JPS5850804A - High-sensitivity fm detection system - Google Patents

Abstract

PURPOSE:To reduce the amount of feedback when an FM input signal has a small frequency shift, and to obtain noise-improved demodulated picture quality, by inserting a nonlinear circuit into a feedback circuit for threshold improvement by a narrow band pass filter. CONSTITUTION:An FM signal modulated by a color TV signal is inputted from a terminal 7 and demodulated through a variable narrow band pass filter 9, a limiter 3, and a discriminator 4, but part of the demodulated signal is fed back to the filter 9 through a low-pass filter 6 for the improvement of a threshold level. When the input FM signal has a small frequency shift, an FM noise component is fed back to cause deterioration in picture quality. To prevent that, a nonlinear circuit 10 which varies in input and output amplitude ratio with an input amplitude level is inserted into the feedback circuit and when the input FM signal has a small frequency shift, the amount of the feedback is reduced, thereby improving the SN ratio of the output demodulated signal.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high-sensitivity FM demodulation method that has a simple configuration and improves the noise characteristics of a demodulated p+ signal.
.

Conventionally, as the simplest method for demodulating a frequency-modulated signal, a frequency demodulation method using a discriminator using a 50 circuit or a delay line has been widely used for a long time. CJ of the FM modulated input signal in this case (
The S/N (signal to fiber ratio) of the FM demodulated (detected) demodulated signal with respect to the carrier power to noise power ratio is
It is expressed as ==cA-I (I is a constant) and is proportional to the Q of the demodulated signal or input signal. On the other hand, the C/N of the FM signal is determined by the noise and the passband width of the bandpass F-wave generator that limits the signal bandwidth, and the demodulation bandwidth B is the maximum frequency deviation of fh1 from the highest modulation frequency of the FM signal. If Δf is B
It is determined as 2 (fh + Δf). Also, according to this method, the above-mentioned C/N can be reduced up to about 10 dB below c/'N.
The relationship with SA is maintained, and at a C/N below that, S
J deteriorates rapidly. This point is called the thread or shhold point.

In particular, the present invention relates to the reception and demodulation of FM waves modulated by TV (video) signals in satellite communications.
In this case, the operating point of the demodulator for satellite communication is set near the threshold point due to limitations on satellite transmission power, stability of the line propagation path, and economics of ground reception equipment. There are many cases. For this reason, sometimes the reception input decreases due to changes in the environmental situation, and the reception point is below the threshold point.9.The demodulated image quality on the TV monitor is
It is significantly disturbed by signal-specific noise. Therefore, particularly in satellite communications, methods of improving the threshold level are considered to be a very, very important issue, and are being widely studied in various fields.

Incidentally, there have been various methods for improving the threshold, and the method shown in FIG. 1 will be described as a first embodiment.

Figure 1 shows a block diagram of the frequency feedback demodulation system, where 1 is a frequency converter, 2 is a narrow band pass F-wave device, 3 is a limiter,
In the circuit, 4 is a frequency discriminator, 5 is a voltage variable frequency oscillator, 6 is a low-pass F wave generator, 7 is an FM signal input terminal, and 8 is an FM signal detection signal output terminal. The FM input signal enters the input terminal 7, passes through the frequency converter 1, the narrow band pass F wave device 2, and the limiter circuit 3, and then is detected by the frequency relay discriminator 4, and its output is the detected signal output. The terminal 8 is removed. At this time, the detected output of the frequency discriminator 4 is input to the voltage variable frequency oscillator 5 after high-frequency i-sounds other than the demodulated signal frequency components are removed by the low-pass F-wave generator 6 .

The output of the voltage variable frequency oscillator 5 is given as a local oscillation wave to the frequency converter 1, and the frequency converter 1 generates a difference frequency component between the FM input signal and the output signal of the voltage variable frequency oscillator 5. Sends to the passing F wave device.

Thereafter, the line is inspected through a limiter circuit 3 and a frequency discriminator 4.

That is, according to the frequency feedback demodulation method explained above, the instantaneous frequency change (deviation, shift) of the FM input signal from the input terminal 7
Correspondingly, the frequency of the local oscillation wave that is the output of the voltage variable frequency oscillator 5 changes in the same direction and is applied to the frequency converter 1. Therefore, the output signal of the frequency converter 1 changes depending on the frequency deviation of the FM input signal. The result is a signal with a sufficiently small frequency deviation. Therefore, the signal component can pass even in the narrow band of the narrow band pass F-wave device 2. Further, the voltage variable frequency oscillator 5 cannot respond to the noise frequency component removed by the low-pass F wave generator 6, and the oscillation frequency takes on a fixed value. That is, in the frequency converter 1, the output of the voltage variable frequency oscillator 5 is used for the FM noise component whose modulation frequency is the same frequency as the component removed by the low-pass wave generator 6 of the y input signal. Since the frequency of a certain local oscillation wave has a fixed value, the FM noise modulation component of the output of the frequency converter 1 has a frequency deviation of the same magnitude as the dynamic noise modulation component of the FM input signal. Therefore, F.M.
Although the noise and sound bandwidth of the input signal are frequency-converted by the frequency converter 1 and sent out as they are, many noise components are removed by the narrow band pass F-wave converter 2. As a result of the above, since the C/N of the FM input signal is improved by the narrow-pass transducer 2 and demodulated by the frequency discriminator 4, the threshold 4. You can change the r level to 11f.

Fig. 2 shows a block diagram of a dynamic tracking filter method which is a second embodiment for improving the threshold. The symbols are the same as in Figure 1. The FM input signal enters through the input terminal 7, passes through the narrow band variable V wave generator 9, further passes through the limiter circuit 3, is subjected to FM detection by the frequency discriminator 4, and outputs the detected signal from the detected signal output terminal 8. is taken out. At this time, the detected signal, which is the output of the frequency discriminator 4, is band-limited by the low-pass p-wave device 6 and applied to the narrow-band variable p-wave device 9, and the narrow-band variable p-wave device 90 passes through the center. Change the frequency. The low-pass wave transducer 6 detects the noise-containing detection signal output from the frequency discriminator 4.
Passes signal frequency components and removes other high-frequency noise components. In this configuration, F input from input terminal 2
A narrowband variable filter 9 follows the instantaneous frequency change of the M signal.
Since the passing center frequency of the filter is controlled to change, among signals with broadband noise, signals with modulation frequency components in the same range as the frequency passing through the low-pass F wave generator 6 are controlled to change by the narrow band variable F. The noise components are effectively extracted by the filter 9, and other noise components are eliminated by the characteristics of the narrow band variable filter 9. As a result, the C/N of the FM input signal is improved, and the frequency discriminator passes through the limiter circuit 3 and is detected by the filter 4, so the threshold level is improved. This is because due to the characteristics specific to FM demodulation of color TV signals, when the frequency deviation of the FM modulation signal is small, the demodulated image quality is disturbed by noise, and in particular color noise causes deterioration of the image quality. As is well known, a color TV signal consists of a luminance signal and a color subcarrier signal, and its spread bandwidth is 4.2 M.
It is a wideband signal that extends up to I (Z). Also, the frequency spectrum of the υ paced signal for TV signals changes significantly depending on the type of subject being transmitted, and in particular, the color and The amplitude of the carrier signal component varies significantly. Normally, emphasis is applied to the FM transmission BK of a TV signal, but in this case, the amplitude of the low frequency part of the TV signal, that is, mainly the luminance signal component, is attenuated, and the amplitude of the wide range part, that is, mainly the color subcarrier component, is attenuated. For this purpose, the amplitude is increased to produce an FM wave modulation signal. Therefore, it is mainly the color subcarrier component that gives the largest frequency shift of the FM modulation signal, and especially when transmitting the color par signal, which is a representative signal among them, the shift is maximum. .

However, in a normal image, the color subcarrier component is very small (almost like a black and white TV signal), and a small frequency shift often occurs. The demodulation of a color TV signal will be explained using the method of the second conventional embodiment (dynamic tracking filter method) as an example. In FIG. 2, in order to faithfully feed back the TV signal demodulated by the frequency discriminator 4 to the narrowband variable filter 9, the band of the low-pass p-wave filter 6 must be at least 4.2.
It must have a passage characteristic up to -. At this time,
The feedback TV signal changes the pass center frequency of the narrowband variable filter 9 according to the frequency shift in the modulation frequency component within 4.2 MHz of the FM input signal,
The frequency shifter 9- effectively passes the frequency-shifted signal component of the FM input signal to the limiter circuit 3 and frequency discriminator 4. However, this signal component and the noise component of the same frequency can also be effectively processed by the narrowband variable E-wavelength converter 9.
is passed through and demodulated.

Now, considering the case where the brightness and color subcarrier components of the transmitted TV signal are low, the frequency shift of the FM modulation signal is small, and the frequency spectrum is also small. In such a situation, the required transmission bandwidth may be inherently narrow. However, since the frequency shift of the FMi tone signal is small at this time, the signal component becomes small in the output of the frequency discriminator 4, but the noise component does not decrease at all. In other words, the demodulated signal at this point has decreased. In this case, since the signal component and the noise component included in the baseband band of 4.2 MHz act on the narrowband variable frequency transducer 9 through the low-pass transducer 6, the same modulation frequency component as these in the FM input signal is transmitted. The signal and noise components contained in the signal are effectively demodulated by the frequency discriminator 4. As a result of these, the demodulated TV monitor! Regarding the image quality above, the noise component is emphasized compared to the signal component, especially when the amplitude of the color subcarrier is small and the color density is low.
Due to these noise effects, dark colors are generated and act as color noise, resulting in a significant deterioration of image quality.

The above is the same in the case of the first conventional embodiment shown in FIG. That is, when the frequency deviation of the FM input signal is small, the signal component at the output of the frequency discriminator 4 becomes small, but the noise component does not decrease at all, and the demodulated signal decreases. and low pass filter 6
Includes signal components through and baseband up to 4.2 MHz. The noise component acts on the voltage variable frequency oscillator 5 to perform local oscillation, and the frequency positioner 1 converts the difference frequency component between the FM input signal and the local oscillation signal into a narrow band pass F.
The signal is sent to the frequency discriminator 2 and demodulated by the frequency discriminator 4 at the subsequent stage. Therefore, noise frequency components up to 4.2 MHz act on the voltage variable frequency oscillator 5, and
．． Since the modulation noise component accompanying the FM input signal at 2 MHz i is effectively demodulated by the frequency discriminator 4, the image quality deteriorates significantly as described above.

In order to eliminate the above-mentioned drawbacks, the present invention inserts a non-linear circuit whose input/output amplitude ratio changes depending on the input amplitude level into the feedback circuit of the conventional threshold improvement method using a narrowband filter. When the frequency shift of the signal is small, the amount of feedback is reduced to improve noise in the demodulated image quality, and will be explained in detail below.

FIG. 3 shows a block diagram showing the first embodiment of the present invention, in which 6 is a low-pass F-wave device, and 10 is a non-linear image whose input/output amplitude ratio changes with respect to the input amplitude level. Passing F
It is fed back to the narrowband variable filter 9 via the wave filter 6, and the “C/′
The effect of improving the threshold level by improving N has been explained.

The problem here is that when the frequency deviation of the FM input signal is very small, the noise components of the input-resistant signal are fed back to the narrowband converter 9 through the feedback circuit, and these FM input signal All of the noise components are effectively passed through the narrowband variable p-wave device 9 and demodulated, resulting in a significant deterioration in image quality. Therefore, if the deviation of the FM input signal is sufficiently small, by automatically opening the feedback circuit, the reduced harmonic signal will not be fed back to the narrowband variable filter 9.
The above problems can be solved. In FIG. 3, a nonlinear circuit 1 is connected to the feedback path from the output end of the frequency discriminator 4 to the narrowband variable ν wave generator 9 via the low-pass F wave generator 6.
0 is inserted, and the low-pass waveform generator 6 and the nonlinear circuit 10 constitute a feedback 5 circuit. Incidentally, the order of connection of the low-pass furnace, the wave device 6, and the nonlinear circuit 1.0 can be changed from that shown in FIG. 3. When the deviation of the FM input signal is small, the frequency discretization is performed by the nonlinear circuit 10! The feedback path from the output end of the converter 4 to the narrowband variable F wave generator 9 is brought close to an open state, and when the deviation of the FM input signal is large, the feedback path is closed by the nonlinear circuit 10, and the demodulated signal is This is to make them return home.

FIG. 4 shows the amplitude S of the output signal relative to the amplitude S of the input signal of the nonlinear circuit 10. shows the characteristics of The nonlinear circuit 10 outputs an output signal S in a region where the amplitude S of the input signal is small. It has a characteristic that an output signal S0 is obtained in a region where the amplitude level of the input signal is large to some extent.

The degree of curvature of the characteristic curve of the nonlinear circuit 10 is determined by the relationship between the desired degree of noise improvement and the distortion tolerance of the demodulated signal.

That is, in the nonlinear circuit 10 having the characteristics shown in FIG. 4, when the deviation of the FM input signal is small, the amplitude Si of the input signal, which is the demodulated signal, is small and the output signal S cannot be obtained, and the narrow band variable F Although the demodulated signal is not fed back to the wave generator 9,
When the amplitude S of the input signal, which is the demodulated signal, becomes large, an output signal S0 is generated, and the demodulated signal is fed back to the narrowband variable filter 9.

Non-linear circuits 1θ having the characteristics shown in FIG. 4 are connected in parallel in the real direction, and this non-linear circuit 10 has the characteristics shown in FIG. 3 through signal input and output terminals Ae and B. It is inserted in series in the return path 5. In the circuit configuration shown in FIG. 5, when the amplitude Si of the input signal is small, the diodes D1. D2 exhibits high resistance, and no current flows through the load R, making it impossible to obtain the output signal S0. However, when the amplitude S of the human input signal increases to a certain extent, the resistance value of either diode D or D2 becomes low. The flow flows. As a result, the amplitude (voltage) So of the output signal is generated in the load R. Therefore, a nonlinear circuit 10 having the characteristics shown in FIG. 4 can be obtained. Also in this case,
Although not shown in FIG. 5, the diode D1. By applying bias voltages to D2, the operating points of the two diodes D, , D2 are arbitrarily adjusted, and the shape of the characteristic curve shown in FIG. 4 is changed. be able to.

Therefore, in the first embodiment of the present invention shown in FIG. The noise component is removed and fed back to the narrowband variable filter 9, and the passing center frequency of the narrowband variable filter 9 is changed. Since the passing center frequency of the narrowband i1P wave generator 9 is controlled to change in accordance with the instantaneous frequency change of the FM signal input from the input terminal 7, it is possible to A signal with a modulation frequency component in the same range as the frequency passing through the bandpass p-wave filter 6 is effectively extracted by the narrowband variable frequency filter 9, and other noise components are eliminated by the characteristics of the narrowband variable filter 9. be done. As a result, the FM input signal is improved and passed through the limiter circuit 3 and detected by the frequency discriminator 4, so that the threshold level is improved. Furthermore, when the deviation of the FM input signal is small, the nonlinear circuit 10 is in an open state, and the demodulated signal with a reduced level is not fed back, and the noise component is emphasized compared to the signal component as described above. This will no longer be the case.

Furthermore, it is also possible to use a band-pass P-wave device that passes a specific frequency in the low-pass F-wave @6D frequency, and for example, it is possible to pass a specific frequency such as a color subcarrier frequency. In addition, in the first embodiment S, feedback of the demodulated signal is increased in the narrowband variable p-wave device 9, the limiter circuit 8, and the frequency disk, and the threshold is improved by improving the Q factor. Therefore, the other symbols are the same as those shown in FIG. Generates a difference frequency component between the local oscillation signal and the j1'M input signal, and generates a narrowband F wave generator 2,
The signal passes through a limiter circuit 3 and is demodulated by a frequency discriminator 4.

The second embodiment also improves demodulation noise retention based on the same principle as described in the first embodiment. That is, F
When the frequency deviation of the M signal is small, the voltage variable frequency oscillator 5 is controlled by the noise component fed back through the low-pass F wave generator 6 in the conventional method shown in FIG. 1, and the change in the oscillation frequency due to the noise is It acts in the same direction as the frequency change due to noise accompanying the FM input signal. Therefore, those noise components effectively pass through the narrow band pass F-wave unit 2 and are demodulated. on the other hand,
Since the frequency deviation due to the signal is small in this case, the feedback of the detected signal component to the voltage variable frequency oscillator 5 is small, and rather the oscillation frequency of the voltage variable oscillator 5 may be kept in a fixed state.These results , when the intensity of the input signal is low, the above-mentioned noise detection component is large, and the nonlinear circuit 1o shown in FIG.
When the frequency deviation of the signal is small and the detection signal of the discriminator 4 is small, the amount of feedback to the voltage variable frequency oscillator 5 is small.As a result, the feedback due to the above-mentioned detection noise, which is a problem, becomes small. The detection noise component from the discriminator 4 is improved, and the demodulated image quality is improved.

Here, the non-linear circuit 1o shown in FIG. 6 is the same as that explained with reference to FIGS. 4 and 5 in the first embodiment, so a description thereof will be omitted.

The present invention uses a non-linear circuit whose input/output amplitude ratio changes depending on the input amplitude level of a signal feedback circuit of an FMi modulation method such as a conventional frequency feedback method or a dynamic tracking filter method. It is changed to i depending on the size of the shift. As a result, FM
When demodulating a modulated signal, not only the threshold level is improved, but also the noise of the demodulated image quality is improved, so it can be used particularly effectively as a demodulator for a satellite reception station.

[Brief explanation of the drawing]

Figure 1 is a block diagram of a conventional demodulation method using frequency feedback, and Figure 2 is a block diagram of a conventional demodulation method using dynamic tracking. 3 is a block diagram of the first embodiment of the present invention, FIG. 4 is an explanatory diagram showing the characteristics of the nonlinear circuit 10, and FIG. 5 is a circuit diagram showing an embodiment of the nonlinear circuit 10. FIG. 6 is a block diagram of a second embodiment of the invention. 1... Frequency converter, 2... Narrow band pass p-wave converter, 3
...Limiter circuit, 4...Frequency discriminator, 500. Voltage variable frequency oscillator, 6...Low pass p wave filter, 2...FM signal input terminal, 8...FM signal detection output terminal, 9...Narrow band variable filter, lO.
...Nonlinear circuit, Dl, D2...diode. Figure 3 Figure 5 Figure 1 Figure 6 17-

Claims (2)

[Claims] (1) Connect the FM signal input terminal, frequency conversion μ, narrow band pass filter, limiter circuit, frequency discriminator, and FM detection output terminal in series, and further connect the output terminal of the frequency discriminator to the signal A voltage variable frequency oscillator is connected to the voltage variable frequency oscillator through a feedback circuit formed by connecting a nonlinear circuit in series with a low-pass P-wave device or a band-pass P-wave device that heats frequency components, and the non-linear circuit is connected to the voltage variable frequency oscillator. The input amplitude (, ) and output amplitude (b) of the linear circuit are expressed as the input amplitude (,
) is increased so that the value of - becomes larger, and the detected signal obtained from the frequency discriminator is sent to the voltage variable frequency oscillator, and the local oscillation is the output of the voltage variable frequency oscillator. A signal is applied to the frequency converter, the frequency converter generates a difference frequency component between the local oscillation signal and the FM input signal, and the difference frequency component is applied to the narrowband F wave generator, limiter circuit, and frequency distributor. A high-sensitivity FM detection method characterized by performing FM detection through a Z liminator. (2) Connect the FM signal input terminal, narrowband variable F wave device, limiter circuit, frequency 4-number discriminator, and FM detection output terminal in series, and further connect the output terminal of the frequency discriminator to the signal frequency component. Low frequency pass iF
The input amplitude of the non-linear circuit is connected to the narrow band variable filter via a feedback circuit formed by connecting a wave filter or a band-pass filter and a non-linear circuit in series, and the input amplitude of the non-linear circuit is (,) and the output amplitude (b) by changing the amount of feedback so that the - value increases as the input amplitude (a) increases, and the detected signal obtained from the frequency discriminator is adjusted to the narrow band range. Feedback to the F wave filter, change the passing center frequency of the narrowband variable filter by the feedback signal to make the FM input signal F wave, the limiter circuit,
A highly sensitive FM detection method characterized by performing FM detection through a frequency discriminator.