JPH0583037A - FM demodulation circuit - Google Patents

Abstract

(57) [Abstract] [Purpose] To prevent the occurrence of beats by reducing the jumping of FM sidebands into the demodulated signal. [Structure] Comparator, variable delay circuit, volume, N
Provided is an FM demodulation circuit which creates a pulse train having twice the frequency of an FM signal by using an OR circuit, makes the amplitude and width of the pulse constant and evenly spaced, and eliminates the FM frequency component.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an FM demodulation circuit which produces an output proportional to a frequency by performing a pulse train having a frequency double that of an input FM signal and integrating the pulse train by a low pass filter. Is.

[0002]

2. Description of the Related Art A conventional FM demodulation circuit of this type has the circuit configuration shown in FIG.
The signal (sine wave) is supplied to the first comparator 23 via the delay circuit 22 and directly to the second comparator 24.

As a result, the outputs of the comparators 23 and 24 have the waveforms shown in A, B, C and D of FIG. 4, respectively. Then, the respective outputs are respectively passed through NOR circuits 25 and 26,
4 becomes-(A + B) and-(B + C), which are further combined by the OR circuit 27 to be-(A + D) + {-in FIG.
(B + C)} waveform output, that is, a pulse train having twice the frequency of the input waveform.

This output pulse train is passed through a low pass filter (L
When the PF) 28 is integrated, an output proportional to the frequency of the input FM signal is obtained and the FM detection is performed.

[0005]

According to the conventional FM demodulation circuit, the FM carrier frequency at the time of demodulation is doubled, but the-(A + D) signal and-(B + C) which are the outputs of the NOR circuits 25 and 26. Since the pulse width, phase, and amplitude of the signal become unbalanced, the frequency component of the FM carrier remains, and this carrier frequency component leaks into the signal band, which may cause a beat.

[0006]

The present invention has been made in view of the above-mentioned drawbacks of the prior art. A pulse train which is twice as many as an input FM signal composed of a sine wave is formed and used as a low-pass filter. In the case of performing an FM detection by obtaining an output proportional to a frequency by performing integration, a first comparator for receiving the sine wave FM signal and obtaining a square wave output having a constant duty ratio, and the sine wave FM A second comparator for receiving a signal to obtain a square wave output having a variable duty ratio, and an input terminal for receiving the output of the first comparator through a variable delay circuit and the other input terminal for the second comparator. First to receive the output of the comparator
NOR circuit, a second NOR circuit that receives the output of the first comparator at one input terminal through a fixed delay circuit, and the output of the first comparator at the other input terminal, and the first NOR circuit. A variable resistor for adjusting the amplitude of the output of the circuit, and an output of the variable resistor and an output of the second NOR circuit to receive the frequency of the FM signal of 2
The present invention provides an FM demodulation circuit including an OR circuit that forms a pulse train having a doubled frequency and supplies the pulse train to the low pass filter. Further, according to the present invention, a variable volume is provided on the output side of the first NOR circuit.

[0007]

Therefore, according to the present invention, the input FM signal is first input to the first and second comparators, and then the signal that has passed through the delay circuit and the signal that does not pass through the NOR circuit are mutually passed through the NOR circuit and the frequency of the input FM signal. Make a pulse train proportional to. At this time, the delay amount of the signal and the comparison level of the comparator are adjusted so that the amplitude and width of the pulse train are constant and the pulse intervals are equal. After that, a signal component is extracted through a low pass filter.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the accompanying drawings. FIG. 1 shows an embodiment of an FM demodulation circuit according to the present invention. In the figure, 1 is an input terminal for an input FM signal, and 2 is a first comparator for receiving the FM signal at its (+) pole and comparing its voltage level with a reference voltage "0" received at its (-) pole. Therefore, duty 50
% Square wave is obtained.

A second comparator 3 receives the FM signal at its (+) pole and compares its voltage level with a variable reference voltage received at the (-) pole, so that the duty can be varied. Reference numeral 4 is a first volume for generating this variable reference voltage.

Numeral 5 is a variable delay circuit capable of changing the delay amount, which receives the output of the first comparator 2 and delays it according to the set delay amount, and outputs it to the next first NOR circuit 6 as a signal. Made to supply.

Reference numeral 7 is a delay circuit having a constant delay amount,
The second output of the comparator 2 is received, delayed by a predetermined amount, and supplied to the second NOR circuit 8 as a signal.

The first NOR circuit 6 receives the output of the second comparator 3 at another input terminal, and the second input terminal
The other input terminal of the NOR circuit 8 is designed to receive the output of the first comparator 2.

Reference numeral 9 is a second volume provided at the output terminal of the first NOR circuit 6.

The output of the first NOR circuit 6 whose voltage is adjusted by the second volume 9 is added to the output of the second NOR circuit 8 at a point (OR circuit) 10 and supplied to the low pass filter 11. It

Since the present invention is configured as described above, when the FM signal is input to the input terminal 1, the outputs of the first and second comparators 2 and 3 are as shown in FIG. The output of the comparator 2 and is variable as indicated by the dotted line by adjusting the first volume 4.

The output of the first comparator 2 is supplied to the first and second NOR circuits 6 and 8 via the variable delay circuit 5 and the delay circuit 7, respectively, and the output of the second comparator 3 is No. 1 NOR circuit 6 is supplied.

The first NOR circuit 6 receives the outputs and supplies an output of-(+) to the addition point 10 via the second volume 9. Further, the second NOR circuit 8 receives the above outputs and supplies the output of-(+) to the addition point 10. At the addition point 10, the outputs of the NOR circuits 6 and 8 are added to each other, and the output of-(+) + {-(+)} that forms a pulse train having twice the frequency of the FM signal is output by the low-pass filter (LPF) 11 Supply to.

Output of the first NOR circuit 6-(+
) Can change the amplitude as shown by the dotted line by adjusting the second volume, and as a result, the output of the addition point 10 changes as shown by the dotted line, resulting in a pulse train having a frequency twice the frequency of the FM signal. Become.

In short, according to the present invention, the amplitude is adjusted by the first volume 4, the variable delay circuit 5 and the second volume 9.
By creating a pulse train of equal width and equally spaced, F
The frequency component of the M signal is erased to become only the second harmonic component.
Therefore, it is possible to reduce the jumping of the FM sideband into the demodulated signal,
It will prevent the generation of beats.

[0020]

Since the present invention is constructed as described above, a pulse train having the same amplitude, width and interval is produced and only the second harmonic component obtained by eliminating the FM frequency component of the input FM signal is obtained. You can

[Brief description of drawings]

FIG. 1 is a block-like electric circuit diagram showing an embodiment of an FM demodulation circuit according to the present invention.

FIG. 2 is a waveform diagram provided for explaining the operation of the FM demodulation circuit of the present invention.

FIG. 3 is a block-like electric circuit diagram showing a conventional FM demodulation circuit.

FIG. 4 is a waveform diagram provided for explaining the operation of a conventional FM demodulation circuit.

[Explanation of symbols]

 1 Input Terminal 2 1st Comparator 3 2nd Comparator 4 1st Volume 5 Variable Delay Circuit 6 1st NOR Circuit 7 Delay Circuit 8 2nd NOR Circuit 9 2nd Volume 11 Low Pass Filter

Claims (2)

[Claims] Claim: What is claimed is: 1. A pulse train twice as large as an FM signal consisting of an input sine wave is produced, and an output proportional to the frequency is obtained by integrating this with a low-pass filter to perform FM detection. A first comparator that receives an FM signal and obtains a square wave output with a constant duty ratio; and a second comparator that receives the sine wave FM signal and obtains a square wave output with a variable duty ratio. A first NOR circuit having an input terminal receiving the output of the first comparator through the variable delay circuit and the other input terminal receiving the output of the second comparator, and one input terminal of the first comparator A second NOR circuit that receives an output through a fixed delay circuit and receives the output of the first comparator at the other input terminal;
A variable resistor for adjusting the amplitude of the output of the NOR circuit, and the output of the variable resistor and the output of the second NOR circuit to form a pulse train having a frequency twice the frequency of the FM signal. An FM demodulation circuit including an OR circuit that supplies a low-pass filter. 2. An FM demodulation circuit, wherein a variable volume is provided on the output side of the first NOR circuit described in claim 1.