CN103338048B - Reception method and unit of radio frequency front end of cognitive spectrum sensing receiver - Google Patents

Abstract

The invention discloses a radio frequency front-end receiving method and unit of a cognitive spectrum sensing receiver, which sets the entire frequency band of DVB-T as a sub-frequency band unit with every xMHz. Through secondary frequency mixing, the received signal is firstly up-converted to the first IF, and then down-converted to the second IF, thereby moving the entire sub-band spectrum to the vicinity of the IF with a bandwidth of xMHz. Then pass through a yMHz bandwidth, and the center frequency slides within a certain range with the change of the voltage. The electronically tuned filter, and sweeps this frequency band. In this way, it is possible to receive signals with a bandwidth of yMHz covered by the entire frequency band of DVB-T. The invention has the characteristics of DVB-T full frequency band coverage, less image interference influence, simple circuit design, simple intelligent operation and easy realization.

Description

Radio frequency front-end receiving method and unit of cognitive spectrum sensing receiver

technical field

The invention belongs to the technical field of communication, and in particular relates to a radio frequency front-end receiving method and unit of a cognitive spectrum sensing receiver based on terrestrial digital television broadcasting (DVB-T) frequency band.

Background technique

Wireless spectrum resource is an extremely limited resource, and it is necessary for the relevant state departments to designate relevant regulations for unified management and authorize spectrum resources within a certain frequency band for specific purposes. With the development of wireless communication technology, people's demands for wireless services are increasingly diversified, and spectrum resources will become increasingly scarce. In order to alleviate the problem of the shortage of spectrum resources, the current management of spectrum resources can be realized by improving the spectrum utilization rate. The common practice adopted internationally is to implement authorized and unlicensed frequency management mechanisms. For authorized frequency bands, non-authorized persons are not allowed to use them at will. In these allocated licensed frequency bands and unlicensed frequency bands, there is an imbalance in utilization of spectrum resources. That is, the utilization rate of spectrum in some licensed frequency bands is very low, while in some unlicensed frequency bands there are many users, the traffic is crowded, and the radio frequency band has basically tended to saturation.

In order to solve this problem, people began to consider if the temporarily vacant spectrum resources can be used, that is, to allow users without licenses to use the allocated licensed frequency bands without any interference to licensed users. The situation will be greatly improved. The application of cognitive radio provides a good solution to improve spectrum utilization. The core idea is that cognitive users can use the frequency band for communication when the licensed user is not using the licensed frequency band without causing interference to the licensed user.

Terrestrial digital television broadcasting (DVB-T) is an important part of people's leisure and entertainment. Often people only occupy part of the frequency bands when watching digital broadcasting TV, while other frequency bands are in an idle state. If this part of the unallocated frequency band can be fully utilized, the spectrum utilization rate will be greatly improved.

In October 2004, IEEE officially established the IEEE802.22 working group, which is the first worldwide air interface standardization organization based on cognitive radio technology. 910MHz) frequency band TV channel, it can automatically detect idle frequency band resources and make use of them.

A key technology of cognitive radio is spectrum sensing technology. How to quickly and accurately detect whether the primary user signal exists on this spectrum is a key issue in spectrum sensing. The core idea of the cognitive radio spectrum sensing receiver is to detect different frequency bands to determine whether the frequency band is free, which requires a multi-band, reconfigurable cognitive radio universal RF front-end hardware platform.

Currently, there are many structural solutions for multi-band radio frequency receivers. For example, the Chinese invention patent whose announcement number is CN101867381 "Using Narrowband Local Oscillator and Switching Filter to Realize Down-Conversion Method of RF Spectrum Analyzer" and the announcement number CN101938613 "Tracking Filter for TV Tuner", they all use the radio frequency The input signal is divided into several bands for down-conversion, and the frequency selection of signals in different bands is realized by using switch filtering. This structure using multiple bandpass filters to achieve frequency selection and frequency tuning is not suitable as the RF front-end circuit of the DVB-T cognitive spectrum detection receiver, because the frequency bandwidth of DVB-T is relatively wide, if the selected sub Fewer frequency bands will affect the probability of free spectrum that can be detected. If more sub-frequency bands are selected, several sub-bands need several filters, which makes the circuit very complicated. In addition, the Chinese invention patent "a multi-band radio frequency receiver" with the announcement number CN101908896 uses radio frequency signals to be mixed twice and down-converted to the required intermediate frequency output. In addition to providing local oscillator signals to the first mixer, the frequency synthesizer , also through a frequency divider to provide the local oscillator signal for the second mixer. However, because it is controlled by a frequency divider, and the frequency divider cannot be very accurate, it is difficult to achieve coverage of the entire DVB-T frequency band. Secondly, if there are many sub-frequency bands, the control of the frequency divider will be very complicated. The Chinese invention patent "Shortwave Spectrum Sensing Method and System" with application number 201210047420.6 uses a tuned filter to refine the shortwave frequency band. And the step-by-step scanning in the refined short-wave frequency band realizes the perception of the received signal, and the short-wave frequency band covers comprehensively. However, due to the limited frequency adjustable range and bandwidth of the tuning filter, it is difficult to realize full-band frequency sweep in the DVB-T frequency band.

Contents of the invention

The technical problem to be solved by the present invention is to provide a radio frequency front-end receiving method and unit of a cognitive spectrum sensing receiver, which has DVB-T full frequency band coverage, less image interference, simple circuit design, simple intelligent operation and easy implementation specialty.

The basic idea of the radio frequency front end of the DVB-T cognitive radio spectrum detection receiver provided by the present invention is to set the entire frequency band of DVB-T as a sub-band unit per xMHz, and set it as CH1, CH2, CH3... CHn. Each sub-frequency band corresponds to a primary local oscillator frequency. Through secondary frequency mixing, the received signal is firstly up-converted to the first IF, and then down-converted to the second IF, so that the entire sub-band spectrum is moved to the vicinity of the IF with a bandwidth of xMHz. Then pass through a yMHz bandwidth, and the center frequency slides within a certain range with the change of the voltage. The electronically tuned filter, and sweeps this frequency band. In this way, it is possible to receive signals with a bandwidth of yMHz covered by the entire frequency band of DVB-T. In the whole system, the host computer controls the first local oscillator frequency and the center frequency of the electric tuning filter, and the second local oscillator frequency is fixed. It is easy to get the signal range currently received by the receiver's RF front end.

In order to solve the above problems, the present invention is achieved through the following schemes:

A radio frequency front-end receiving method of a cognitive spectrum sensing receiver comprises the following steps:

(1) The low-pass filter receives signals in the DVB-T frequency band and filters out signals outside the frequency band;

(2) The low noise amplifier amplifies the received signal and suppresses the noise power;

(3) Divide the DVB-T frequency range into n sub-frequency bands, namely CH1, CH2, CH3...CHn, where each sub-frequency band is xMHz;

(4) Under the control of the host computer, the first local oscillator generator generates a first-level local oscillator frequency for each sub-frequency band;

(5) The first mixer up-converts the sub-band signal to be received at the primary local oscillator frequency, and moves the sub-band spectrum to a fixed xMHz bandwidth IF frequency band;

(6) Pass the up-converted signal through an IF filter with a bandwidth of xMHz to suppress image interference, and amplify its power through an IF amplifier;

(7) The second local oscillator generator generates a fixed secondary local oscillator frequency;

(8) The second mixer mixes the signal amplified by the IF amplifier at the secondary local oscillator frequency, and moves the entire sub-band spectrum to the IF frequency band with a bandwidth of xMHz through the second frequency mixing ;

(9) Send the mixed signal to the automatic gain controller for automatic gain control;

(10) Send the signal after the automatic gain control to the tuned filter network; the tuned filter network includes a transformer group for boosting impedance matching transmission line, an electrical tuning filter, a tuning filter amplifier, and a transformer group for dropping impedance matching transmission line;

(10.1) The boost impedance matching transmission line transformer group sends the signal after automatic gain control to boost impedance matching and sends it to the electric tuning filter;

(10.2) Under the control of the host computer, the numerical control DC stabilized power supply realizes the tuning and filtering function by changing the voltage of the electric tuning filter;

(10.3) The electronically tuned filter performs frequency sweeping with a center frequency of yMHz bandwidth within a certain range, and the electrically tuned filter filters the signal after the raised impedance is matched;

(10.4) The tuned filter amplifier amplifies the filtered signal;

(10.5) Reduced impedance matching The transmission line transformer group performs reduced impedance matching on the amplified signal;

(11) The upper computer performs data processing on the primary local oscillator frequency data generated by the first local oscillator generator and the center frequency data of the electric tuning filter, and obtains the frequency band received by the RF front end with a bandwidth of yMHz, and performs data processing on the upper computer displayed on the machine, where

The frequency received by the RF front-end = the primary local oscillator frequency - the secondary local oscillator frequency + the center frequency of the electric tuning filter;

(12) Send the frequency band signal received by the radio frequency front end to the baseband processing unit of the receiver to detect whether the frequency spectrum is free.

In the above method, the electric tuning filter adopts a parallel resonant circuit of a wire-wound inductor and a varactor diode, and the digitally controlled DC stabilized voltage power supply realizes tuning by changing the voltage of the varactor diode under the control of the host computer filter function.

In the above method, the range of the DVB-T frequency band is between 50-878 MHz.

In the above method, the bandwidth of each sub-frequency band, namely x, ranges from 8 MHz to 16 MHz.

In the above method, the center frequency of the electric tuning filter is between 10 MHz and 30 MHz.

In the above method, the bandwidth of the electronically tuned filter, that is, the value range of y is between 1 MHz and 4 MHz.

A radio frequency front-end unit of a cognitive spectrum sensing receiver, mainly composed of a low-pass filter, a low-noise amplifier, a first local oscillator generator, a first mixer, an IF filter, an IF amplifier, and a second local oscillator A generator, a second mixer, an automatic gain controller, a tuned filter network, a first single-chip microcomputer, a second single-chip microcomputer and a host computer. The tuned filter network includes a boost impedance matching transmission line transformer group, an electrical tuning filter, a tuned filter amplifier, a falling impedance matching transmission line transformer group and a digitally controlled DC stabilized voltage power supply. The input end of the low-pass filter is connected with the receiving antenna. The output end of the low-pass filter is sent to one of the input ends of the first mixer after passing through the low-noise amplifier, and the upper computer is connected to the data input end of the first local oscillator generator through the first single-chip microcomputer, and the first local oscillator generator The output terminal is connected to another input terminal of the first mixer. The output terminal of the first mixer is connected to one of the input terminals of the second mixer after passing through the IF filter and the IF amplifier in turn, and the output terminal of the second local oscillator generator is connected to the other of the second mixer input. The output terminal of the second mixer is sent to the input terminal of the boost impedance matching transmission line transformer group of the tuned filter network through the automatic gain controller. The output end of the rising impedance matching transmission line transformer group is connected to the input end of the electric tuning filter, the output end of the electric tuning filter is connected to the input end of the falling impedance matching transmission line transformer group through the tuned filter amplifier, and the output end of the falling impedance matching transmission line transformer group is connected to Baseband processing unit. The host computer is connected to the data input end of the digitally controlled DC stabilized voltage power supply via the second single chip computer, and the voltage output of the numerically controlled DC stabilized voltage power supply is connected to the voltage of the electric tuned filter.

In the above unit, the electric tuning filter adopts a parallel resonant circuit of a wire-wound inductance and a varactor diode.

Compared with prior art, the present invention has following advantage:

1. The RF front-end circuit is specially suitable for the structure of DVB-T spectrum sensing receiver, which refines the DVB-T frequency band, and performs the step-by-step frequency sweep function of yMHZ bandwidth in the refined frequency band, so as to realize the detection of the received signal Perception, and can achieve full frequency band coverage of DVB-T.

2. The second frequency conversion scheme of the first up-conversion is adopted, and a fixed frequency filter and a tuned filter are used, and the frequency bands of the two filters are custom-designed, which can make the whole system have a better choice and passband characteristics, to achieve better image suppression effect and less distortion effect.

3. The local oscillator signal that provides the first upper side frequency is controlled by the host computer, which can output the desired local oscillator frequency conveniently and accurately.

4. The numerically controlled DC voltage stabilizer for controlling the electric tuning filter uses a single-chip microcomputer as the core, and it uses the upper computer control to replace the traditional digital potentiometer control, so as to control the filtering frequency band of the tuning filter more simply, conveniently and accurately.

5. Although the RF front-end circuit also needs two frequency conversions, since the high and high frequency after the first mixing is fixed, the second mixing is also fixed, and there is no need to change the circuit; therefore, the entire RF front-end What the circuit system needs to control is to provide the first mixed frequency local oscillator signal and the electric tuning filter, both of which can be intuitively controlled in real time through the host computer and display the received frequency band, making the whole circuit very simple; through this Two-part control, you can intuitively know which part of the frequency band is used for spectrum detection, which is convenient for the subsequent work of cognitive radio equipment.

Description of drawings

Fig. 1 is a schematic diagram of the circuit system of the present invention.

Fig. 2 is a channel block diagram of the present invention.

Fig. 3 is a system flow chart of the present invention.

Fig. 4 is a block diagram of the electrical tuning filter of the present invention.

Detailed ways

A radio frequency front-end unit of a cognitive spectrum sensing receiver, as shown in Figure 1, mainly consists of a low-pass filter, a low-noise amplifier, a first local oscillator generator, a first mixer, an IF filter, an IF It consists of an amplifier, a second local oscillator generator, a second mixer, an automatic gain controller, a tuned filter network, a first single-chip microcomputer, a second single-chip microcomputer and a host computer. The tuned filter network includes a boost impedance matching transmission line transformer group, an electrical tuning filter, a tuned filter amplifier, a falling impedance matching transmission line transformer group and a digitally controlled DC stabilized voltage power supply. The input end of the low-pass filter is connected with the receiving antenna. The output end of the low-pass filter is sent to one of the input ends of the first mixer after passing through the low-noise amplifier. The data input terminal of the oscillator, the output terminal of the first local oscillator generator is connected with the other input terminal of the first mixer. The output terminal of the first mixer is connected to one of the input terminals of the second mixer after passing through the IF filter and the IF amplifier in turn, and the output terminal of the second local oscillator generator is connected to the other of the second mixer input. The output terminal of the second mixer is sent to the input terminal of the boost impedance matching transmission line transformer group of the tuned filter network through the automatic gain controller. The output end of the rising impedance matching transmission line transformer group is connected to the input end of the electric tuning filter, the output end of the electric tuning filter is connected to the input end of the falling impedance matching transmission line transformer group through the tuned filter amplifier, and the output end of the falling impedance matching transmission line transformer group is connected to Baseband processing unit. The upper computer is connected to the input end of the second single-chip microcomputer through the serial port, and the output end of the second single-chip microcomputer is connected to the data input end of the digitally controlled DC stabilized voltage power supply, and the voltage output of the numerically controlled DC stabilized voltage power supply is connected to the voltage of the electric tuned filter.

The upper computer is written in VB language, which contains three parts, which are the first-level local oscillator control part, the electric tuning filter control part, and the receiver receiving frequency band display part.

For the first local oscillator generator, the first phase-locked loop uses the ADF4350 chip, which is characterized by an internal integrated first voltage-controlled oscillator (VCO), which can output frequency ranges from 137.5MHZ to 4400MHZ. It is very suitable as the local oscillator part of the RF front end. It contains 8 registers inside, and the corresponding local oscillator frequency can be generated by setting the values of these 8 registers. What these 8 registers adopt is the serial data input, can carry on the control with the one-chip computer. The host computer can control the single-chip microcomputer to input data to the phase-locked loop. According to each sub-frequency band CHn, the host computer corresponds to a local oscillator frequency. When needed, just input the desired sub-band value and press the corresponding button to move the corresponding sub-band to the low frequency band.

For the tuned filter network, it mainly includes two transmission line transformer network matching, an electrical tuned filter, a tuned filter amplifier branch and a digitally controlled DC regulated power supply. The above-mentioned electric tuning filter uses a parallel resonant circuit of a wire-wound inductor and a varactor diode, as shown in FIG. 4 . The tuning filter network is the focus of the RF receiving front end, and its performance directly determines the detection probability and sensitivity of the spectrum sensing system. The Q value of the resonant circuit is very high due to the realization of the narrow-bandwidth electrical tuning filter directly realizing the yMHZ bandwidth at the low frequency with the bandwidth of xMHz. If the input and output impedance and the load are both 50 ohms, the attenuation of the filter will be very slow, the passband bandwidth will be very wide, and the Q value of the resonant circuit will not be high, thus failing to meet the requirements of narrow-band filtering. Therefore, the input and output impedance and load can be increased. Among them, the transmission line transformer group with rising impedance matching and the transmission line transformer group with falling impedance matching adopt the transmission line transformer chip TC4-1W and ADT16-1T of Mini-Circuits Company, and realize the impedance conversion from 50 ohm to 3.2K ohm through two-stage cascading So as to achieve the purpose of improving the quality factor Q value of the resonant circuit of the electric tuning filter. After passing through the impedance matching network, the electric tuning filter can realize yMHZ bandwidth sliding in the low frequency with a bandwidth of xMHz by controlling the voltage at both ends of the varactor diode. And through a numerically controlled DC stabilized voltage power supply, the control of the electric tuned filter can be realized. The numerically controlled DC regulated power supply also uses the host computer to control the single-chip microcomputer here, and the center frequency of the electric tuning filter can be controlled by controlling the output voltage of the numerically controlled DC regulated power supply.

The RF front-end receiving method of the cognitive spectrum sensing receiver realized by the above-mentioned RF front-end unit, as shown in Figure 2, is characterized in that it comprises the following steps:

(1) Receive the DVB-T frequency band signal through the low-pass filter of the RF front-end unit, and filter out the signals outside the frequency band.

(2) The low noise amplifier of the RF front-end unit amplifies the received signal and suppresses the noise power.

(3) Set the DVB-T frequency range of 50-878MHz as a sub-frequency band per x MHz, and divide it into n sub-frequency bands, namely CH1, CH2, CH3...CHn, each sub-frequency band corresponds to a local oscillator frequency, the first The vibration part is controlled by the host computer. Through the first mixer of the radio frequency front-end unit, the sub-band spectrum to be received is up-converted, and the sub-band spectrum is moved to a high-medium frequency band with a fixed xMHz bandwidth.

(4) Pass the up-converted signal through an IF filter with a bandwidth of xMHz to suppress image interference, and amplify its power through an IF amplifier. In this embodiment, the bandwidth of each sub-frequency band, namely x, ranges from 8 MHz to 16 MHz.

(5) The frequency generated by the second mixer of the RF front-end unit and the secondary local oscillator is then down-mixed. The frequency of the secondary local oscillator is fixed.

(6) Send the mixed signal to the automatic gain control module of the radio frequency front-end unit for automatic gain control.

(7) Send the signal after automatic gain control to the tuned filter network. The tuning filter network includes a boosting impedance matching transmission line transformer group for boosting impedance matching, an electrical tuning filter, a tuning filter amplifier, and a dropping impedance matching transmission line transformer group for dropping impedance matching. The bandwidth of the electronically tuned filter is yMHz, and it can change the voltage so as to realize the center frequency sliding between certain ranges. Control the electrical tuned filter by the numerically controlled DC stabilized power supply of the upper computer with the single-chip microcomputer as the core, so that the frequency sweep with the center frequency of yMHz bandwidth can be performed within the range of intermediate frequency with the bandwidth of xMHz. In this embodiment, the electric tuning filter uses a parallel resonant circuit of a wire-wound inductance and a varactor diode, and the digitally controlled DC stabilized voltage power supply is controlled by a host computer by changing the voltage of the varactor diode. Realize tuning filter function. The center frequency of the electrically tuned filter is between 10MHz and 30MHz. The bandwidth of the electronically tuned filter, that is, the value range of y is between 1 MHz and 4 MHz.

(8) The host computer data controlling the first local oscillator frequency and the host computer data controlling the center frequency of the electric tuning filter are transmitted to the PC for data processing. According to the formula: RF front-end receiving frequency = first local oscillator frequency - secondary local Vibration frequency + center frequency of electric tuning filter. The frequency band received by the RF front-end with a bandwidth of yMHz can be obtained.

(9) The host computer can control the first local oscillator frequency and the center frequency of the electric tuning filter accordingly, and display the corresponding received frequency band.

(10) Send the received signal to the baseband processing module to detect whether the frequency spectrum is free.

Through steps (7) (8), it can be obtained whether the spectrum in the frequency band received by the receiver is free. Then repeat the previous operation to scan the entire DVB-T frequency band.

As shown in Fig. 3, in the preferred embodiment of the present invention, we set DVB-T as a sub-band unit every 8Mhz. Taking the CH1 frequency band as an example for analysis, it is necessary to perform spectrum detection on the CH1 sub-frequency band (50-58MHz) to analyze which part of the spectrum is idle. The radio frequency signal first passes through a low-pass filter and a low-noise amplifier, and performs up-conversion for the first time with the first local oscillator signal generated by the phase-locked loop in the first mixer. Wherein, the local oscillator frequency corresponding to the sub-frequency band CH1 is 968 MHz. Here, you only need to input the desired sub-band value in the host computer, press the corresponding button, and the local oscillator will generate the local oscillator frequency corresponding to the sub-band. Through up-conversion, the entire CH1 frequency spectrum is moved to 910-918MHz. The frequency-converted signal passes through an intermediate frequency filter with a center frequency of 915MHz and a bandwidth of 8MHz to suppress image interference, and then passes through an intermediate frequency amplifier to amplify its power. Then, the second mixer of the RF front-end unit mixes with a 938MHz frequency generated by the secondary local oscillator, and down-converts the frequency to 20-28MHz. The signal after the secondary mixing is sent to the automatic gain control module of the radio frequency front-end unit for automatic gain control. Finally, through the tuned filter network, the electrical tuned filter module has a bandwidth of 1MHz, and the center frequency slides between 20 and 28MHz as the voltage changes. The center frequency can be controlled by pressing the corresponding button on the host computer. Here, first press the key corresponding to the center frequency at 21MHz, so that the index of the electronically tuned filter is 1MHz in bandwidth and 21Mhz in center frequency. Through formula calculation, we can know the frequency band received by the current RF front-end receiver: RF front-end receiving frequency = first local oscillator frequency - second-level local oscillator frequency (938) + center frequency of electric tuning filter, where the first local oscillator frequency is 968MHz, the center frequency of the electronically tuned filter is 21MHz; brought into the formula is: RF front-end receiving frequency = 968-938+21=51MHz; so the frequency received by the current RF front-end is a signal with a bandwidth of 1MHz and a center frequency of 51MHz . The upper computer will automatically run the formula and display the result on the upper computer. The received signal is sent to the baseband processing module of the subsequent stage to detect the idle spectrum. If it is in an idle state, this section of spectrum can be used. If it is in a non-idle state, then repeat the above steps, and press the button corresponding to 22MHz on the upper computer of the electrokinetic tuning filter module, and you can get the signal that the frequency accepted by the RF front-end is 1MHz in bandwidth and the center frequency is in the range of 52MHz. Then send it to the follow-up for baseband processing. In this way, spectrum idle detection can be performed on the CH1 sub-frequency band (50-58 MHz). Similarly, if you want to detect the spectrum idleness of other sub-bands, you only need to press the key corresponding to the sub-band on the first local oscillator host computer, and then repeat the previous operation.

Claims (8)

1. The radio frequency front-end receiving method of cognitive spectrum perception receiver, it is characterized in that comprising the following steps: (1) The low-pass filter receives signals in the DVB-T frequency band and filters out signals outside the frequency band; (2) The low noise amplifier amplifies the received signal and suppresses the noise power; (3) Divide the DVB-T frequency band range into n sub-frequency bands, namely CH1, CH2, CH3...CHn, wherein each sub-frequency band is xMHz; (4) Under the control of the host computer, the first local oscillator generator generates a first-level local oscillator frequency for each sub-frequency band; (5) The first mixer performs up-conversion of the sub-band signal to be received under the primary local oscillator frequency, and moves the sub-band spectrum to a fixed high-medium frequency band of xMHz bandwidth; (6) The signal after the up-conversion is passed through a bandwidth of xMHz IF filter to suppress the image interference, and it is power amplified through the IF amplifier; (7) The second local oscillator generator generates a fixed secondary local oscillator frequency; (8) The second mixer mixes the signal amplified by the IF amplifier at the secondary local oscillator frequency, and moves the entire sub-band spectrum to the IF frequency band with a bandwidth of xMHz through the second frequency mixing ; (9) Send the signal mixed by the second mixer to the automatic gain controller for automatic gain control; (10) Send the signal after the automatic gain control into the tuned filter network; the tuned filter network includes a rising impedance matching transmission line transformer group, an electric tuning filter, a tuning filter amplifier, and a falling impedance matching transmission line transformer group; wherein (10.1) The boost impedance matching transmission line transformer group sends the signal after automatic gain control to boost impedance matching and sends it to the electric tuning filter; (10.2) Under the control of the host computer, the numerical control DC stabilized power supply realizes the tuning and filtering function by changing the voltage of the electric tuning filter; (10.3) The electric tuning filter performs a frequency sweep with a center frequency of yMHz bandwidth within its tuning range, and the electric tuning filter filters the signal after the boost impedance matching; (10.4) The tuned filter amplifier amplifies the filtered signal; (10.5) Reduced impedance matching The transmission line transformer group performs reduced impedance matching on the amplified signal; (11) The host computer performs data processing on the first-order local oscillator frequency data of the frequency generated by the first local oscillator generator and the center frequency data of the electric tuning filter, and obtains the frequency band received by the radio frequency front end with a bandwidth of yMHz, and performs data processing on the host computer displayed on the machine, where The frequency received by the RF front end = the primary local oscillator frequency - the secondary local oscillator frequency + the center frequency of the electric tuning filter; (12) Send the frequency band signal received by the radio frequency front end to the baseband processing unit of the receiver to detect whether the frequency spectrum is free. 2. the radio frequency front-end receiving method of cognitive spectrum sensing receiver according to claim 1, is characterized in that: The electrical tuning filter uses a parallel resonant circuit of a wire-wound inductance and a varactor diode, and the digitally controlled DC stabilized voltage supply realizes the tuning and filtering function by changing the voltage of the varactor diode under the control of the host computer. 3. the radio frequency front-end receiving method of cognitive spectrum sensing receiver according to claim 1, is characterized in that: The range of the DVB-T frequency band is between 50-878 MHz. 4. the radio frequency front-end receiving method of cognitive spectrum sensing receiver according to claim 1, is characterized in that: The bandwidth of each sub-band, that is, the value range of x is between 8 MHz and 16 MHz. 5. the radio frequency front-end receiving method of cognitive spectrum sensing receiver according to claim 1, is characterized in that: The center frequency of the electric tuning filter is between 10MHz and 30MHz. 6. the radio frequency front-end receiving method of cognitive spectrum sensing receiver according to claim 1, is characterized in that: The bandwidth of the electronically tuned filter, that is, the value range of y is between 1 MHz and 4 MHz. 7. The RF front-end unit of the cognitive spectrum sensing receiver is characterized in that it is mainly composed of a low-pass filter, a low-noise amplifier, a first local oscillator generator, a first mixer, an IF filter, an IF amplifier, The second local oscillator generator, the second mixer, the automatic gain controller, the tuned filter network, the first single-chip microcomputer, the second single-chip microcomputer and the host computer; the tuned filter network includes the boost impedance matching transmission line transformer group, electric tuning filter, tuned filter amplifier, reduced impedance matching transmission line transformer group and digitally controlled DC stabilized power supply; the input end of the low-pass filter is connected to the receiving antenna; the output end of the low-pass filter is sent to the first mixing frequency after passing through the low-noise amplifier One of the input terminals of the device, the upper computer is connected to the data input terminal of the first local oscillator generator through the first single chip microcomputer, and the output terminal of the first local oscillator generator is connected to the other input terminal of the first mixer; the first mixer The output end of the frequency converter is connected to one of the input ends of the second mixer after passing through the IF filter and the IF amplifier in turn, and the output end of the second local oscillator generator is connected to the other input end of the second mixer; The output end of the second mixer is sent to the input end of the rising impedance matching transmission line transformer group of the tuning filter network through the automatic gain controller; The output end of the filter is connected to the input end of the reduced-impedance matching transmission line transformer group through the tuned filter amplifier, and the output end of the reduced-impedance matching transmission line transformer group is connected to the baseband processing unit; , and the voltage output of the digitally controlled DC stabilized power supply is connected to the voltage of the electric tuning filter. 8. The radio frequency front-end unit of cognitive spectrum sensing receiver according to claim 7, is characterized in that, The electric tuning filter adopts a parallel resonant circuit of a wire-wound inductance and a varactor diode.