KR0132897B1 - Switching mode decoding system - Google Patents

Abstract

A demodulation system of a switching mode which can demodulate two signals by single signal processing process is provided to prevent a loss of picture and bit noise without using a mixer. The system comprises: a tuner(10) for outputting an IF(intermediate frequency) by mixing a RF(radio frequency) and a local generating frequency; a demodulator(30) of switching mode for demodulating a base band signal by mixing the IF(intermediate frequency) and the synchronous signal; and a micro-controller(40) for controlling a synchronous signal generator(OSC1,OSC2) in the demodulator(30) selectively switching by applying a driving signal.

Description

Switching Demodulation System

1A and 1B show IF signals of a conventional mixer method.

2 is a block diagram of a conventional mixer demodulator.

3 is a block diagram of the mixer shown in FIG.

4A, 4B and 4C are characteristic diagrams of a double balance mixer used as the mixer of FIG.

5a and 5b show the input and output spectrum of the mixer.

6 shows frequency characteristics of an IF bandpass filter.

7 is a block diagram of an AM coherent demodulator according to the present invention.

8 is a block diagram of a switched demodulation system according to the present invention.

Figure 9 is a block diagram of a tuner IF mixer

10 is a control circuit for generating a synchronization signal according to the present invention.

11A and 11B show IF spectra of the L 'channel of the mixer method and the switching method.

12A and 12B show IF spectra of the L channel of the mixer method and the switching method.

* Explanation of symbols for main parts of the drawings

10: Tuner 20: IF filter

30: switching demodulator 40: micom

50: local carrier signal generator 60: filter

The present invention relates to a demodulation system, particularly a switching demodulation system capable of demodulating two different signals with one signal processing process. The broadcasting method of SECAM L system, which is generally adopted in France in Europe, has a channel bandwidth of 7 MHz for VHF and 8 MHz for UHF, and the image modulation method is used for suppressing a part of one side band among two side bands. VSB (Vestigial Side-Band) POSITIVE modulation method and voice modulation method use AM method. In addition, unlike other systems, this system has an upper heterodyne in the VHF Low Channel and a lower heterodyne in the VHF High and UHF Channels. Therefore, the system has a different configuration in the demodulator circuit. That is, if the demodulator is designed based on the intermediate frequency (IF) signal of the L (VHF-H, UHF) channel, the intermediate frequency signal of the L '(VHF-Low) channel is not processed and L' (VHF- Low) When designed based on the intermediate frequency signal of the channel, the intermediate frequency signal of the L channel is not processed. Therefore, in order to solve this problem, conventionally, a mixer is used to configure a demodulator, thereby solving the circuit configuration problem, and also enabling the PAL-B / G and SECAM-L systems to be used. It has brought a lot of development. However, by using the mixer, due to the characteristics of the mixer itself, the signal power was greatly affected by loss of noise and noise. In other words, one of the important items for evaluating the picture quality in TV is the video noise ratio, which is closely related to the performance of the demodulator. Therefore, even if the broadcast station transmits clear picture quality, viewers cannot see good picture quality unless the receiver demodulates the performance properly. Also, a problem in the conventional mixer demodulator is a decrease in productivity. Local oscillating frequency adjustment of the mixer is difficult, which increases the probability of quality problems due to poor operator adjustment during operation, resulting in low productivity. Accordingly, an object of the present invention is to provide a demodulator of a switching method capable of demodulating two different signals using one signal processing process without using a mixer. Another object of the present invention is to provide a demodulation system of a switching method configured by adopting the above-described switching demodulator. In order to achieve the above object, the present invention provides a tuner that tunes to a specific frequency of a radio frequency (RF) signal input through an antenna and mixes with a local oscillation frequency of an internal oscillator to output an intermediate frequency signal; Synchronizing signal generating means for generating two synchronizing signals having different frequencies in order to demodulate a baseband signal by mixing the synchronizing signals of the intermediate frequency signals from the tuner; Switching means for selecting any one of two synchronization signals of said synchronization signal generating means by a control signal; A switching demodulator including a multiplier for multiplying the input signal with a synchronization signal applied through the switching means to demodulate an input signal; A microcomputer for generating a corresponding oscillation frequency by applying PLL data to the tuner and applying a driving signal to the synchronizing signal generating means in the demodulator of the switching method so as to selectively switch the synchronizing signal generating means. It is to provide a demodulation system of the switching method, characterized in that. Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. Prior to the description of the present invention, the configuration of the mixer-based demodulation system and its problems will be described in detail. In the SECAM L system, the intermediate frequency signal output from the tuner is output as shown in FIG. At this time, 1Aeh is the signal output of the intermediate frequency in the case of the L 'channel and Figure 1b is the signal output of the intermediate frequency of the L channel. In FIG. 1, Fp is a Picture Carrier Frequency, Fc is a Color Carrier Frequency, and Fs is a Sound Carrier Frequency. As shown in FIGS. 1a and 1b, the outputs of the intermediate frequencies in the L (VHF-High, UHF) channel and the L '(VHF-Low) channel are inverted and outputted (Fp, Fc in the L' channel). , Fs, and in the L channel, Fs, Fc, and Fp.) In the back demodulator, the intermediate frequencies of the L channel and the L 'channel cannot be processed by the same method. Therefore, in order to make the intermediate frequency signal output from the tuner the same shape, a mixer capable of processing signals of a frequency band different from that of the first mixer is required at the rear of the mixer for the intermediate frequency output from the tuner. A schematic block diagram of a demodulator having such a two stage mixer configuration is shown in FIG. The mixer type demodulator of FIG. 2 includes an IF mixer 1 inside the tuner for mixing the intermediate frequency signals of the tuner, another mixer 2 for processing only signals of other channels among the output signals of the mixer 1, and A filter 3 for filtering the output signal from the mixer 2 and a demodulator 4 for demodulating the filtered signal. Such a mixer-type demodulator mixes only the L (VHF-Low) channel signal of the L system among the output signals from the mixer 1 that mixes only the L '(VHF-High, UHF) channel signal of the L system. A separate mixer 2 is required to receive and operate the L 'enable signal for driving the mixer 2 from the microcomputer. Only the signal input / output relationship of the mixer 2 used here will be described with reference to FIG. IF (A) signal is input to the mixer 2 and the signal is passed as it is when it is a signal of L (VHF-High, UHF) channel, and is applied from the microcomputer when it is a signal of L '(VHF-Low) channel. The enable signal is operated and the 72.3MHz local oscillation signal is mixed with the input signal to output the converted IF (B) signal as shown in Table 1. This output signal is applied to the demodulator and demodulated.

Therefore, as the L channel signal and the L 'channel signal pass through the mixer 2, the IF signal of the same frequency is always output, so that the rear end can process and demodulate the signal regardless of the channel. The mixer 2 used here is a double balanced mixer, which has a relatively low conversion loss, obtains a wideband characteristic, and has a high isolation compared to other mixers. However, when the mixer can be used to process the signals of different channels in the same manner as described above, a problem arises that the quality of the image is degraded by the mixer used basically. That is, due to the characteristics of the mixer, the conversion loss cannot be ignored, and this conversion loss represents the difference in dB between the input power input to the mixer and the output power of the sideband from the output port. In general, even in an ideal mixer, a single side band (SSB) 3dB conversion loss is basically generated, and part of the output is lost as heat due to the diode's series resistance and mismatching in the system. Conversion loss occurs. Looking at the characteristics of the double balance mixer used in this mixer method with reference to Figure 4 as follows. FIG. 4a shows the relationship between input frequency versus isolation at intermediate and radio frequencies, and FIG. 4b shows the relationship between local oscillation signal output and loss, and FIG. 4c shows the relationship between input frequency and loss. The relationship is shown. As shown, the intermediate frequency should be 30 to 40dB of isolation around 100MHz, and the local oscillation signal output of the mixer should be about 4 to 8dBm, and using this mixer, the conversion loss is about 6 to 8dB as shown in Figure 4c. . Harmonic components are generated in the mixing process of the double balance mixer having such characteristics, and are most affected by intermodulation components. This component can be expressed as follows.

fp = 33.4MHz, fc = 37.83MHz fs = 39.9MHz fL = 72.3MHz f ₀ : Intermediate Modulation Frequency As shown in equation (1), calculating the intermediate modulation component results in components existing in the same frequency band as the desired signal. These components act as noise on the desired signal, which adversely affects the image quality. 5 shows the mixer input intermediate frequency spectrum and the output intermediate frequency spectrum when the mixer is applied. FIG. 5A is the spectrum of the input signal before being processed by the mixer and FIG. 5B is the spectrum of the output signal after being processed by the mixer. In the spectrum of FIG. 5B, the noise portion (dotted line in the figure), which was not seen in the input intermediate frequency spectrum of FIG. . Therefore, when we put everything together, we can see that using the mixer adversely affects the picture quality. Another problem with mixer demodulators is their poor productivity. As mentioned earlier, since the local oscillation frequency of the mixer 2 has to be adjusted to 72.3 MHz, an adjustment process is added. If the adjustment process is not performed properly, signal processing is not performed properly, resulting in poor quality. As a result, poor work quality leads to poor quality, which results in poor productivity. In order to improve the problems of the conventional mixer-type demodulator, considering all the methods for demodulating the SECAM-L broadcasting system, the following three methods can be considered.

(1) Demodulation using a mixer

(2) Demodulation with two signal processing processes

(3) Demodulation with one signal processing process without using a mixer

Here, the first method is the demodulation method by the mixer mentioned above, which is a problem in terms of quality and productivity. The second method processes the intermediate frequency signals of the two channels independently, so in terms of quality, It is more advantageous than that, but it is not preferable in terms of manufacturing cost and productivity due to the complexity of the circuit. The third method compensates for the shortcomings of the first method in that it does not use a mixer, and the second method uses one signal processing process instead of two separate signal processing processes. It can be seen as the most ideal way to compensate for the shortcomings. However, in order to configure the demodulator of the SECAM-L broadcasting system using this method, the following two basic conditions must be satisfied.

(1) Intermediate frequency signal of L '(VHF-Low) channel and L (VHF-H, UHF) channel should exist in the same frequency band.

(2) A detection circuit capable of demodulating two different intermediate frequency signals is to be provided.

The first of these two conditions is considered in terms of reducing circuit complexity and manufacturing costs. If two intermediate frequency signals exist in different bands, signal processing is difficult in the same process, and thus, different circuits will increase complexity and manufacturing cost by having different processes. Therefore, the frequencies of two intermediate frequency signals should be made to exist in the same frequency band. In this case, the intermediate frequency signal (hereinafter referred to as IF signal) is an output generated by mixing a high frequency signal (hereinafter referred to as RF signal) received through an antenna and a local oscillation signal from the tuner's IF mixer. The IF signal can be made to exist in the desired frequency band by changing the local frequency of the oscillator signal of the IF mixer if it is considered not to interfere with the local RF frequency. Here, when examining the method of changing the oscillation frequency of the tuner IF mixer, the tuner currently used in the present invention is a frequency synthesizer (F / S) tuner, the phase synchronization data (PLL Data: clock applied from the microcomputer to the tuner). The oscillation frequency can be changed by changing the signal, data signal and strobe signal). The PLL data applied from the microcomputer to the tuner is set for each channel to fit the data processing format of the PLL IC built into the F / S tuner and embedded in the microcomputer. Then, when the user selects the desired channel, the corresponding data is stored in the F / S tuner. It is supplied to the PLL IC, and the PLL IC generates an oscillation frequency corresponding to the supplied data value. The PLL IC is characterized by being configured to control the oscillation frequency by digital data value. Therefore, the oscillation frequency (Fosc) is generated by PLL data sent from the microcomputer.

Becomes In these relations, M and S can be changed to determine the Fosc value. At this time, M value is composed of 10-bit binary code and S value is composed of 5-bit binary code. In order to obtain a desired Fosc for each channel, corresponding data, that is, M and S, are set in advance and stored in a memory device (ROM) of the microcomputer in advance. When the user sets up the channel, the memorized data is outputted to the tuner to make the Fosc. This signal is used as the local oscillation signal of the IF mixer. The signal is mixed with the RF signal coming from the antenna. Output We will use this tuner to determine the IF signal frequency that satisfies the first condition. As mentioned above, the IF signal is composed of Fp, Fc and Fs. In general, the IF signal frequency refers to Fp. Also, as shown in FIG. 1, in SECAM broadcasting, Fc exists with a difference of 4.43 MHz from Fp and Fs exists with a difference of 6.5 MHz from Fp in an L-system. In order to determine the frequency of the IF signal, the RF frequency band of the used area is examined and the unobstructed range is as follows. The demodulator used here is used in France and the RF frequencies in France are shown in Table 2.

As shown in Table 2, the lowest RF signal frequency is 47.75MHz, so the IF signal frequency can be determined in the lower range. In general, since the IF signal frequency used mainly in PAL / SECAM broadcasting system demodulator using lower heterodyne is 38.9MHz, the saw filter used as IF bandpass filter is mostly 38.9MH. Therefore, in consideration of these points, the IF signal frequency is determined to be 38.9 MHz in the L (VHF-H, UHF) channel of the SECAM-L broadcasting system. Next, in order to determine the IF signal frequency of the L '(VHF-Low) channel, the frequency characteristics of the IF bandpass filter are first examined. Since the IF signal frequency of the L (VHF-H, UHF) channel has already been determined to be 38.9MHz, the IF bandpass filter used here is also used for 38.9MHz. Figure 6 shows the frequency characteristics of the 38.9MHz sawtooth filter used in the present invention. As can be seen from the frequency characteristics of the sawtooth filter shown in FIG. 6, it can be seen that the attenuation ratios of Fp and Fc are the same, and from this point, the IF signal Fp 'of the L' (VHF-Low) channel is L (VHF-High). If we decide 34.4MHz equal to Fc of UHF) channel, Fc 'becomes 38.9MHz like Fp, and the frequency characteristic of Fp (Fp') and Fc (Fc ') is 5dB equally in L channel or L' channel. The signal is attenuated and passed, and other signals are suppressed, so that the characteristics of the original IF filter can be applied as it is. As a result, the IF signal frequency that can satisfy the first condition mentioned above can be determined as shown in Table 3.

The IF signal frequencies thus determined exist in the same frequency band, so that the signals can be processed with the same characteristics in the same process. This is the first most important feature of the switched demodulator configuration. Next, we will look for a detection circuit that can demodulate these two IF signals. The AM coherent demodulator used in the prior art can demodulate the baseband signal by multiplying the same synchronization signal frequency as the IF signal frequency. Therefore, if the synchronization signal generation part of the demodulator is applied, it is considered that a synchronization signal corresponding to two different IF signals can be generated. In other words, if the signal frequency is L '(VHF-Low) channel or L (VHF-H, UHF) channel according to the input IF signal, the synchronization signal frequency may be changed according to the signal. The problem of distinguishing IF signal is easy to find out because it creates L 'channel and L channel IF signal by using microcomputer. In other words, when the L 'channel is selected in the microcomputer, a' high 'signal is output, and in other channels, a' low 'signal is output and the signal is used as a control signal to change the synchronization signal frequency. Thus, a block diagram of a switching AM coherent demodulator according to the present invention that can satisfy the second condition is shown in FIG. As shown in FIG. 7, the switching AM coherent demodulator includes a switching unit configured to select oscillators OSC1 and OSC2 having different oscillation frequencies according to an input signal, and an oscillation signal output from the switching unit. Is multiplied by the input signal and passed through the low pass filter to obtain a demodulated baseband signal. This is the second important feature of the switching scheme, which allows for demodulation of the two incoming signals. Therefore, when the IF signal of the L 'channel is input in the demodulator of the switching method shown in FIG. 7, the microcomputer sends a control signal to switch the oscillator OSC2 to be connected, and when the IF signal of the L channel is input, the oscillator OSC1. To be connected. The generation of such a synchronization signal and a description of the control circuit therefor will be described with reference to FIG. 12. 10 is a control circuit for generating a synchronization signal adopted in the present invention to change the frequency of the synchronization signal by turning the diode D1 on and off in accordance with the L 'enable signal. That is, when the L 'enable signal is high, the diode D1 is turned on, and the capacitor C2 is added to the resonant circuit composed of the resistors, the capacitors, and the inductors R1, C1, and L1 to change the synchronization signal. In the present invention, OSC1 is a synchronization signal oscillation block generated as a resonant circuit composed of R1, C1, L1, and OSC2 is another resonant circuit composed of R1, C1, L1, C2 to generate a second synchronization signal. Accordingly, it can be seen that two synchronization signals can be generated according to the L 'enable signal. Therefore, when L 'enable is low, it is a resonant circuit composed of R1, C1, L1 and generates 38.9MHz synchronization signal. When L' is enabled, C2 is added to R1, C1, L1, and 33.4MHz synchronization signal is generated. Will let you. Table 4 shows the control signal transmitted according to the synchronization signal frequency and the type of channel from the microcomputer.

Therefore, it is possible to realize a demodulator configuration that can satisfy the second condition mentioned above. That is, a detection circuit capable of demodulating two different IF signals through the above circuit configuration may be provided. The demodulation system of the switching method according to the present invention newly constructed by satisfying both of the above conditions so far is shown in FIG. Referring to the components of the present invention according to FIG. 8 briefly, an F / S tuner 10 for receiving an RF signal and outputting an IF signal through an antenna, an IF filter 20 for filtering an IF signal, and filtering the filtered signal. To generate a synchronization signal and a demodulator driving signal for controlling the switch demodulator 30 and the F / S tuner 10 and the demodulator 30 according to the present invention for demodulating the baseband signal by mixing with the synchronization signal. It consists of the microcomputer 40. In other words, when the RF signal is input through the antenna, the PLL data of the channel selected by the microcomputer 40 'is transmitted to the F / S tuner 10 and the tuner is shown in Table 3 below. IF signal is output. The IF signal is input to the demodulator 30 of the switching method through the filter 20 and a control signal is applied to generate a corresponding synchronization signal from the microcomputer. Therefore, in the switching demodulator, the IF signal and the synchronization signal are mixed to output the baseband signal. At this time, the switching method is not switching using a general pulse, and the switching operation occurs only at the boundary between the L 'channel and the L channel. That is, when changing from C channel (63.75MHz) to 5 channel (175MHz) in France, L 'enable signal should be changed from' high 'level to' low 'level, and vice versa. You can change it from level to low level. Thus, the L 'enable signal maintains the' high 'level in the L' channel and the 'low' level in the L channel. Therefore, the noise related to switching may not be considered. The demodulation of the switching method according to the present invention can demodulate a signal having two different frequency bands without two signal processing processes without using a conventional mixer but with one signal processing process. We looked at the system. The demodulation process of this switching demodulation system will be analyzed theoretically. This type of signal processing analysis can be analyzed in the same way as the basic demodulator. First, the front-end output waveform input to the tuner can be expressed as follows.

Where C (t) is the desired carrier waveform and N (t) is the bandpass noise (receiver front end), and if it is assumed that it is a pure AM carrier without phase modulation (θ (t) = 0), then The general form is:

Where a (t) = C [1 + m (t)] and C is an amplitude constant. And the general form of bandpass noise

Substituting Equations 3) and 4) into Equation 2),

Becomes 9 is a tuner IF mixer, when Xc (t) is inputted, it is a local oscillation signal. This mix produces sum and difference frequencies If the sum frequency is omitted, the mixer output signal is obtained by considering only the difference frequency component.

If we rearrange the left side as Xif (t) in (6)

In Equation 7), Nm (t) is the output noise of the tuner IF mixer.

And the noise component is

Becomes Equation (10) assumes that Nc (t) and Ns (t) are independent of each other at Nm (t) and that this is a Gaussian noise process, its power spectrum is

Where N ₀ = kTeqG1, k is the Boltzmann constant, Teq is the equivalent noise temperature, and G1 is the total receiver gain from the antenna to the demodulator input. In equations 10) and 11) The output signal frequency of the fif tuner IF mixer should be determined as shown in Table 3. Therefore, if the tuner IF mixer output is expressed separately from the L 'channel and the L channel, the IF signal is first displayed in the L' channel. And

And if L channel is IF signal

Becomes Where fifL '= 34.4 MHz and fifL = 38.9 MHz.

Here, the most important point in the switching method is to convert the IF frequency to 38.9MHz in the L channel and 34.4MHz in the L 'channel.

This is to solve the problem that signal processing is difficult in the same process because the IF signal is different in the L channel and the L 'channel as mentioned above. An IF signal as shown in Equations 12) and 13 is input to the switching AM coherent demodulator of FIG. 7 along with an IF band pass filter having the characteristics as shown in FIG. So when L 'channel is tuned, OSC2 sync signal And demodulate the original signal, and when the L channel is tuned, the OSC1 sync signal And demodulate the original signal. This demodulation process is expressed by the equation. First, when the IF signal of L 'channel is inputted, the output of the demodulator is Say

to be. If this signal passes through the baseband filter and the signal component without noise is SO (t)

Becomes Where a (t) = C [1 + m (t)], so a pure baseband signal without distortion

Becomes And in (14), output of noise component Say

Becomes In Eq. 17), the second and third terms are removed because they are outside the baseband filter range. Therefore, the output noise

to be. In the same way, it can be easily inferred from the IF signal processing formula of the L 'channel even when the IF signal of the L channel is input. Therefore, the demodulator output at this time If you say

And the noise component If you say

Equations 19) and 20) passed through the baseband filter yield the same results as Equations 16 and 18, respectively.

So far, as a result of the signal processing of the switched demodulator, it is confirmed that both L channel and L 'channel can be demodulated identically. Therefore, it can be seen that the problems caused by the mixer can be solved because the demodulation can be performed without using the mixer compared to the conventional mixer method.

As such, it can be clearly seen that the present invention can process signals having different frequency bands by one process without using an existing mixer. In addition, the signal-to-noise ratio (SNR) of the switching method and the mixer is compared.

First, find the SNR of the switching demodulator.

Pure signal power

here

to be. Refer to Eq. 12) to obtain the noise power spectrum of the final output.

And noisy power

here

Becomes Therefore, at the final output of the demodulator, the SNR is

Is expressed in dB,

Becomes As shown here, this type of SNR is the same as a general demodulator, so only general considerations such as tuner IF mixer noise and filter noise need to be considered.

Next, the SNR of the mixer type demodulator is as follows. The mixer type demodulator (FIG. 2) is a mixer 2 added to the conventional demodulator. Therefore, in order to examine the noise of the mixer 2, the SSB noise figure, which is widely used as an index indicating the performance of the mixer, will be examined first. This SSB noise diagram shows the input signal SNR and the output signal SNR of the mixer in dB.

to be. Therefore, in the mixer method, the SNR can be expressed as SNR of the basic demodulator minus SSB NF (dB) due to the addition of the mixer, which is expressed in dB by referring to equations (25) and (26).

to be. In general, the SSB noise level of the mixer is regarded as (conversion loss + 05dB). As a result, comparing the mixer method SNR 25) and the switching method SNR 27), the switching method than the mixer method results in a higher image quality than the mixer.

This means that the switching method improves 3 ~ 8dB in terms of SNR in consideration of the conversion loss due to the use of a mixer, and it can be confirmed from the experimental data, and it is also good in terms of image quality because it is not affected by the harmonic components generated during mixing. I think I got the result.

So far, we have compared and reviewed the mixer demodulator and the switched demodulator. The results are summarized in Table 4 below.

As can be seen from the table comparing the performance of the mixer-type demodulation system and the switching-type demodulation system, it can be seen that the signal / noise ratio of the switching and demodulation demodulator according to the present invention is improved by 3 to 8 dB. Also, on the screen, bit noise appeared in the mixer method, but this phenomenon was not seen in the switching method. Comparing the IF spectrum of the L channel of the mixer method (Fig. 11a) and the switching method (Fig. 11b) shown in Fig. 11, the IF signal of the mixer method has a large number of noise components (dotted lines in Fig. 11a). ) Can be seen. In particular, a noise component having a difference of about 10 dB from a color component exists near 0.9 MHz of the color carrier signal. This noise component appears as a color beat phenomenon on the screen, resulting in deterioration of image quality. In addition, a large number of noise components exist around the picture carrier with a difference of about 30 dB and directly affect the original signal components. In general, noise components and signal components are not affected by noise components only when isolation is maintained above 40dB. In the case of the switching IF signal, almost no noise component can be seen as in the mixer method, and the image quality is also improved. In FIG. 12, the L channel IF spectrum of the mixer method (Fig. 12a) and the switching method (Fig. 12b) is compared with each other. In the L channel, the mixer does not use a mixer, so it is similar to the switching method.

In addition, even in the case of the baseband signal waveform demodulated finally in the mixer and the switching method, the mixer includes much more noise components than the signal waveform of the switching method.

In conclusion, it can be seen that the switching method does not use a mixer, thereby preventing bit noise due to harmonics and loss of image quality due to conversion loss. In addition, in terms of productivity and cost, the switching method does not use a mixer, which eliminates the adjustment process and eliminates the defects, thereby increasing productivity and reducing the cost of the mixer.

Claims (13)

Synchronizing signal generating means for generating two synchronizing signals having different frequencies; Switching means for selecting any one of two synchronization signals of said synchronization signal generation means by said signal; And a multiplier for multiplying the input signal with a synchronization signal applied through the switching means to demodulate an input signal. 2. The demodulator of claim 1, wherein the demodulator further comprises a low pass filter for low pass filtering the output signal multiplied by the multiplier. 3. The synchronizing signal generating means according to claim 1, wherein the synchronizing signal generating means includes first and second resonant circuits connected to the voltage-controlled oscillator and the voltage-controlled oscillator, respectively, for generating synchronizing signals having different frequencies. Switching demodulator. 4. The demodulator according to claim 3, wherein the first resonant circuit generates a synchronous signal having a frequency of 38.9 MHz and the second resonant circuit generates a synchronous signal having a frequency of 33.4 MHz. 5. The demodulator according to claim 4, wherein said second resonant circuit includes a capacitor connected to said first resonant circuit for changing the frequency of said first resonant circuit. A tuner tuned to a specific frequency of a radio frequency (RF) signal input through an antenna, and mixed with a local oscillation frequency of an internal oscillator to output an intermediate frequency signal; A demodulator of claim 1 for demodulating a baseband signal by mixing an intermediate frequency signal from said tuner and a synchronization signal; And applying a driving signal to the synchronizing signal generating means in the demodulator of the switching method by applying phase synchronizing (PLL) data to the tuner to control the synchronizing signal generating means to be selectively switched. Switching demodulation system, characterized in that. 7. The demodulation system according to claim 6, further comprising a low pass filter provided between the tuner and the switching demodulator for low pass filtering the intermediate frequency signal from the tuner. 8. The demodulation system according to claim 6 or 7, wherein the tuner is a frequency synthesizer tuner. The demodulation system according to claim 6 or 7, wherein the oscillation frequency generated by the phase synchronization data applied from the micom to the tuner is generated by the following relational expression. 10. The method of claim 9, wherein the modulus (M and S) is set in advance so as to obtain a desired oscillation frequency for each channel and stored in the microcomputer, and if the channel is set, the modulus value is provided to the tuner Switching demodulation system. A method for demodulating a SECAM L type signal using signals existing in different first and second frequency bands (L channel and L 'channel), the middle of the first and second frequency bands. A first step of placing the frequency signal in the same frequency band; Determining two signals that lie in the same frequency band; And a third step of multiplying the output signal in the first step with the synchronization signal by varying the frequency of the synchronization signal according to the determination result in the determination step. 12. The demodulation method according to claim 11, wherein the intermediate frequency signal of the second frequency band is transferred to the first frequency band in the first step. The intermediate frequency signal of the second frequency band processed in the first step has a picture carrier frequency (Fp) of 34.4 MHz, a color carrier frequency of 38.83 MHz, and a voice carrier frequency (Fs) of 40.9 MHz. Demodulation method characterized in that.