JPS6253084B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a frequency demodulation method that improves the noise characteristics of a demodulated signal of an FM modulated wave with a simple configuration.

Conventionally, as the simplest method for demodulating a frequency-modulated signal, a frequency demodulation method using a discriminator using an LC circuit or a delay line has been known and widely used for a long time.

In this case, the signal-to-noise ratio S/N (hereinafter referred to as S/N) of the FM demodulated (detected) signal to the carrier power-to-noise power ratio C/N (hereinafter referred to as C/N) of the FM-modulated input signal ) is S/N=
It is expressed as (C/N)·FI (here, FI indicates the degree of FM improvement), and the S/N of the demodulated signal is proportional to the C/N of the input signal.

On the other hand, the C/N of the input signal is determined by the noise used on the input side of the FM demodulator and the passband width of a bandpass filter for limiting the signal band. Normal C/N
The above-mentioned C/N vs. S/N relationship is maintained up to approximately 10 dB, and when the C/N of the input signal is lower than that, the S/N of the demodulated signal deteriorates rapidly. This point of change is called the threshold point.

Generally, in communications that transmit TV signals, such as satellite communications, an FM modulation method is used to transmit the signals. In this case, the received signal level is often set near the threshold point due to limitations on the transmission power of the satellite in the communication line and the problem of a stable satellite communication propagation path. Therefore, the received input decreases due to changes in environmental conditions such as rain and snow, and the noise in the demodulated signal increases rapidly, sometimes to the point where no demodulated image can be obtained. Therefore, it is considered to be a very important problem to improve the noise characteristics using a simple method when receiving satellites.Especially in simple satellite receiving equipment such as broadcasting satellite communication, this problem has led to the development of noise improvement methods using simple configurations. is considered an extremely important issue.

The present invention provides a solution to the above problem,
By using a non-linear resonant system that combines a resonant circuit and a non-linear resistor, the input signal level is reduced from a specific value by connecting a band-pass waver with a fixed reference bandwidth to the above-mentioned non-linear resonant system. Accordingly, the passband width of the signal and noise is automatically narrowed, the C/N entering the frequency discriminator is improved, and the S/N (demodulated image quality) of the demodulated signal is improved.

FIG. 1 is a diagram showing a demodulation system according to a first embodiment of the present invention, in which reference numeral 1 denotes a reference bandpass waver having a bandpass characteristic that is flat within a fixed band and provides steep attenuation outside the band; 2 is a nonlinear resonant system whose Q value changes according to the input signal level and the passband width; 3 is a frequency discriminator; 4 is an amplifier whose input/output amplitude characteristics change nonlinearly to some extent;
A circuit such as a mixer, 5 a circuit such as an AGC amplifier or a limiter, 6 an FM signal input terminal, and 7 an FM signal demodulation (detection) signal output terminal.

FM input signal is input from input terminal 6 and output terminal 7
obtained as a demodulated signal.

As explained earlier, in normal satellite communication
The FM method is used to transmit TV signals. In this case, due to limitations on satellite transmission power and stability of satellite communication lines, the received signal level is set within a range that is several dB higher than the threshold level at most. Therefore, the received signal level may sometimes fall below the threshold level due to the influence of rain, snow, etc. At this time, the S/N of the demodulated signal rapidly deteriorates, and the quality of the demodulated image on the monitor television deteriorates significantly, making it impossible to identify the image.

This threshold level normally occurs when the C/N of the input signal is about 10 dB, and this is due to the bandwidth B of the reference bandpass converter 1, which determines the passband width of the signal and noise used in the previous stage of the demodulation system. Determined by _p .

The bandwidth B _p is normally set to about B _p 〓2(Δ+h) in order to sufficiently pass the modulated signal energy and maintain good waveform distortion characteristics of the demodulated signal. Here, Δ represents the highest frequency deviation, and h represents the highest modulation frequency.

By the way, when looking at the image quality on a monitor of a TV signal transmitted and demodulated using the FM method, the good image quality is mainly due to deterioration of image quality due to thermal noise etc. that enters the demodulation system, and distortion of the phase and amplitude characteristics of the transmission path. It is determined by the distortion of the image due to the distortion of the image, and is greatly influenced by the bandwidth B _p . In general, narrowing the bandwidth B _p is advantageous in terms of noise due to thermal noise, etc., but distortion in image quality increases.

Now, when the bandwidth B _p is selected to be about B _p 〓2(Δ+h), looking at the demodulated image quality of the TV input signal near the threshold point, it is found that the image quality deterioration due to thermal noise is dominant. , the deterioration of image quality due to transmission path distortion is hardly noticeable. When the signal level is below the threshold point, the image quality deteriorates significantly as the level decreases, and the image is rapidly disturbed by thermal noise, to the point where it becomes unrecognizable.

Therefore, if the C/N of the input signal is improved by narrowing the passband width of the signal in response to the decrease in the level of the input signal near the threshold point,
At the cost of an increase in image quality distortion, image quality deterioration due to thermal noise is greatly improved, and overall good demodulated image quality is obtained. This effect is very large because the S/N of the demodulated signal deteriorates rapidly with respect to the C/N of the input signal below the threshold.

The principle of the present invention is that, at points where the input signal is at or below the threshold point determined by the reference bandpass transducer 1, the passband width is changed to the reference bandpass waveform by the nonlinear resonant system 2 having the configuration of the present invention. The input signal level is set to be narrower than the bandwidth of the receiver 1, and as the signal level decreases, the passband width is automatically compressed to improve the C/N of the input signal, thereby reducing noise in the demodulated image quality. It measures improvement. Among them, the reference bandpass wave generator 1 and the nonlinear resonant system 2
Due to the continuous connection of , even if a simple non-linear resonant system 2 is used, a steep out-of-band attenuation characteristic can be obtained as an overall band-pass characteristic.

FIG. 2 shows an example of the non-linear resonant system 2, which is constructed from parallel resonant circuits.

8 is a capacitor, 9 is a coil, 101, 102
11 is a transmission line, 12 is a signal input terminal, and 13 is a signal output terminal.

In this case, the resonant frequency approximately matches the signal frequency. In addition, in this example, the nonlinear resistance element 101,
Although a variable resistance diode is used as the variable resistance element 102, other variable resistance elements such as a transistor may also be used.

An input signal enters through a signal input terminal 12, is influenced by the nonlinear resonance system 2, and is sent out from a signal output terminal 13.

When two nonlinear resistance elements 101 and 102 are connected in parallel as shown in FIG. 2, the overall characteristic of voltage V vs. current I of the nonlinear resistance elements 101 and 102 is as shown in FIG. 3. . Also, this is the voltage V
The resistance value Rd of the non-linear resistance elements 101 and 102 is expressed as shown in FIG.

In the resistance characteristics of the nonlinear resistance elements 101 and 102 shown in FIG. 4, when a high frequency input signal 16 is applied as shown in the figure, when the level e of the input signal 16 is small, the nonlinear resistance elements 101 and 102
indicates a high effective load resistance Rdeff. Also, input signal 1
When the level e of 6 increases, the nonlinear resistance element 10
The effective load resistance Rdeff of 1,102 drops rapidly.

Therefore, the characteristics of the effective load resistance Rdeff of the nonlinear resistance elements 101 and 102 with respect to the level e of the input signal 16 are shown in FIG. On the other hand, when C is the capacitance of the capacitor 8 and ω _p is the angular frequency of the resonant circuit, the Q value of the parallel resonant circuit is expressed as Q=ω _p CRdeff. Therefore, the input signal 16 has a level e
The Q value is high in a region where the value is small, and as the level e of the input signal 16 increases, the Q value decreases.

That is, the passage characteristic of the signal from the signal input terminal 12 to the signal output terminal 13 in FIG.
The transmission (pass) band characteristics are as shown in FIG. 6. Also, FIG. 7 shows the passband width B for the level e of the input signal 16 at this time.
shows the characteristics of

Let the passband _width of the reference bandpass transducer 1 of the demodulation system in _{FIG .} do. At this time, the level of the nonlinear resonant system 2 is set so that the passband width is larger than B _p with respect to the input signal level e _p in the input signal level vs. passband width characteristic shown in FIG.

Therefore, the reception bandwidth of the entire demodulation system shown in FIG. 1 becomes the bandwidth B _p of the reference bandpass transducer 1 in a state where the signal level is equal to or higher than e _p .

Furthermore, when the receiving input signal level decreases, the passband width of the nonlinear resonant system 2 becomes narrower, and therefore the receiving bandwidth of the entire demodulation system becomes narrower than the bandwidth B _p of the reference bandpass waver 1. The C/N is improved compared to the case where the bandwidth B _p is .

The degree of compression of the signal and noise passing bandwidth for the input signal of the entire demodulation system shown in FIG. It is adjusted according to the input/output characteristics of the circuit 4.

Further, the discriminator 3 is constituted by an ordinary LC circuit or delay line.

As explained above, in the first embodiment, the nonlinear resonant system 2 uses a parallel resonant circuit and nonlinear resistance elements 101 and 102 to automatically change the passband width depending on the input signal level, and the nonlinear resonant system 2 has a fixed band width. A reference bandpass transducer 1 having a pass characteristic is connected in cascade, and the reception bandwidth of the demodulation system is changed according to the level of the reception input signal. That is, for a received input signal that is equal to or higher than the threshold determined by the reference bandpass waveform generator 1, it is demodulated using the reception bandwidth of the reference bandpass waveform generator 1 to achieve an optimal state regarding waveform distortion of the demodulated signal, and the reception input signal is demodulated using the reception bandwidth of the reference bandpass waveform generator 1. When the signal becomes near or below the threshold, the passband width of the nonlinear resonant system 2 is narrowed to improve the C/N. As a result, the influence of thermal noise is significantly reduced even at low received input signal levels, and the noise characteristics of the demodulated signal (demodulated image quality) are improved.

In the first embodiment, a case was explained in which a parallel resonant circuit was used as the nonlinear resonant system 2, but similar effects can be obtained in a resonant system consisting of a series resonant circuit and a nonlinear resistance element, and the following will be explained. explain.

Figure 8 shows the nonlinear resonant system 2 in the second embodiment.
The configuration is shown below. The entire demodulation system is the same as that shown in FIG. In FIG. 8, 103 and 104 are nonlinear resistance elements, and 14 is a nonlinear resistance element 103 and 10.
4 is a DC bias power supply, 15 is a return path for DC bias, and other symbols are the same as in FIG.

The non-linear resistance elements 103 and 104 are connected in series in opposite directions to the signal transmission path, and are each biased in the forward direction by the DC bias power supply 14.

Nonlinear resistance element 1 now forward biased
When 03 and 104 are connected in series, P in Figure 8
The overall characteristic of the combined voltage V versus current I of the nonlinear resistance elements 103 and 104 seen from a point is as shown in FIG. Also, this is expressed as voltage V vs. nonlinear resistance element 103,1
If the resistance value R _d is expressed as 04, it will be as shown in FIG.

Nonlinear resistance elements 103, 104 with this characteristic
In contrast, a high frequency input signal 17 is applied as shown in FIG.

When the level e of the input signal 17 is small, the nonlinear resistance elements 103 and 104 have a low effective load resistance.
Indicates Rdeff. Furthermore, as the level e of the input signal 17 increases, the effective load resistance of the nonlinear resistance elements 103 and 104 increases rapidly. Therefore, the nonlinear resistance elements 103 and 104 for the level e of the input signal 17
The characteristics of the effective load resistance Rdeff are shown in FIG.

On the other hand, the nonlinear resonant system 2 in this embodiment is the eighth
As shown in the figure, a series resonant circuit consisting of a capacitor 8 and a coil 9 and non-linear resistance elements 103 and 104
It consists of Therefore, when expressed using the same symbol as in the first embodiment, the Q value of the series resonant circuit is Q=5ω
It is given as _p C/Rdeff. Therefore, when the level e of the input signal 17 is small, the effective load resistance
Since Rdeff is small, the Q value becomes large, and when the level e of the input signal 17 becomes large, the effective load resistance
Rdeff increases and the Q value decreases.

That is, the passage characteristics of the signal from the signal input terminal 12 to the signal output terminal 13 in FIG. 8 change depending on the level e of the input signal 17, and the transmission (passage)
The band characteristics are as shown in FIG. 6, the same as in the first embodiment.

Therefore, when the level of the received input signal is high, it is demodulated using the reception bandwidth of the reference bandpass waver 1 to achieve the optimum state regarding waveform distortion of the demodulated signal, and when the level of the received input signal is low, non-linear resonance occurs. System 2
The passband width is narrowed and the C/N ratio is improved, and the noise characteristics of the demodulated signal are also improved as in the first embodiment.

The present invention utilizes a demodulation system that automatically narrows the receiving bandwidth when the input signal level becomes low by cascading a reference band pass waveformer 1 and a simple nonlinear resonant system 2 consisting of a resonant circuit and a nonlinear resistance element. form a system. Therefore, when the received signal level is low, C/
N and improve the noise characteristics of the demodulated signal.

Therefore, it can be effectively used as an S/N improvement method especially for simple satellite receivers that receive TV modulated signals.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of an embodiment of the present invention, FIG. 2 is a circuit diagram showing a nonlinear resonant system in the first embodiment,
3 to 7 are explanatory diagrams showing the operation of the first embodiment, and FIG. 3 shows the voltage V of the nonlinear resistance element.
Fig. 4 is an explanatory diagram showing the characteristics of the resistance value R d of the nonlinear resistance element versus voltage V. Fig. 5 is an explanatory diagram showing the characteristics of the resistance value R _d of the nonlinear resistance element versus the input signal level e. FIG. 6 is an explanatory diagram showing the characteristics of the resistance Rdeff, FIG. 6 is an explanatory diagram showing the transmission band characteristics of a nonlinear resonant system, and FIG. 7 is an explanatory diagram showing the pass band width characteristics with respect to the input signal level e. Figure 8 is a circuit diagram showing a non-linear resonance system in the second embodiment;
11 are explanatory diagrams showing the operation of the second embodiment, FIG. 9 is an explanatory diagram showing the characteristics of voltage V vs. current I of the nonlinear resistance element, and FIG. 10 is an explanatory diagram showing the characteristics of voltage V vs. nonlinear resistance element. FIG. 11 is an explanatory diagram showing the characteristics of the resistance value R _d of the linear resistance element, and FIG. 11 is an explanatory diagram showing the characteristics of the effective load resistance Rdeff of the nonlinear resistance element with respect to the level e of the input signal. 1... Reference band pass wave device, 2... Non-linear resonant system, 3... Frequency discriminator, 4... Circuits such as amplifiers and mixers, 5... Circuits such as AGC amplifiers and limiters, 6... FM Signal input terminal, 7...
...Demodulated signal output terminal, 8...Capacitor, 9...
Coil, 101, 102, 103, 104...Nonlinear resistance element, 11...Transmission line, 12...Signal input terminal, 13...Signal output terminal, 14...DC bias power supply, 15...DC bias return path, 16,
17...Input signal.

Claims (1)

[Scope of Claims] 1. A nonlinear resonant system consisting of at least a reference bandpass transducer with a fixed bandwidth, an element including a nonlinear resistance, and a parallel resonant circuit or a series resonant circuit, and a frequency discriminator. and the resonant circuit are connected in cascade in the transmission direction, and as the input signal level to the resonant circuit increases, the Q value of the resonant circuit decreases, the passband width widens, and the input signal level decreases. Accordingly, the non-linear resonant system in which the Q value of the resonant circuit is increased and the passband width is narrowed allows the FM determined by the bandwidth of the reference bandpass waver to be
The passband width of the non-linear resonant system is set to be wider than the passband width of the reference band pass waver for a received signal equal to or higher than the threshold level, with a boundary near a threshold level of the modulated received signal; For a received signal below the threshold level, the passband width of the nonlinear resonant system is made narrower than the passband width of the reference band pass waver, and the receive bandwidth is adjusted in accordance with the level of the received signal. It is characterized by changing the noise characteristics and improving the noise characteristics.
FM signal demodulation method.