low-pass filter (6) through the three single-pole double-throw switches (4). Specifically, the RF signal first passes through the 90° hybrid (3), and then the two single-pole double-throw switches are used to select the phase-shifting channel according to the states of the low local oscillator and the high local oscillator, and finally a single-pole double-throw switch is used to select the main channel.

2. The circuit for achieving high image frequency rejection through one - stage down - conversion according to claim 1, wherein, A stage of image-frequency filter (1) is added before the IQ mixer (2).

3. A circuit for achieving high image frequency rejection through a single down-conversion according to claim 2, wherein, The 1dB bandwidth of the image-frequency filter is greater than or equal to BW and less than or equal to BW + IF, where BW is the bandwidth of the RF signal and IF is the intermediate frequency obtained through mixing.

4. A circuit for achieving high image frequency rejection through a single down-conversion according to claim 1, characterized in that, The local oscillator signal is divided into two segments of low local oscillator and high local oscillator by the frequency source (7). The low local oscillator is applicable to the low end of the RF signal, and the high local oscillator is applicable to the high end of the RF signal.

5. A circuit for achieving high image frequency rejection through a single down-conversion according to claim 4, characterized in that The local oscillator signal is divided into two segments of low local oscillator and high local oscillator by the frequency source (7), which are respectively: LOL = (F0 - 0.5×BW - IF) ~ (F0 - IF) and LOH = (F0 + IF) ~ (F0 + 0.5×BW + IF), where: F0 is the center frequency, BW is the bandwidth of the RF signal, and IF is the intermediate frequency obtained through mixing; when the frequency of the RF signal RFL = (F0 - 0.5×BW) ~ F0, the low local oscillator is used for mixing; when the frequency of the RF signal is RFH = F0 ~ (F0 + 0.5×BW), the high local oscillator is used for mixing.

6. A circuit for achieving high image frequency rejection through one - stage down - conversion according to claim 1, characterized in that, The high-frequency components at the far end obtained through mixing are filtered out by the low-pass filter (6) of the intermediate frequency.

7. A circuit for achieving high image frequency rejection through a single down-conversion according to claim 2, characterized in that The image-frequency filter (1) is replaced by 1 low-pass filter and 1 high-pass filter.

8. A circuit for achieving high image frequency rejection through a single down-conversion according to claim 1, characterized in that The low-pass filter (6) is replaced by a band-pass filter.

9. A circuit for achieving high image frequency rejection through one-time down-conversion according to claim 1, characterized in that, The three single-pole double-throw switches (4) are replaced by 1 double-pole double-throw switch.

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