JPH02211706A - Fm demodulating circuit - Google Patents

Abstract

PURPOSE:To eliminate a need of a switching circuit and a frequency converting circuit by allowing the series resonance point and a parallel resonance point of a ceramic resonator constituting a phase shift circuit to corresponding to the frequency of an FM signal to be demodulated by an external circuit. CONSTITUTION:A phase shift circuit 2 consists of capacitors C1 and C3, a coil L2, resistances R1 and R2, and a ceramic resonator (x). The ceramic resonator (x) is used in the phase shift circuit 2 and resonance points corresponding to a center frequency f1 to f2 of the FM signal to be demodulated by the external circuit are given to obtain an S curve shown in the figure. The phase is detected by a multiplier 1, and an RA demodulation output is obtained through a low pass filter 3. Thus, a simple circuit constitution is realized where a switching circuit and a frequency converting circuit are not required for the purpose of selecting a desired signal from plural FM signals to obtain the demodulation output.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an FM demodulation circuit, and more particularly to an FM demodulation circuit suitable for a detection circuit that uses a ceramic resonator in a phase shift circuit of a quadrature wave circuit.

[Conventional technology]

Conventionally, when demodulating different FM signals, there is a method of switching the phase shift circuit corresponding to each frequency according to the desired FM signal (Japanese Patent Application Laid-Open No. 6O-27209), or a method of demodulating different FM signals.
The frequency of the signal is converted into a single FM signal by a frequency conversion circuit consisting of a local oscillation circuit and a frequency mixing circuit, and demodulation is performed using a phase shifter corresponding to the converted FM signal.

[7IIM to be solved by the invention] When switching the phase shift circuit, which is one method of the above-mentioned prior art,
A switching circuit for this switching and a frequency discrimination circuit for selecting a desired FM signal are required, which makes the circuit complicated, resulting in an increase in the number of parts, a decrease in reliability, and an increase in cost. In order to eliminate these problems, it is possible to leave the switching circuit to the user's operation of opening and closing a switch, but this inevitably increases the cost of the switching device and complicates the user's operations. In the case of using a frequency conversion circuit, which is the other method, the circuit becomes complicated, the number of parts increases, the reliability decreases, and the cost increases, similarly to the above. In addition, since this method uses an oscillation circuit and a frequency mixing circuit internally, it has the disadvantage of being affected by interference waves generated by unnecessary radiation from the oscillation circuit and distortion from the frequency mixing circuit. The object of the present invention is to construct an inexpensive and simple circuit that does not require a switching circuit or a frequency conversion circuit when demodulating FM signals of different frequencies.

[Means to solve the problem]

The above purpose is to use a ceramic resonator in the phase shift circuit of the quadrature wave circuit, and to make the series resonance point and parallel resonance point of this ceramic resonator correspond to the center frequencies of the input FM signals having different frequencies. This is achieved by

[Effect]

The present invention provides a quadrature waveform in which an FM signal and an FM signal obtained by passing the phase of this F'M signal through a phase shift circuit are inputted to a multiplier, and an FM demodulated output is obtained from the phase difference between these two signals. In the circuit, the series resonance point and parallel resonance point of the ceramic resonator constituting the phase shift circuit are made to correspond to the frequency of the FM signal to be demodulated by the external circuit. Therefore, when one desired signal is input from among a plurality of FM signals with different frequencies, the phase shift circuit has a resonance point at the same frequency as this desired signal, so the center frequency of this desired FM signal A phase difference occurs around the center, and an FM demodulated output of the desired signal is obtained. For this reason, the present invention selects a desired signal from among a plurality of FM signals and uses a switching circuit to obtain a demodulated output.
This provides a simple circuit configuration that does not require a frequency conversion circuit.

〔Example〕

An embodiment of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 shows a quadrature circuit for demodulating a single FM signal. 1 is a multiplier, 2 is a phase shift circuit, and 3 is a low-pass filter. One of the FM signals with center frequency f0 inputted from the four points is inputted as is to the multiplier 1, and the other is inputted to the multiplier through the phase shift circuit 2.

The capacitor Cm and the coil f in the phase shift circuit 2.

has a resonance point. At this time, a phase difference as shown in FIG. 2 occurs between the input points 4 and 5 of the multiplier 1, and after one-phase detection, the signal is passed through the low-pass filter 3 to obtain an FM demodulated output.

As an embodiment of the present invention, the center frequency f and fl, f
Figure 3 shows an FM waveform circuit that demodulates one of three FM signals with different frequencies.Similar to Figure 1, the FM demodulation circuit includes a multiplier 1, a phase shift circuit 2, It is composed of a low-pass filter 3. The phase shift circuit is composed of capacitors C, ,C, coil L2, resistors R, ,R1, and ceramic resonator X. An equivalent circuit of the ceramic resonator X is shown in FIG. The ceramic resonator is coil L. .

The capacitors c, , and Co resistors R0 are equivalently placed.

The impedance and phase relationship of this circuit is shown in Figure 5. At 0 frequency f, the coil LO and the capacitor C0
There is a series resonance point due to the frequency f1, and a parallel resonance point due to the coil L0 and capacitors C0 and C4 exists at the frequency f1. In general, ceramic resonators have c a > c
Due to the relationship o, f, and f are very close. Therefore, when demodulating a single FM signal in the prior art,
To use this ceramic resonator, connect the coils in series, f, and f.

The present invention uses only f, apart from .

The system uses series resonance points such as chi, which have not been used in the past, to demodulate multiple FM signals. FIG. 6 shows the phase shift differences at the four points and five points shown in FIG. 3.

Frequency f. In , there is a series resonance between the external circuit, capacitor C3゜, ceramic resonator This is a series resonance point between the coil L2 and the C capacitance of the ceramic resonator. FIG. 7 shows the actance characteristics when the coil L is connected in series with the ceramic resonator × and the capacitor C□. The resistor Rt indicates the damping resistance of the series resonance point, and the resistor RU indicates the damping resonance point of the parallel resonance point, and is designed to draw a figure-eight curve necessary for phase detection. Using a ceramic resonator, the center frequency of the FM signal to be demodulated is set using an external circuit.

By providing resonance points corresponding to each f1, it is possible to obtain a figure-eight curve as shown in Figure 6. As a result, phase shift detection is performed in multiplier 1, and FM demodulation is performed through low-pass filter 3. I get the output.

〔Effect of the invention〕

According to the present invention, there is no need for a switching circuit or a frequency conversion circuit.
It is possible to construct an inexpensive and simple circuit that does not generate interference signals and can demodulate FM signals of a plurality of different frequencies.

[Brief explanation of the drawing]

Figure 1 is a basic configuration diagram of the quadrature wave circuit, Figure 2 is a frequency-phase characteristic diagram of the 4 points and 5 points in Figure 1, and Figure 3 is an embodiment of the quadrature wave circuit of the present invention. The figure shows the equivalent circuit of a ceramic resonator, Figure 5 shows the frequency-phase and impedance characteristic diagram of Figure 4, and Figure 6 shows the 4 points and 5 points of Figure 3.
FIG. 7 is a frequency-phase characteristic diagram of a point and an actance characteristic diagram of a point. 1... Multiplier, 2... Phase shift circuit, 3...
Low-pass filter, 4... FM signal input, 5... Phase shifted Yamanin 5 + Ri-:

Claims (1)

[Claims] 1. Input an FM modulated signal (hereinafter referred to as FM signal) and a signal obtained by shifting the phase of the FM signal through a phase shift circuit consisting of a ceramic resonator to a multiplier, and
F
In the M demodulation circuit, the series resonance point and the parallel resonance point of the ceramic resonator are selected at arbitrary frequencies, and different FM signals corresponding to the frequencies are demodulated.
Demodulation circuit.