JPH0422587Y2 - - Google Patents

Description

[Detailed Description of the Invention] (Field of Industrial Application) The present invention relates to an FSK signal demodulation device that converts an FSK modulated signal into a binary signal.

(Prior Art) In a CATV system, as shown in FIG. 5, a CATV signal transmitted through a trunk cable 1 is branched at a tap 2 and supplied to each terminal device 3. Then, by the terminal device 3
A television broadcast signal of a receivable channel is selected from among the CATV signals and subjected to appropriate signal processing, and the processed signal is received by the television receiver 4.

In such a CATV system, the terminal device 3
Digital data necessary for billing and prevention of eavesdropping, etc. is collected together with television broadcast signals.
It is transmitted superimposed on the CATV signal. Usually, this digital data is FSK modulated.

FIG. 6 shows a block circuit diagram of an example of a data receiver that demodulates this FSK-modulated digital data. In FIG. 6, a band pass filter 5 extracts a band signal of digital data from the CATV signal, the extracted signal is amplified by a high frequency amplifier 6, and further provided to a mixer 8 via another filter 7. This mixer 8 includes a local oscillator 9
An intermediate frequency signal is extracted from the frequency-converted signal by a filter 10 and supplied to an intermediate frequency amplifier 11. Then, the FSK-modulated intermediate frequency signal output from the intermediate frequency amplifier 11 is demodulated by the FM demodulator 12, and this demodulated signal is further converted into digital data by the binary signal converter 13 and output. Here, since the demodulated signal obtained by demodulating the FSK modulated signal by the FM demodulator is not a rectangular wave, it is converted into digital data of a binary signal by the binary signal converter 13. These FM demodulator 12 and binary signal converter 13 form an FSK signal demodulator.

Here, a circuit diagram of a conventional FSK signal demodulator is shown in FIG. In FIG. 7, the FM demodulator 12
The demodulated signal output from is smoothed by a large time constant circuit consisting of resistors R1 and R2 and capacitor C1, and the average voltage Va of the demodulated signal is outputted from voltage comparator 14.
is given to the negative input terminal of Further, the demodulated signal is applied to the positive input terminal of the voltage comparator 14 via a small time constant circuit consisting of capacitors C2 and C3 and resistor R3. Furthermore, the demodulated signal is given to a noise squelch circuit 15. The output terminal of the voltage comparator 14 is connected to the base of the transistor Tr1, and the output is taken out from the collector of the transistor Tr1 to the output terminal 16. Further, another transistor Tr2 controlled by the output of the noise squelch circuit 15 is interposed between the base of this transistor Tr1 and the ground. Note that the noise squelch circuit 15 extracts noise components with a frequency higher than the frequency of the demodulated signal using a high-pass filter, and when the noise components extracted by the high-pass filter are at a predetermined level or higher, the transistor
It is configured to make Tr2 conductive.

In such a configuration, the FM demodulator 12 to the eighth
As shown in Figure a, a demodulated signal containing low-level noise components is output, and this demodulated signal is converted into a waveform from which only the noise components are removed by the time constant circuit and sent to the voltage comparator 14.
is applied to the positive input terminal of , and also Va
The average voltage of this demodulated signal is applied to the negative input terminal of the voltage comparator 14, as shown in FIG. Therefore, the voltage comparator 14 outputs a rectangular waveform as shown in FIG. Output. Here, the level of the noise component superimposed on the demodulated signal is low, and the noise squelch circuit 15 does not operate.

Furthermore, when there is no FSK modulated signal, a noise component with a relatively high frequency and a high level is output from the FM demodulator 12 as shown in FIG. 9a. This noise component and the average voltage Va of this noise component are compared by a voltage comparator 14 to produce a binary signal output as shown in FIG. 9d. However, the noise squelch circuit 15 operates to make the transistor Tr2 conductive and keep the collector of the transistor Tr1 at a high level, so that the binary signal due to the noise component is output to the output terminal 16.
is not output to .

(Problem to be solved by the invention) However, when receiving an unmodulated signal,
As shown in Figure 10a, low level noise components
It is output from the FM demodulator 12. Then, the average voltage Va of the noise component and the noise component are compared by the voltage comparator 14 to generate a binary signal based on the noise component as shown in FIG. 10d. At this time, since the level of the noise component as a demodulated signal is low, the noise squelch circuit 15 does not operate, and the 10th
There is a problem in that the binary signal of the noise component shown in FIG. d is inverted as is and outputted to the output terminal 16.

The purpose of the present invention was to solve the above-mentioned problems of the FSK signal demodulator, and to provide an FSK signal demodulator that does not output a binary signal of noise components even when receiving an unmodulated signal. It is something to do.

(Means to be solved by the invention) In order to achieve this purpose, the FSK of the invention
The signal demodulator includes an FM demodulator that demodulates the FSK modulated signal, and a pseudo signal that is set at a predetermined level and has a frequency higher than the frequency of the demodulated signal, and superimposes the demodulated signal output from the FM demodulator. A superimposition circuit that outputs a signal, a voltage comparator that compares the superimposed signal with an average voltage of the superimposed signal or the demodulated signal and outputs a binary signal, and is provided on the output side of the voltage comparator. and a low-pass filter having a cut-off frequency lower than the frequency of the pseudo signal and higher than the frequency of the demodulated signal.

(Function) Since a pseudo signal with a frequency higher than the frequency of this demodulated signal is superimposed on the demodulated signal output from the FM demodulator in the superimposition circuit, when an unmodulated signal is received, a binary signal based on the pseudo signal is output from the voltage comparator. frequency. Since the frequency component of the pseudo signal is attenuated by a low-pass filter provided on the output side of the voltage comparator, no output is generated at the output terminal. Furthermore, when receiving the modulated signal, the level of the pseudo signal is lower than that of the demodulated signal, so the superimposed signal is not significantly modified by the demodulated signal, and is converted into a binary signal by the voltage comparator and sent to the output terminal. Output.

(Description of Embodiments) Hereinafter, embodiments of the present invention will be described with reference to FIGS. 1 to 4. Figure 1 shows the present invention.
It is a circuit diagram of an example of an FSK signal demodulation device,
FIG. 2 is a characteristic diagram of the low-pass filter in FIG. 1, FIG. 3 is a waveform diagram explaining the operation when receiving a modulated signal, and FIG. 4 is a diagram explaining the operation when receiving a non-modulated signal. FIG. In Figure 1,
Circuits that are the same as those in FIG. 7 are given the same reference numerals, and redundant explanation will be omitted.

The differences between Figure 1 and Figure 7 are as follows:
A pseudo signal outputted from the oscillator 17 is superimposed on the demodulated signal outputted from the FM demodulator 12 in a superimposition circuit 18, and each superposed signal is given to the voltage comparator 14 via a time constant circuit. Further, the collector of the transistor Tr1 is connected to the output terminal 16 via the low-pass filter 19.

Then, the frequency fo of the pseudo signal is set to a high frequency, for example, about four times the frequency f of the demodulated signal,
Furthermore, the level of the pseudo signal is set to be sufficiently lower than the level of the demodulated signal, and higher than the level of the noise component of the demodulated signal when the unmodulated signal is received. Furthermore, the low-pass filter 19 has an upper cutoff frequency that is slightly lower than the frequency fo of the pseudo signal and much higher than the frequency f of the demodulated signal.

In this configuration, the operation when receiving a modulated wave signal will first be explained. The FM demodulator 12 outputs a demodulated signal on which a slight noise component is superimposed as shown in FIG. A superimposed signal like this is output. Then, this superimposed signal and its average voltage Va are calculated by the voltage comparator 1.
4, and a binary signal as shown in FIG. 3d is output. Here, the noise squelch circuit 15 is not operating, and the low-pass filter 19 passes a binary signal of the frequency of the demodulated signal, and the output terminal 16
In this case, the binary signal shown in FIG. 3d is inverted almost unchanged and output. Note that since the level of the pseudo signal is lower than that of the demodulated signal, the superimposed signal is only a slightly modified version of the demodulated signal, and the binary signal of the superimposed signal is different from the signal obtained by directly converting the demodulated signal into a binary signal. Almost the same. Furthermore, low pass filter 1
By setting the cutoff frequency 9 to a much higher frequency that is more than three times the frequency f of the demodulated signal, the binary signal is hardly transformed in the low-pass filter 19.

Next, when there is no signal, the FM demodulation circuit 12
A noise component having a relatively high frequency and a large level is output from the noise squelch circuit 15, and the noise squelch circuit 15 operates as in the conventional device shown in FIG. 7, and the collector of the transistor Tr1 is held at a high level.

Furthermore, when receiving an unmodulated signal, only a slight noise component is output from the FM demodulator 12 as shown in FIG. 4a, and the pseudo signal shown in FIG. A superimposed signal as shown in FIG. 4c is output. Then, this superimposed signal and its average voltage Va are compared by a voltage comparator 14, and a binary signal having the frequency fo of the pseudo signal is output as shown in FIG. 4d. However, the binary signal shown in FIG. 4d is attenuated by the low-pass filter 19, and no signal is output to the output terminal 16.

Therefore, a binary signal of a demodulated signal is output when a modulated signal is received, and no binary signal is output when there is no signal or when an unmodulated signal is received.

In the embodiment shown in FIG. 1, the voltage comparator 14 is configured to compare the superimposed signal and its average voltage, but instead of comparing the average voltage of the superimposed signal, it is compared with the average voltage of the demodulated signal. It's okay.

(Effects of the invention) As explained above, according to the FSK signal demodulation device of the invention, no binary signal is output when receiving an unmodulated signal, and a binary signal with a noise component is output as in the conventional device. This has an excellent effect in that it does not cause the problem of being damaged.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of an embodiment of the FSK signal demodulator of the present invention, Figure 2 is a characteristic diagram of the low-pass filter in Figure 1, and Figure 3 is the operation when receiving a modulated signal. FIG. 4 is a waveform diagram explaining the operation when receiving an unmodulated signal, FIG. 5 is a schematic diagram of the CATV system, and FIG. 6 is a waveform diagram explaining the operation when receiving an unmodulated signal. 1 is a block circuit diagram of an example of a data receiver that can be used.
7 is a circuit diagram of the conventional FSK signal demodulation device shown in FIG. 6, FIG. 8 is a waveform diagram illustrating the operation of the device shown in FIG. 7 when receiving a modulated signal, and FIG. , FIG. 10 is a waveform diagram illustrating the operation when there is no signal, and FIG. 10 is a waveform diagram illustrating the operation when receiving a non-modulated signal. 12...FM demodulator, 14...Voltage comparator, 1
6... Output terminal, 17... Oscillator, 18... Superposition circuit, 19... Low pass filter, R1 to R3...
...Resistor, C1-C3...Capacitor.

Claims (1)

[Scope of utility model registration request] FM demodulator that demodulates FSK modulated signal and this
a superimposition circuit that superimposes a pseudo signal having a frequency higher than the frequency of the demodulation signal and set at a predetermined level on a demodulation signal output from an FM demodulator to output a superposition signal; a voltage comparator that compares the average voltage of the demodulated signal and outputs a binary signal; and a voltage comparator that is provided on the output side of the voltage comparator and has a cut-off frequency that is lower than the frequency of the pseudo signal and higher than the frequency of the demodulated signal. An FSK signal demodulation device comprising: a low-pass filter having a low-pass filter;