Clock synchronization frequency modulation continuous terahertz frequency sweeping source nonlinear error elimination system

Abstract

The invention discloses a clock synchronization frequency modulation continuous terahertz frequency sweep source nonlinear error elimination system, which comprises two clock synchronization receiving and transmitting heterogenous frequency modulation continuous terahertz system error elimination architecture designs, and realizes reference signal generation by using double-source frequency mixing and single-source difference frequency respectively. The two architectures both select 10MHz as the frequency of a reference signal and an intermediate frequency signal, adopt the same 100MHz constant-temperature crystal oscillator as a reference clock of the frequency-modulated continuous wave frequency source, and provide local oscillators for the reference signal and the intermediate frequency signal; and the intermediate frequency signal and the reference signal are subjected to frequency mixing, so that the obtained difference frequency signal has the characteristic that a clock synchronization error, a modulation nonlinear error and the like are eliminated, and the difference frequency signal can be used for terahertz in-situ nondestructive testing and imaging after IQ demodulation. The invention provides an implementation method for a low-frequency aperiodic intermediate-frequency time domain signal required by a frequency spectrum type terahertz in-situ nondestructive testing and imaging technology.

Description

Technical Field

[0001] The present invention relates to the technical field of terahertz in-situ nondestructive testing and imaging, and in particular to a clock-synchronized frequency-modulated continuous terahertz sweeping source nonlinear error elimination system. Background Art

[0002] The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.

[0003] Terahertz waves have shown great potential in many fields due to their unique frequency characteristics. Among them, terahertz nondestructive testing technology, compared with traditional nondestructive testing technologies such as infrared radiation, X-rays, and ultrasound, has unique advantages such as low energy, high spatial resolution, and broadband spectral analysis capabilities. It can penetrate non-metallic and non-polar materials that conventional testing technologies cannot penetrate, thus providing a reliable technical method for non-metallic and non-polar material defect imaging, thickness measurement, etc. However, terahertz in-situ nondestructive testing and imaging technology faces many challenges.

[0004] A terahertz radar system generally includes an IQ modulation and demodulation module, a RF local oscillator module, and a power amplifier multiplier module. With the current development level of terahertz hardware, it is inevitable to introduce multiple frequency multipliers to achieve RF output in the terahertz band and a swept bandwidth with a certain bandwidth. In this case, due to the modulation nonlinearity problem brought by the system itself, the target energy will be spread to other frequency bands, so the modulation nonlinearity problem becomes an important factor restricting the system resolution. To overcome the above problems, early methods focused on optimizing individual components, and subsequently developed supplementary technologies based on software algorithms. However, such methods are highly dependent on massive training data, and the algorithms are complex and real-time performance is difficult to guarantee. Therefore, the development of a new systematic nonlinear error elimination architecture design is of great significance to promoting the development of terahertz radar systems and terahertz in-situ nondestructive testing imaging technology.

[0005] The present invention designs a system structure in which the two sources of transmission and reception are independent and heterogeneous. Under a larger sweep bandwidth and a shorter sweep period, the clock synchronization error caused by the two independent sources is one of the important factors affecting the system performance error. Therefore, a clock-synchronized frequency-modulated continuous terahertz sweep source nonlinear error elimination system is proposed. The intermediate frequency signal and the local oscillator reference signal are mixed to offset the phase difference introduced by the different phases of the two sources. The system nonlinear error elimination is achieved through the reference channel link architecture design, and the transmission, reception and signal processing of high-stability and high-reliability broadband terahertz frequency-modulated continuous waves are further realized for spectrum-type terahertz in-situ nondestructive testing and imaging technology. Summary of the invention

[0006] The purpose of the present invention is to provide a clock-synchronized frequency-modulated continuous terahertz frequency-sweeping source nonlinear error elimination system to solve the problems existing in the prior art.

[0007] The technical solution of the present invention is as follows:

[0008] A clock-synchronous frequency-modulated continuous terahertz frequency-sweeping source nonlinear error elimination system, comprising: a dual-channel broadband frequency-sweeping source module, a terahertz transmitting module and a terahertz receiving module;

[0009] The dual-channel broadband swept frequency source module generates two RF signal outputs with a frequency difference based on the same low phase noise constant temperature crystal oscillator; wherein the first RF signal is input to the terahertz transmitting module, and the second RF signal is input to the terahertz receiving module; and a reference signal is generated based on the two RF signals or one RF signal;

[0010] The terahertz transmitting module transmits a signal after frequency multiplication of the first radio frequency signal through a frequency multiplication link;

[0011] The terahertz receiving module multiplies the second RF signal through a frequency multiplication link as a local oscillator signal, mixes it with the received external signal carrying sample information, and then down-converts it to obtain an intermediate frequency signal;

[0012] The intermediate frequency signal is used as input, the reference signal is used as the local oscillator, and the difference frequency signal obtained by secondary mixing is converted into I / Q two-way digital signals and output to the terminal for signal processing.

[0013] Furthermore, the dual-channel broadband swept frequency source module comprises:

[0014] Frequency source generation module, which generates 0.7-1GHz frequency modulated continuous wave frequency source based on 100MHZ constant temperature crystal oscillator;

[0015] The power divider divides the 0.7-1GHz frequency modulated continuous wave frequency source into two paths. Both signals are multiplied by 16 and output to RF1OUT and RF2OUT. The difference frequency of the two paths is set to 4.583MHz. Among them, RF1OUT is input to the terahertz transmitting module; RF2OUT is input to the terahertz receiving module.

[0016] A reference channel unit generates a 10 MHz reference signal based on two RF signals or one RF signal.

[0017] Further, when a 10 MHz reference signal is generated based on RF1OUT and RF2OUT, the reference channel unit includes:

[0018] Coupler, RF1OUT and RF2OUT enter the reference channel unit through the coupler;

[0019] A mixer, wherein the coupled signal is mixed by the mixer and then outputs a 4.583MHz intermediate frequency signal;

[0020] 6-frequency multiplier, primary filter, 4-frequency multiplier and secondary filter, the 4.583MHz intermediate frequency signal is multiplied by 6 to obtain a 27.5MHz intermediate frequency signal, which is filtered and amplified and then multiplied by 4 to obtain a 110MHz intermediate frequency signal;

[0021] The internal 100MHz crystal oscillator, mixer and third-order filter, the 110MHz intermediate frequency signal and the internal 100MHz crystal oscillator signal are mixed twice to obtain a 10MHz reference signal, and the 10MHz reference signal is output after filtering and amplification.

[0022] Further, when a 10 MHz reference signal is generated based on RF1OUT, the reference channel unit includes:

[0023] The internal 100MHz crystal oscillator and the 100MHz crystal oscillator PLL are phase-locked to obtain a 10MHz signal. The 10MHz signal is output as a reference signal on the one hand, and mixed with RF1OUT on the other hand to introduce a 10MHz fixed difference frequency, which is then input into the terahertz transmitting module.

[0024] Furthermore, in the terahertz transmitting module, RF1OUT undergoes 24-fold frequency multiplication, filtering and amplification operations, and then transmits a 268-384 GHz terahertz signal through the antenna.

[0025] Furthermore, in the terahertz receiving module, RF2OUT is multiplied by 12, filtered and amplified, and then mixed with the received terahertz signal containing sample information through a second harmonic mixer to obtain a 110MHz intermediate frequency signal; the 110MHz intermediate frequency signal is output as a 10MHz intermediate frequency signal through a down-conversion channel unit.

[0026] Furthermore, the down-conversion channel unit comprises:

[0027] The internal 100MHz crystal oscillator, 110MHz intermediate frequency signal and 100MHz crystal oscillator signal are mixed to obtain a 10MHz intermediate frequency signal.

[0028] Furthermore, in the terahertz transmitting module, the signal mixed with RF1OUT undergoes 24-fold frequency multiplication, filtering and amplification operations, and then transmits a 268.240-384.240 GHz terahertz signal through the antenna.

[0029] Furthermore, in the terahertz receiving module, RF2OUT is mixed with the received terahertz signal containing sample information through a second harmonic mixer after 12-fold frequency multiplication, filtering and amplification to obtain a 240MHz intermediate frequency signal; the 240MHz intermediate frequency signal is output as a 10MHz intermediate frequency signal through the down-conversion channel unit.

[0030] Furthermore, the down-conversion channel unit comprises:

[0031] The internal 100MHz crystal oscillator uses a 100MHz crystal oscillator PLL to phase-lock a 250MHz signal. The 250MHz signal is used as the local oscillator to mix with the 240MHz intermediate frequency signal to obtain a 10MHz intermediate frequency signal.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] In actual imaging systems, nonlinear amplitude and phase modulation will inevitably be introduced, and the time-frequency diagram of the difference frequency signal obtained by mixing will no longer be a straight line, making it impossible to accurately obtain target information. Therefore, this system compensates for the nonlinear error caused by the system in the intermediate frequency signal through a secondary mixing reference signal, thereby meeting the requirements of spectral ellipsometric imaging signals. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a principle block diagram of a clock-synchronized frequency-modulated continuous terahertz sweep source nonlinear error elimination system;

[0035] Figure 2 This is the principle block diagram of the dual-source mixing reference frequency modulation continuous terahertz system architecture;

[0036] Figure 3 This is a principle block diagram of the single-source difference-frequency reference frequency-modulated continuous terahertz system architecture. DETAILED DESCRIPTION

[0037] It should be noted that relational terms such as "first" and "second" are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the sentence "comprise a ..." do not exclude the existence of other identical elements in the process, method, article or device including the elements.

[0038] The features and performance of the present invention are further described in detail below in conjunction with the embodiments.

[0039] Embodiment 1

[0040] This embodiment proposes a clock-synchronized frequency-modulated continuous terahertz sweeping source nonlinear error elimination system architecture design. Based on a clock-synchronized transceiver heterogeneous frequency-modulated continuous terahertz transceiver system with a center frequency of 330 GHz, two switchable system error elimination architectures are developed, which use dual-source mixing and single-source difference frequency to achieve reference signal generation, respectively. The clock-synchronized broadband sweeping source uses the same high-precision, low-phase noise constant-temperature crystal oscillator as the system clock source, which is divided into two paths and multiplied to the terahertz frequency and then transmitted through the transmitting antenna. After being received by the receiving antenna, it is mixed with the local oscillator reference signal to offset the phase difference introduced by the different phases of the dual sources and achieve system coherence.

[0041] This embodiment also proposes a non-coherent dual-source to achieve a coherent system structure. The clock synchronous transceiver heterogeneous frequency modulated continuous terahertz dual-arm transceiver system is connected to the transmitting source and the local oscillator signal source through a coupler. The RF reference signal is connected. The intermediate frequency signal needs to be mixed with the local oscillator reference signal to offset the phase difference introduced by the different phases of the dual sources, thereby achieving system coherence and solving the synchronization and phase consistency problems of different signal sources.

[0042] In this embodiment, please refer to Figure 1-3 , a clock-synchronized frequency-modulated continuous terahertz frequency-sweeping source nonlinear error elimination system, comprising: a dual-channel broadband frequency-sweeping source module, a terahertz transmitting module and a terahertz receiving module;

[0043] The dual-channel broadband swept frequency source module generates two RF signal outputs with a frequency difference based on the same low phase noise constant temperature crystal oscillator; wherein the first RF signal is input to the terahertz transmitting module, and the second RF signal is input to the terahertz receiving module; and a reference signal is generated based on the two RF signals or one RF signal; that is, this embodiment carries out two error elimination architecture designs for heterogeneous frequency modulation continuous terahertz systems for transmission and reception, and uses dual-source mixing and single-source difference frequency respectively to realize the generation of reference signals;

[0044] The terahertz transmitting module transmits a signal after frequency multiplication of the first radio frequency signal through a frequency multiplication link;

[0045] The terahertz receiving module multiplies the second RF signal through a frequency multiplication link as a local oscillator signal, mixes it with the received external signal carrying sample information, and then down-converts it to obtain an intermediate frequency signal;

[0046] The intermediate frequency signal is used as input, the reference signal is used as the local oscillator, and the difference frequency signal obtained by secondary mixing is converted into I / Q two-way digital signals and output to the terminal for signal processing;

[0047] It should be noted that the terahertz transmitting module and the terahertz receiving module are the terahertz imaging front ends and can be used for scanning imaging. The terahertz transmitting module adopts a system architecture of frequency doubling and amplification. The reference intermediate frequency is generated by a dual-channel sweeping frequency source. The terahertz signal of the terahertz transmitting module is reflected to the receiving antenna after passing through the sample under test. The terahertz receiving module generates a test intermediate frequency through mixing. The amplitude and phase information of the reference intermediate frequency and the test intermediate frequency can be extracted. The nonlinear error caused by the system in the intermediate frequency signal is further compensated by the secondary mixing reference signal, thereby meeting the requirements of spectral ellipsometric imaging signals.

[0048] In this embodiment, specifically, the dual-channel broadband swept frequency source module includes:

[0049] Frequency source generation module, which generates 0.7-1GHz frequency modulated continuous wave frequency source based on 100MHZ constant temperature crystal oscillator;

[0050] The power divider divides the 0.7-1GHz frequency modulated continuous wave frequency source into two paths, and both signals are output RF1OUT and RF2OUT after 16-fold frequency multiplication, and the difference frequency of the two paths is set to 4.583MHz; RF1OUT is input to the terahertz transmitting module; RF2OUT is input to the terahertz receiving module; that is, two independent 0.7-1GHz frequency modulated continuous wave frequency sources are output according to the frequency difference of 0.2865MHz, and after 16-fold frequency multiplication, two RF signal outputs with a frequency difference of 4.583MHz are obtained;

[0051] A reference channel unit, wherein the reference channel unit generates a 10MHz reference signal based on two RF signals or one RF signal; namely, dual-source mixing and single-source difference frequency; it should be noted that, compared with single-source difference frequency, dual-source mixing has the advantages of good image suppression effect; dual-source mixing is to solve the problem of matching the terahertz band frequency with the 10MHz intermediate frequency, and 10MHz intermediate frequency signal output is achieved by mixing and down-converting with a crystal oscillator; and compared with dual-source mixing, single-source difference frequency has the advantages of simple structure and high integration.

[0052] In this embodiment, the nonlinear error caused by the system in the intermediate frequency signal is compensated by the secondary mixing reference signal, so as to meet the requirements of the spectrum type ellipsometric imaging signal. The principle block diagram of the secondary mixing unit is shown in FIG. Figure 1 An example of the difference frequency signal generation of the dual-source mixing and single-source difference frequency modulation continuous terahertz system: the 10MHz intermediate frequency signal obtained by down-conversion is used as the input, the reference signal is used as the local oscillator, and a mixer is used for secondary mixing. The difference frequency signal is converted into two I / Q digital signals to realize terahertz imaging for the terminal.

[0053] Embodiment 2

[0054] Example 2 provides a specific technical solution for dual-source mixing. Figure 1 and Figure 2 When a 10 MHz reference signal is generated based on RF1OUT and RF2OUT (dual source mixing), the reference channel unit includes:

[0055] Coupler, RF1OUT and RF2OUT enter the reference channel unit through the coupler;

[0056] A mixer, wherein the coupled signal is mixed by the mixer and then outputs a 4.583MHz intermediate frequency signal;

[0057] 6-frequency multiplier, primary filter, 4-frequency multiplier and secondary filter, the 4.583MHz intermediate frequency signal is multiplied by 6 to obtain a 27.5MHz intermediate frequency signal, which is filtered and amplified and then multiplied by 4 to obtain a 110MHz intermediate frequency signal;

[0058] The internal 100MHz crystal oscillator, mixer and third-order filter, the 110MHz intermediate frequency signal is mixed with the internal 100MHz crystal oscillator signal to obtain a 10MHz reference signal, which is then filtered and amplified before output;

[0059] That is, the two RF signals pass through the coupler and enter the mixer of the reference channel unit to get a 4.583MHz intermediate frequency signal. The 4.583MHz intermediate frequency signal is filtered and amplified after a 6-fold frequency increase (27.5MHz), filtered and amplified, and then filtered and amplified after a 4-fold frequency increase (110MHz). It is mixed with the internal 100M crystal oscillator signal for the second time to get a 10MHz reference signal output. The reference signal is finally filtered and amplified before output.

[0060] In this embodiment, specifically, in the terahertz transmitting module, RF1OUT is subjected to 24-fold frequency multiplication, filtering and amplification operations, and then transmits a 268-384 GHz terahertz signal through an antenna.

[0061] In this embodiment, specifically, in the terahertz receiving module, RF2OUT is mixed with the received terahertz signal containing sample information through a second harmonic mixer after 12-fold frequency multiplication, filtering and amplification to obtain a 110 MHz intermediate frequency signal; the 110 MHz intermediate frequency signal is output as a 10 MHz intermediate frequency signal through a down-conversion channel unit.

[0062] In this embodiment, specifically, the down-conversion channel unit includes:

[0063] The internal 100MHz crystal oscillator, 110MHz intermediate frequency signal and 100MHz crystal oscillator signal are mixed to obtain a 10MHz intermediate frequency signal.

[0064] Embodiment 3

[0065] Example 3 provides a specific technical solution for single-source frequency difference. Figure 1 and Figure 3 When a 10 MHz reference signal is generated based on RF1OUT, the reference channel unit comprises:

[0066] The internal 100MHz crystal oscillator and the 100MHz crystal oscillator PLL are phase-locked to obtain a 10MHz signal. The 10MHz signal is output as a reference signal on the one hand, and mixed with RF1OUT on the other hand to introduce a 10MHz fixed difference frequency, which is then input into the terahertz transmitting module.

[0067] In this embodiment, specifically, in the terahertz transmitting module, the signal mixed with RF1OUT undergoes 24-fold frequency multiplication, filtering and amplification operations, and then transmits a 268.240-384.240 GHz terahertz signal through the antenna.

[0068] In this embodiment, specifically, in the terahertz receiving module, RF2OUT is mixed with the received terahertz signal containing sample information through a second harmonic mixer after 12-fold frequency multiplication, filtering and amplification to obtain a 240 MHz intermediate frequency signal; the 240 MHz intermediate frequency signal is output as a 10 MHz intermediate frequency signal through a down-conversion channel unit.

[0069] In this embodiment, specifically, the down-conversion channel unit includes:

[0070] The internal 100MHz crystal oscillator uses a 100MHz crystal oscillator PLL to phase-lock a 250MHz signal. The 250MHz signal is used as the local oscillator to mix with the 240MHz intermediate frequency signal to obtain a 10MHz intermediate frequency signal.

[0071] The above-mentioned embodiments only express the specific implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the protection scope of the present application. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the technical solution concept of the present application, and these all belong to the protection scope of the present application.

[0072] This background section is provided to generally present the context of the invention, and the work of the presently named inventors, the work to the extent described in this background section, and aspects of this section that did not constitute prior art at the time of application are neither explicitly nor implicitly admitted to be prior art to the present invention.

Claims

1. A clock-synchronized frequency-modulated continuous terahertz sweep source nonlinear error elimination system, characterized in that: include: Dual-channel broadband swept frequency source module, terahertz transmitting module and terahertz receiving module; The dual-channel broadband swept frequency source module generates two RF signal outputs with a frequency difference based on the same low phase noise constant temperature crystal oscillator; wherein the first RF signal is input to the terahertz transmitting module, and the second RF signal is input to the terahertz receiving module; and a reference signal is generated based on the two RF signals or one RF signal; The terahertz transmitting module transmits a signal after frequency multiplication of the first radio frequency signal through a frequency multiplication link; The terahertz receiving module multiplies the second RF signal through a frequency multiplication link as a local oscillator signal, mixes it with the received external signal carrying sample information, and then down-converts it to obtain an intermediate frequency signal; The intermediate frequency signal is used as input, the reference signal is used as the local oscillator, and the difference frequency signal obtained by secondary mixing is converted into I / Q two-way digital signals and output to the terminal for signal processing.

2. The clock-synchronized frequency-modulated continuous terahertz frequency-sweeping source nonlinear error elimination system according to claim 1, characterized in that: The dual-channel broadband frequency sweep source module comprises: Frequency source generation module, which generates 0.7-1GHz frequency modulated continuous wave frequency source based on 100MHZ constant temperature crystal oscillator; The power divider divides the 0.7-1GHz frequency modulated continuous wave frequency source into two paths. Both signals are multiplied by 16 and output to RF1OUT and RF2OUT. The difference frequency of the two paths is set to 4.583MHz. Among them, RF1OUT is input to the terahertz transmitting module; RF2OUT is input to the terahertz receiving module. A reference channel unit generates a 10 MHz reference signal based on two RF signals or one RF signal.

3. The clock-synchronized frequency-modulated continuous terahertz sweep source nonlinear error elimination system according to claim 2, characterized in that: When a 10 MHz reference signal is generated based on RF1OUT and RF2OUT, the reference channel unit includes: Coupler, RF1OUT and RF2OUT enter the reference channel unit through the coupler; A mixer, wherein the coupled signal is mixed by the mixer and then outputs a 4.583MHz intermediate frequency signal; 6-frequency multiplier, primary filter, 4-frequency multiplier and secondary filter, the 4.583MHz intermediate frequency signal is multiplied by 6 to obtain a 27.5MHz intermediate frequency signal, which is filtered and amplified and then multiplied by 4 to obtain a 110MHz intermediate frequency signal; The internal 100MHz crystal oscillator, mixer and third-order filter, the 110MHz intermediate frequency signal and the internal 100MHz crystal oscillator signal are mixed twice to obtain a 10MHz reference signal, and the 10MHz reference signal is output after filtering and amplification.

4. The clock-synchronized frequency-modulated continuous terahertz sweep source nonlinear error elimination system according to claim 2, characterized in that: When a 10 MHz reference signal is generated based on RF1OUT, the reference channel unit includes: The internal 100MHz crystal oscillator and the 100MHz crystal oscillator PLL are phase-locked to obtain a 10MHz signal. The 10MHz signal is output as a reference signal on the one hand, and mixed with RF1OUT on the other hand to introduce a 10MHz fixed difference frequency, which is then input into the terahertz transmitting module.

5. The clock-synchronized frequency-modulated continuous terahertz sweep source nonlinear error elimination system according to claim 3, characterized in that: In the terahertz transmitting module, RF1OUT undergoes 24-fold frequency multiplication, filtering and amplification before transmitting 268-384 GHz terahertz signals through the antenna.

6. The clock-synchronized frequency-modulated continuous terahertz frequency-sweeping source nonlinear error elimination system according to claim 5, characterized in that: In the terahertz receiving module, RF2OUT is multiplied by 12, filtered and amplified, and then mixed with the received terahertz signal containing sample information through a second harmonic mixer to obtain a 110MHz intermediate frequency signal; the 110MHz intermediate frequency signal is output as a 10MHz intermediate frequency signal through the down-conversion channel unit.

7. The clock-synchronized frequency-modulated continuous terahertz sweep source nonlinear error elimination system according to claim 6, characterized in that: The down-conversion channel unit comprises: The internal 100MHz crystal oscillator, 110MHz intermediate frequency signal and 100MHz crystal oscillator signal are mixed to obtain a 10MHz intermediate frequency signal.

8. The clock-synchronized frequency-modulated continuous terahertz sweep source nonlinear error elimination system according to claim 4, characterized in that: In the terahertz transmitting module, the signal mixed with RF1OUT undergoes 24-fold frequency multiplication, filtering and amplification, and then transmits a 268.240-384.240 GHz terahertz signal through the antenna.

9. The clock-synchronized frequency-modulated continuous terahertz frequency-sweeping source nonlinear error elimination system according to claim 8, characterized in that: In the terahertz receiving module, RF2OUT is multiplied by 12, filtered and amplified, and then mixed with the received terahertz signal containing sample information through a second harmonic mixer to obtain a 240MHz intermediate frequency signal; the 240MHz intermediate frequency signal is output as a 10MHz intermediate frequency signal through the down-conversion channel unit.

10. The clock-synchronized frequency-modulated continuous terahertz sweep source nonlinear error elimination system according to claim 9, characterized in that: The down-conversion channel unit comprises: The internal 100MHz crystal oscillator uses a 100MHz crystal oscillator PLL to phase-lock a 250MHz signal. The 250MHz signal is used as the local oscillator to mix with the 240MHz intermediate frequency signal to obtain a 10MHz intermediate frequency signal.

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