JPS5855684B2 - FM signal pulse count demodulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a pulse count type demodulation device for FM signals.

Conventionally, it is known that there is a pulse count type demodulator as one type of demodulator for demodulating an FM signal.

That is, as shown in FIG. 1, the limiter circuit 2 limits the amplitude of the FM signal applied to the input terminal 1, the pulse conversion circuit 3 converts it into a pulse signal of a constant width, and then the signal is sent to the output terminal 5 via the integration circuit 4. It is used to obtain audio signals.

In the conventional example described above, a single monostable multi-vibration circuit such as one of which is shown in FIG. 2, for example, is used as the pulse conversion circuit 3.

However, in Fig. 2, the output pulse width is determined by the capacitor C1, a differential pulse with a time constant determined by the resistor R1, and the threshold level of the circuit. Since a differential pulse having a polarity opposite to the differential pulse at the leading edge is generated at the trailing edge, the voltage at the connection point at each leading edge of the output pulse is not constant due to the frequency change of the FM signal. As a result, the output pulse width becomes inconsistent and demodulation distortion occurs.

For example, if the FM signal is f(t)=f□+△f cosp
t, the signal waveform at each part in Fig. 2 is cosp
When t = l, as shown in Fig. 4, cospt knee 1
In this case, the output pulse width of D changes as shown in FIG.

Therefore, in order to improve such demodulation distortion, the time constant of the monostable multi-bi break can be made smaller to make the output pulse width smaller, but if this is done, the level of the demodulated output becomes smaller and the S/N ratio deteriorates.

On the other hand, if the time constant of the monostable multi-bi break is increased in order to improve the S/'N, demodulation distortion will increase.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a pulse count type demodulator for FM signals that solves the above-mentioned drawbacks and improves both demodulation distortion and S/N.

FIG. 3 is a block diagram showing one embodiment of the present invention, in which the configuration of the pulse conversion circuit 3 is particularly improved, and the other configurations are the same as in FIG. 1.

FIG. 6 is a signal waveform diagram at each part of FIG. 3.

In FIG. 3, the pulse conversion circuit 3 has two cascade-connected
It is composed of two monostable multi-bi breaks 6.7 and a logic operation element 8 which obtains the logical sum of each output pulse of the monostable multi-bi breaks 6.7.

The FM signal applied to the input terminal 1 is amplitude limited by the limiter circuit 2 to become a pulse signal as shown in FIG.

The monostable multi-bi break 6 operates at the leading edge of the pulse signal shown in Figure 6E, and the capacitor C is connected as shown in Figures 6F and G.
An output pulse having a width T1 determined by the time constant of the resistor R2 and the resistor R2 is obtained at the Q and Q terminals.

The monostable multi-bi break 7 operates at the trailing edge of the Q output of the monostable multi-bi break 6, and has a width T2 determined by the time constant of the capacitor C3 and resistor R3, as shown in FIG. 6H.
An output pulse of is obtained at the Q terminal.

Therefore, if the output pulse of the monostable multi-bi break 7 and the output pulse of the monostable multi-bi break 6 are applied to a logic operation element, for example, NAND 8, which obtains the logical sum of the input signals, the output pulse has the width of the two output pulses T1 and T2. The output pulse of FIG. 6I having a constant width T3 of the sum is obtained.

The output pulse from the logical operation element 8 is integrated by an integrating circuit 4 such as a low-pass filter, and an audio signal is obtained at an output terminal 5.

That is, since a plurality of monostable multivibrators are used in the pulse conversion circuit 3 and an output pulse with a width equal to the sum of their respective output pulse widths is obtained, the time constant of each monostable multivibrator can be made small. demodulation distortion is improved.

Further, by increasing the number of monostable multi-bi-breaks without increasing the time constant, a desired pulse width to be integrated can be obtained, and the S/N can also be improved.

Note that the logical operation element 8 is not limited to the NAND of the above embodiment, but may be any type as long as it can obtain the logical sum of input signals.

In addition, if each monostable multi-bi break has an operation time delay, each output pulse is delayed by a delay circuit using an inverter etc. so that the leading and trailing edges of each output pulse match, and then the logical sum is performed. Just take it.

As described above, according to the present invention, a plurality of monostable multi-by-breaks are used in the pulse conversion circuit of a pulse count type demodulator, and an output pulse with a constant width is obtained by adding the output pulse widths of the monostable multi-by-breaks. , it is possible to improve both demodulation distortion and S/N, which were contradictory conditions when using one conventional monostable multi-by-break.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a conventional FM signal pulse count demodulator, FIG. 2 is an example of the pulse conversion circuit in FIG. 1, FIG. 3 is a block diagram showing an embodiment of the present invention, and FIG. 5 is a signal waveform diagram at each part in FIG. 2, and FIG. 6 is a signal waveform diagram at each part in FIG. 3. 1...Input terminal, 2...Limiter circuit, 3...Pulse conversion circuit, 4...Integrator circuit, 5...Output terminal , 6, 7... Monostable multivibrator, 8... Logical operation element.

Claims (1)

[Claims] In a pulse count demodulation device that demodulates an I FM signal into an audio signal via a limiter circuit, a pulse conversion circuit, and an integration circuit, the pulse conversion circuit is operated at the trailing edge of each output pulse in the next stage, and the FM signal is A logical operation for obtaining the logical sum of a plurality of cascaded monostable multi-bi breaks whose time constant is sufficiently small to ignore changes in the output pulse width with respect to frequency changes, and the output pulses of each of the monostable multi-bi breaks. 1. A pulse count type demodulator for an FM signal, characterized in that the output of the limiter circuit is converted into a wide pulse obtained by ORing the output pulses of each of the monostable multi-bibreaks.