Down converter

Abstract

The utility model discloses a down converter. The down converter comprises a preprocessing module, a first down-conversion preprocessing module, a first down-conversion module, a second down-conversion preprocessing module, a second down-conversion module, a frequency selection and temperature compensation module, a third down-conversion preprocessing module, a third down-conversion module and a third down-conversion post-processing module which are connected in sequence. According to the invention, the problems of signal distortion and signal-to-noise ratio of a traditional down converter can be effectively solved, so that the signal-to-noise ratio and purity of a down-conversion signal can be improved.

Description

Technical Field

[0001] The present application belongs to the technical field of frequency converters, and in particular relates to a down converter. Background Art

[0002] Since the existing down-converter generally includes fewer frequency processing links, it is easy to cause the final output down-converted signal to have nonlinear distortion and low signal-to-noise ratio problems during the process of down-converting the high-frequency signal. Utility Model Content

[0003] The main purpose of the present application is to provide a down converter, aiming to solve the problems of signal distortion and low signal-to-noise ratio in the output signal of the existing down converter.

[0004] To achieve the above objectives, this application proposes the following technical solutions:

[0005] A down converter comprises: a preprocessing module, a first down-conversion pre-processing module, a first down-conversion module, a second down-conversion pre-processing module, a second down-conversion module, a frequency selection and temperature compensation module, a third down-conversion pre-processing module and a third down-conversion module; wherein the input end of the preprocessing module inputs a signal to be down-converted, the output end of the preprocessing module is connected to the input end of the first down-conversion pre-processing module, the output end of the first down-conversion pre-processing module is connected to the input end of the first down-conversion module, the output end of the first down-conversion pre-processing module is connected to the input end of the second down-conversion pre-processing module, the output end of the second down-conversion pre-processing module is connected to the input end of the second down-conversion module, the output end of the second down-conversion module is connected to the input end of the frequency selection and temperature compensation module, the output end of the frequency selection and temperature compensation module is connected to the input end of the third down-conversion pre-processing module, the output end of the third down-conversion pre-processing module is connected to the input end of the third down-conversion module, and the output end of the third down-conversion module outputs a down-conversion signal.

[0006] Optionally, the pre-processing module includes: a limiter, the input end of the limiter is input with the signal to be down-converted, and the output end of the limiter is connected to the input end of the first down-conversion pre-processing module.

[0007] Optionally, the first down-conversion pre-processing module includes: a first amplifier and a first filter, wherein the input end of the first amplifier is connected to the output end of the limiter, the output end of the first amplifier is connected to the input end of the first filter, and the output end of the first filter is connected to the input end of the first down-conversion module.

[0008] Optionally, the first down-conversion module includes: a first mixer, a first input end of the first mixer is connected to the output end of the first filter, a second input end of the first mixer inputs a first local oscillation signal, and an output end of the first mixer is connected to the input end of the second down-conversion pre-processing module.

[0009] Optionally, the second down-conversion pre-processing module includes: an equalizer, a second filter and a second amplifier, wherein the input end of the equalizer is connected to the output end of the first mixer, the output end of the equalizer is connected to the input end of the second filter, the output end of the second filter is connected to the input end of the second amplifier, and the output end of the second amplifier is connected to the input end of the second down-conversion module.

[0010] Optionally, the second down-conversion module includes: a second mixer, a first input end of the second mixer is connected to the output end of the second amplifier, a second input end of the second mixer inputs a frequency synthesis signal, and an output end of the second mixer is connected to the input end of the frequency selection and temperature compensation module.

[0011] Optionally, the frequency selection and temperature compensation module includes: a third filter and a temperature compensating attenuator, wherein the input end of the third filter is connected to the output end of the second mixer, the output end of the third filter is connected to the input end of the temperature compensating attenuator, and the output end of the temperature compensating attenuator is connected to the output end of the third down-conversion pre-processing module.

[0012] Optionally, the third down-conversion pre-processing module includes: a third amplifier and a fourth filter, wherein the input end of the third amplifier is connected to the output end of the temperature-compensated attenuator, the output end of the third amplifier is connected to the input end of the fourth filter, and the output end of the fourth filter is connected to the input end of the third down-conversion module.

[0013] Optionally, the third down-conversion module includes: a third mixer, a first input end of the third mixer is connected to the output end of the fourth filter, a second input end of the third mixer inputs a second local oscillation signal, and an output end of the third mixer outputs a down-conversion signal.

[0014] Optionally, the down converter also includes a down-conversion post-processing module, which includes: a fifth filter, a fourth amplifier and a sixth filter, wherein the input end of the fifth filter is connected to the output end of the third mixer, the output end of the fifth filter is connected to the input end of the fourth amplifier, and the output end of the fourth filter is connected to the output end of the sixth filter.

[0015] Compared with the prior art, the present invention can bring the following beneficial effects:

[0016] By introducing a preprocessing module, this application can effectively suppress excessive input signal strength and avoid overloading of subsequent circuits; by adopting a multi-level down-conversion strategy, it can not only improve the flexibility and accuracy of frequency conversion, but also reduce the difficulty of frequency conversion at each step through step-by-step processing; by setting corresponding processing modules before and after each level of down-conversion, the signal can be optimized before down-conversion. This application can effectively solve the signal distortion, dynamic range limitation and stability problems existing in traditional down-converters, thereby improving the signal-to-noise ratio and purity of the down-converted signal. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 is a circuit block diagram of a down converter provided by an embodiment of the present application;

[0018] Figure 2 is a circuit structure diagram of a down converter provided by another embodiment of the present application;

[0019] Figure 3 is a circuit block diagram of a down converter provided by another embodiment of the present application;

[0020] Figure 4 This is a schematic diagram of the circuit structure of a down converter provided in another embodiment of the present application.

[0021] The following are the descriptions of the reference numerals:

[0022] 10. Pre-processing module; 11. Limiter; 20. First down-conversion pre-processing module; 21. First amplifier; 22. First filter; 30. First down-conversion module; 31. First mixer; 40. Second down-conversion pre-processing module; 41. Equalizer; 42. Second filter; 43. Second amplifier; 50. Second down-conversion module; 51. Second mixer; 60. Frequency selection and temperature compensation module; 61. Third filter; 62. Temperature compensated attenuator; 70. Third down-conversion pre-processing module; 71. Third amplifier; 72. Fourth filter; 80. Third down-conversion module; 81. Third mixer; 90. Third down-conversion post-processing module; 91. Fifth filter; 92. Fourth amplifier; 93. Sixth filter. DETAILED DESCRIPTION

[0023] The following will be combined with the embodiments of this application Figures 1 to 4 , the technical solutions in the embodiments of the present application are clearly and completely described. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present application.

[0024] It should be noted that all directional indications in the embodiments of the present application (such as up, down, left, right, front, back, etc.) are only used to explain the relative position relationship, movement status, etc. between the components under a certain specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication will also change accordingly.

[0025] In this application, unless otherwise clearly specified and limited, the terms "connection", "fixation", etc. should be understood in a broad sense. For example, "fixation" can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements, unless otherwise clearly defined. For ordinary technicians in this field, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0026] In addition, if there are descriptions involving "first", "second", etc. in the embodiments of the present application, the descriptions of "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying their relative importance or implicitly indicating the number of technical features indicated. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In addition, the meaning of "and / or" appearing in the full text includes three parallel schemes. Taking "A and / or B" as an example, it includes scheme A, or scheme B, or a scheme that satisfies both A and B. In addition, the technical solutions between the various embodiments can be combined with each other, but it must be based on the ability of ordinary technicians in the field to implement. When the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection required by this application.

[0027] Figure 1 is a circuit block diagram of a down converter according to an implementation mode of the present application, such as Figure 1 As shown, the down converter comprises:

[0028] Preprocessing module 10, first down-conversion pre-processing module 20, first down-conversion module 30, second down-conversion pre-processing module 40, second down-conversion module 50, frequency selection and temperature compensation module 60, third down-conversion pre-processing module 70 and third down-conversion module 80;

[0029] in,

[0030] The input end of the preprocessing module 10 inputs the signal to be down-converted.

[0031] The output end of the pre-processing module 10 is connected to the input end of the first down-conversion pre-processing module 20.

[0032] The output end of the first down-conversion pre-processing module 20 is connected to the input end of the first down-conversion module 30.

[0033] The output end of the first down-conversion module 30 is connected to the input end of the second down-conversion pre-processing module 40.

[0034] The output end of the second down-conversion pre-processing module 40 is connected to the input end of the second down-conversion module 50.

[0035] The output end of the second down-conversion module 50 is connected to the input end of the frequency selection and temperature compensation module 60.

[0036] The output end of the frequency selection and temperature compensation module 60 is connected to the input end of the third down-conversion pre-processing module 70.

[0037] The output end of the third down-conversion pre-processing module 70 is connected to the input end of the third down-conversion module 80.

[0038] The output end of the third down-conversion module 80 outputs a down-conversion signal.

[0039] By introducing a preprocessing module, the present application can effectively prevent high-intensity pulses or noise in the input signal from causing damage to subsequent circuits, and also help maintain the linear characteristics of the entire down-conversion process and avoid nonlinear distortion; by adopting a multi-frequency conversion module design, the flexibility and accuracy of signal frequency conversion can be improved; by optimizing the signal before each level of down-conversion, the purity and signal-to-noise ratio of the final output signal can be ensured.

[0040] In summary, the present application can effectively solve the problems of signal distortion and signal-to-noise ratio existing in the traditional down-converter, thereby improving the signal-to-noise ratio and purity of the signal in the down-conversion process of the down-converter.

[0041] In another exemplary embodiment, Figure 2 As shown, the preprocessing module 10 includes:

[0042] Limiter 11,

[0043] The signal to be down-converted is input to an input end of the limiter 11 , and an output end of the limiter 11 is connected to an input end of the first down-conversion pre-processing module 20 .

[0044] In this embodiment, it is assumed that the signal to be down-converted is a radio frequency signal of 7140MHz to 7235MHz. Since the radio frequency signal may contain instantaneous high-intensity pulses or noise, it is necessary to limit the radio frequency signal by a limiter 11 before down-converting the radio frequency signal. Through the limiting process, high-intensity pulses or noise can be prevented from entering the subsequent circuit, thereby protecting subsequent amplifiers, mixers and other sensitive components from the impact of excessive pulse signals. In addition, the limiter 11 helps to maintain the linear working range of the down-conversion circuit, and can avoid intermodulation distortion and other nonlinear distortions caused by excessive amplitude of the radio frequency signal during the mixing process.

[0045] In another exemplary embodiment, Figure 2 As shown, the first down-conversion pre-processing module 20 includes:

[0046] The first amplifier 21 and the first filter 22,

[0047] in,

[0048] The input end of the first amplifier 21 is connected to the output end of the limiter 11.

[0049] The output end of the first amplifier 21 is connected to the input end of the first filter 22.

[0050] The output end of the first filter 22 is connected to the input end of the first down-conversion module 30 .

[0051] In this embodiment, as mentioned above, the RF signal often has energy loss after being limited by the limiter 11, so an amplifier needs to be set to amplify it. By setting the first amplifier 21 after the limiter 11, the RF signal amplitude can be increased to ensure that the RF signal still maintains sufficient signal strength after the limiting process, thereby meeting the input requirements of the subsequent processing module.

[0052] In addition, although the limiter 11 can effectively limit the amplitude of the RF signal, it may introduce some distortion or spurious signals. The first filter 22 is immediately after the first amplifier 21 and can effectively filter out the noise and unnecessary frequency components introduced by the limiting or amplification process to ensure the purity of the RF signal.

[0053] In another exemplary embodiment, Figure 2 As shown, the first down-conversion module 30 includes:

[0054] The first mixer 31,

[0055] The first input end of the first mixer 31 is connected to the output end of the first filter 22.

[0056] The first local oscillation signal is input into the second input terminal of the first mixer 31.

[0057] The output end of the first mixer 31 is connected to the input end of the second down-conversion pre-processing module 40 .

[0058] In this embodiment, the first input end of the first mixer 31 is connected to the first local oscillator, and the first local oscillator oscillates to generate a first local oscillation signal. Exemplarily, the frequency of the first local oscillation signal is 5115 MHz, and after mixing with the 7140 MHz to 7235 MHz RF signal by the first mixer 31, a first mixed signal with a frequency of 2025 MHz to 2120 MHz can be generated.

[0059] In another exemplary embodiment, Figure 2 As shown, the second down-conversion pre-processing module 40 includes:

[0060] an equalizer 41, a second filter 42 and a second amplifier 43,

[0061] in,

[0062] The input end of the equalizer 41 is connected to the output end of the first mixer 31.

[0063] The output end of the equalizer 41 is connected to the input end of the second filter 42.

[0064] The output end of the second filter 42 is connected to the input end of the second amplifier 43.

[0065] An output end of the second amplifier 43 is connected to an input end of the second down-conversion module 50 .

[0066] In this embodiment, the first mixing signal may generate distortion during the transmission process, especially phase distortion and amplitude distortion. By setting an equalizer 41 after the first down-conversion module 30, the generated distortion can be corrected so that the first mixing signal achieves a flat frequency response within the passband, thereby enhancing the overall quality of the first mixing signal.

[0067] Furthermore, by setting the second filter 42, it is possible to filter out unnecessary frequency components in the first mixing signal, especially the image frequency and spurious signals near the intermediate frequency signal. By selectively passing the required intermediate frequency signal, noise and interference are reduced, the first mixing signal can be further purified and the signal-to-noise ratio can be improved.

[0068] Furthermore, after being equalized and filtered, the amplitude of the first mixing signal will be attenuated, so a second amplifier 43 needs to be provided to restore the power level of the first mixing signal to ensure that the first mixing signal still has sufficient strength in subsequent processing.

[0069] In another exemplary embodiment, Figure 2 As shown, the second down-conversion module 50 includes:

[0070] The second mixer 51,

[0071] The first input terminal of the second mixer 51 is connected to the output terminal of the second amplifier 43.

[0072] The second input terminal of the second mixer 51 inputs a frequency synthesis signal.

[0073] The output end of the second mixer 51 is connected to the input end of the frequency selection and temperature compensation module 60 .

[0074] In this embodiment, the second input end of the second mixer 51 is connected to the frequency synthesizer for receiving a frequency synthesis signal generated by the frequency synthesizer (exemplarily, the frequency of the frequency synthesis signal is 1675MHz to 1770MHz). After the frequency synthesis signal is mixed with the first mixing signal by the second mixer, a second mixing signal with a frequency of 350MHz can be generated.

[0075] In another exemplary embodiment, Figure 2 As shown, the frequency selection and temperature compensation module 60 includes:

[0076] The third filter 61 and the temperature compensation attenuator 62,

[0077] in,

[0078] The input end of the third filter 61 is connected to the output end of the second mixer 51.

[0079] The output end of the third filter 61 is connected to the input end of the temperature compensation attenuator 62.

[0080] The output end of the temperature-compensated attenuator 62 is connected to the output end of the third down-conversion pre-processing module 70 .

[0081] In this embodiment, since the second mixing signal may contain spurious and image frequency components in addition to the target intermediate frequency signal, it is necessary to set a third filter 61 after the second mixer 51 to selectively pass the target intermediate frequency signal therein while suppressing the spurious and image frequency components, so as to improve the purity and signal-to-noise ratio of the second mixing signal.

[0082] Furthermore, although the third filter 61 can filter and optimize the second mixing signal, it cannot accurately control the level of the second mixing signal to the ideal value required by the subsequent circuit. If the filtered second mixing signal is directly connected to the subsequent circuit with strict requirements on the input level, the signal strength may be insufficient. Therefore, by setting the temperature compensation attenuator 62, based on its temperature compensation characteristics, even if there is an ambient temperature fluctuation, the second mixing signal level can be accurately transmitted, thereby ensuring that the entire down converter has a good dynamic range and linearity.

[0083] In addition, even after filtering, there may still be strong transient or spike signals in the second mixing signal, which may threaten the subsequent circuit. The temperature compensated attenuator 62 can be used as a protection link to buffer these transient signals through a preset attenuation amount, thereby protecting the subsequent circuit from damage.

[0084] In another exemplary embodiment, Figure 2 As shown, the third down-conversion pre-processing module 70 includes:

[0085] The third amplifier 71 and the fourth filter 72,

[0086] in,

[0087] The input end of the third amplifier 71 is connected to the output end of the temperature compensation attenuator 62.

[0088] The output end of the third amplifier 71 is connected to the input end of the fourth filter 72.

[0089] The output end of the fourth filter 72 is connected to the input end of the third down-conversion module 80 .

[0090] In this embodiment, the temperature compensating attenuator 62 may cause the power of the second mixing signal to decrease while adjusting the attenuation of the second mixing signal according to the temperature change. By arranging the third amplifier 71 after the temperature compensating attenuator 62, this part of the power loss can be compensated, thereby ensuring that the signal strength of the second mixing signal can meet the requirements of subsequent processing and transmission. In addition, by arranging the third amplifier 71, it can also play the role of impedance matching to ensure the smooth transmission of the second mixing signal from the temperature compensating attenuator 62 to the subsequent filter and the third mixer 81, avoiding signal reflection and power loss.

[0091] Furthermore, the fourth filter 72 is provided before the third mixer 81, which can be regarded as a pre-selection filter, allowing only signals within the target frequency range to pass, thereby reducing unnecessary mixing products, thereby improving the mixing efficiency and purity of the output signal of the third mixer 81. In addition, the fourth filter 72 is provided after the third amplifier 71, which can also prevent unexpected high-frequency signals or interference, thereby protecting the third mixer 81 and subsequent sensitive circuits from damage.

[0092] In another exemplary embodiment, Figure 2 As shown, the third down-conversion module 80 includes:

[0093] The third mixer 81,

[0094] The first input end of the third mixer 81 is connected to the output end of the fourth filter 72.

[0095] The second input terminal of the third mixer 81 inputs the second local oscillation signal.

[0096] The output terminal of the third mixer 81 outputs a down-converted signal.

[0097] In this embodiment, the second input end of the third mixer 81 is connected to the second local oscillator for receiving a second local oscillation signal generated by the second local oscillator (exemplarily, the frequency of the second local oscillation signal is 280 MHz). After the second local oscillation signal and the second mixing signal are mixed by the third mixer 81, a third mixing signal with a frequency of 70 MHz can be generated.

[0098] Figure 3 is a circuit block diagram of a down converter provided by another exemplary embodiment of the present application, such as Figure 3 As shown, the down converter also includes a down conversion post-processing module 90, specifically, as Figure 4 As shown, the down-conversion post-processing module 90 includes:

[0099] The fifth filter 91, the fourth amplifier 92 and the sixth filter 93,

[0100] in,

[0101] The input end of the fifth filter 91 is connected to the output end of the third mixer 81.

[0102] The output end of the fifth filter 91 is connected to the input end of the fourth amplifier 92.

[0103] The output end of the fourth filter 92 is connected to the output end of the sixth filter 93 .

[0104] In this embodiment, after the third mixing, the third mixed signal output by the third mixer 81 may contain new frequency components, so it is necessary to set a fifth filter 91 to filter the third mixed signal to filter out all irrelevant frequency components including spurious responses, so that the third mixed signal is further purified. In addition, the third mixer 81 may generate intermodulation distortion during the mixing process, and by setting the fifth filter 91, the distortion of the third mixed signal can be reduced.

[0105] It should be noted that after being filtered by the fifth filter 91 , the signal strength of the third mixed signal may decrease, so the fourth amplifier 92 needs to be provided to increase the signal power of the third mixed signal.

[0106] In addition, the fourth amplifier 92 will also amplify the noise in the process of amplifying the third mixed signal. By setting the sixth filter 93 before the final output of the down-converted signal, these noises can be effectively suppressed, especially the broadband noise introduced by the internal or external environment of the fourth amplifier 92, so as to ensure the purity and signal-to-noise ratio of the final output down-converted signal. In addition, the fourth amplifier 92 may introduce nonlinear distortion in the amplification process, and the sixth filter 93 can correct these distortions to a certain extent.

[0107] In summary, as the last link of the down-converter signal chain, by providing the down-conversion post-processing module 90, it is possible to prevent external interference or internal stray signals from affecting subsequent circuits, thereby ensuring that the final output signal has a purity that meets predetermined specifications.

[0108] The above are only preferred embodiments of the present application, and are not intended to limit the patent scope of the present application. Any equivalent structure or equivalent process transformation made using the contents of the present application specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present application.

Claims

1. A down converter, characterized in that: The down converter comprises: A preprocessing module, a first down-conversion pre-processing module, a first down-conversion module, a second down-conversion pre-processing module, a second down-conversion module, a frequency selection and temperature compensation module, a third down-conversion pre-processing module and a third down-conversion module; in, The input end of the preprocessing module inputs the signal to be down-converted, The output end of the pre-processing module is connected to the input end of the first down-conversion pre-processing module, The output end of the first down-conversion pre-processing module is connected to the input end of the first down-conversion module. The output end of the first down-conversion module is connected to the input end of the second down-conversion pre-processing module. The output end of the second down-conversion pre-processing module is connected to the input end of the second down-conversion module. The output end of the second down-conversion module is connected to the input end of the frequency selection and temperature compensation module. The output end of the frequency selection and temperature compensation module is connected to the input end of the third down-conversion pre-processing module. The output end of the third down-conversion pre-processing module is connected to the input end of the third down-conversion module. The output end of the third down-conversion module outputs a down-conversion signal.

2. The down converter according to claim 1, characterized in that: The preprocessing module comprises: Limiter, The input end of the limiter inputs the signal to be down-converted, and the output end of the limiter is connected to the input end of the first down-conversion pre-processing module.

3. The down converter according to claim 2, characterized in that: The first down-conversion pre-processing module comprises: a first amplifier and a first filter, in, The input end of the first amplifier is connected to the output end of the limiter, The output end of the first amplifier is connected to the input end of the first filter, The output end of the first filter is connected to the input end of the first down-conversion module.

4. The down converter according to claim 3, characterized in that: The first down-conversion module comprises: The first mixer, The first input terminal of the first mixer is connected to the output terminal of the first filter, The first local oscillation signal is input into the second input terminal of the first mixer. The output end of the first mixer is connected to the input end of the second down-conversion pre-processing module.

5. The down converter according to claim 4, characterized in that: The second down-conversion pre-processing module comprises: an equalizer, a second filter and a second amplifier, in, The input end of the equalizer is connected to the output end of the first mixer, The output end of the equalizer is connected to the input end of the second filter, The output end of the second filter is connected to the input end of the second amplifier, The output end of the second amplifier is connected to the input end of the second down-conversion module.

6. The down converter according to claim 5, characterized in that: The second down-conversion module comprises: The second mixer, The first input terminal of the second mixer is connected to the output terminal of the second amplifier, The second input terminal of the second mixer inputs a frequency synthesis signal, The output end of the second mixer is connected to the input end of the frequency selection and temperature compensation module.

7. The down converter according to claim 6, characterized in that: The frequency selection and temperature compensation module includes: The third filter and temperature compensated attenuator, in, The input end of the third filter is connected to the output end of the second mixer, The output end of the third filter is connected to the input end of the temperature compensated attenuator, The output end of the temperature compensated attenuator is connected to the output end of the third down-conversion pre-processing module.

8. The down converter according to claim 7, characterized in that: The third down-conversion pre-processing module comprises: The third amplifier and the fourth filter, in, The input end of the third amplifier is connected to the output end of the temperature compensated attenuator, The output end of the third amplifier is connected to the input end of the fourth filter, The output end of the fourth filter is connected to the input end of the third down-conversion module.

9. The down converter according to claim 8, characterized in that: The third down-conversion module comprises: The third mixer, The first input end of the third mixer is connected to the output end of the fourth filter, The second input terminal of the third mixer inputs a second local oscillation signal, The output terminal of the third mixer outputs a down-converted signal.

10. The down converter according to claim 9, characterized in that: The down converter further includes a down conversion post-processing module, and the down conversion post-processing module includes: a fifth filter, a fourth amplifier and a sixth filter, in, The input end of the fifth filter is connected to the output end of the third mixer, The output end of the fifth filter is connected to the input end of the fourth amplifier, The output end of the fourth filter is connected to the output end of the sixth filter.

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