JPS58146107A - Frequency modulation demodulator - Google Patents

Abstract

PURPOSE:To use the demodulator at a high frequency band and to form the demodulator on a printed circuit board, by connecting the middle point of a series circuit of two capacitors connecting in parallel with a secondary coil to an input terminal. CONSTITUTION:A parallel resonance circuit consisting of a primary capacitor 4 and a primary coil 5 is connected between the input terminal 1 and ground, and two capacitors 12, 13 connected in series are connected across the secondary coil 6 mutually coupled with the primary coil 5 to form a dual tuning circuit. The mutual connecting point of the two capacitors 12, 13 is connected to the input terminal 1 to lead the input signal at the primary side, allowing to operate the circuit as Foster-Seeley type frequency discriminator.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a Fosno-Schiele frequency discriminator, which is one of the FM demodulation circuits, and can be used in higher frequency bands, and the circuit can be assembled on a printed circuit board. The object of the present invention is to provide an FM demodulator.

The Fosno-Schiele frequency discriminator uses a double-tuned circuit to convert changes in the frequency of an input signal into changes in phase to perform FM demodulation. The principle of operation will be explained using figures. As shown in Fig. 1, the M-coupled double-tuned circuit consists of capacitors 4, 11 and coils 5, 6, and both the secondary side and secondary side of the tuned circuit are connected to the input FM.
It is designed to resonate at the center frequency fO of the signal. Further, the input signal of the input terminal 1 is led to the middle point of the coil 6 on the secondary side. Outputs to output terminals 2 and 3 of the frequency discriminator are obtained by peak-detecting the terminal voltage on the secondary side of the double-tuned circuit using a diode 718 and a capacitor 9, 10, respectively.

FIG. 2 is a vector diagram showing the operation of the Fosno-Schiele frequency discriminator. The voltage vector of the signal input to the input terminal 1 is represented by Gm with the ground potential as a reference. When the frequency f of the input signal is equal to the center frequency fO of the FM signal, the voltage phase difference between the primary and secondary sides of the M-coupled double-tuned circuit is 9σ, so the voltage vector G on the primary side is The voltage vectors C1 on the secondary side intersect at right angles as shown in FIG. 2a. Furthermore, since point A is the midpoint of coil 6, distribution B-
There will be person C, and IGB1=1GCI.

Since the outputs to the output terminals 2 and 3 are the peak detection outputs of the terminal voltage on the secondary side of the double-tuned circuit, they are equal in this case.

Next, when the frequency f of the input signal differs from the center frequency fO of the FM signal, the phase difference between the primary side and the secondary side of the double-tuned circuit changes as shown in Figure 2, c. Changes in output voltage according to frequency changes are obtained, and F
M demodulation can be performed. FIG. 2 shows the case where the frequency is greater than fO, and FIG. 2C shows the case where the frequency is smaller than fo.

As explained above, the conventional Foster-Schiele frequency discriminator applies the voltage on the primary side to the middle point of the coil 6 on the secondary side I of the double-tuned circuit, thereby changing the frequency discriminator according to the frequency change of the input signal. FM demodulation is performed by fully utilizing the change in phase difference between the voltages between the primary and secondary sides. When the frequency of the input signal is low, such as several υMHz or less, the coil 6.6 that makes up the double-tuned circuit has a large number of turns, and a bobbin transformer with a core is usually used, so the terminal should be taken out at the midpoint of the secondary winding. is easy. However, in cylindrical frequency circuits where the frequency of the input signal exceeds 100 MHz, an air-core coil with a small number of turns of inductance is usually used. Therefore, it is M-connected, f'1.
When an air-core coil is used in a circuit configuration in which a terminal is taken out from the center of a double-tuned circuit, it is difficult to create a circuit on a printed circuit board because it involves aerial wiring.

It is an object of the present invention to eliminate the above-mentioned drawbacks of the conventional example and to configure a high frequency FM demodulation circuit on a printed circuit board. It is easy to configure a double-tuned circuit by placing two air-core coils facing each other on a printed circuit board, but
It is difficult to take out the connection terminal from the midpoint of one air-core coil because it requires all the air wiring. Further, when the inductance on the secondary side of the double-tuned N path is constructed with two air-core coils, it is difficult to closely perform M coupling with the negative side. Therefore, in the present invention, as shown in FIG. 2, two capacitors 12, 13 connected in series are provided between both ends of the secondary winding of the double-tuned circuit, and these two capacitors 12. By configuring the input signal on the primary side to be guided to the connection point 13, a circuit that satisfies the operating principle of a Foster-Schiele frequency discriminator can be obtained. That is, the double-tuned circuit has a capacitor 4 on the primary side. By the coil 6,
The secondary side is composed of a coil 6° capacitor 111 12.13, and if the values of capacitors 12 and 13 are chosen equally, the input signal on the primary side will be selected at the midpoint between both ends of the tuned circuit on the secondary side. Therefore, it can be considered that the operation is the Foster-Schiele type frequency discriminator explained with reference to FIG. As a practical matter, the values of capacitors 12 and 13 should be selected so that the linearity of the frequency discriminator is the best, and it is not necessarily the case that the best linearity is when the values of capacitors 12 and 13 are equal. I can not say.

As described above, according to the present invention, the Foster-Schiele frequency discriminator, which has conventionally been used exclusively in frequency bands below several tens of MHz, can be used in higher frequency bands, and moreover, the circuit can be assembled on a printed circuit board. It is something that can be done.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a conventional Foster-Schiele frequency discriminator, FIG. 2 is a vector diagram for explaining the operating principle of the Foster-Schiele frequency discriminator, and FIG. 3 is a diagram of the present invention. FIG. 2 is a circuit diagram of an FM demodulator according to an embodiment of the present invention. 1...Input terminal, 2, 3...Output terminal, 4゜11.12.13...Capacitor, 6,
6... Coil, 7.8... Diode, 9.10... Capacitor.

Claims (1)

[Claims] A parallel resonant circuit consisting of a primary capacitor and a primary coil is connected between the input terminal and the ground, and two
A double-tuned circuit is constructed by connecting a secondary capacitor in parallel to the secondary coil, and a detection circuit that detects the voltage at both ends of the secondary coil is provided. Connect a series circuit of capacitors and connect these two (
D': FM demodulator in which the interconnection point of the j7 balance wheel is connected to the input terminal.