JPH04343503A - Fm detector - Google Patents

Abstract

PURPOSE:To eliminate the necessity of coil adjusting work and to prevent a distortion rate due to variation of a carrier from being deteriorated by using a variable capacitor 9 based upon voltage control instead of a variable coil. CONSTITUTION:An FM detection circuit 2 outputs a frequency difference between an IF input signal and an output from an oscillator 10 to be oscillated by resonation based upon a coil 8 and a variable capacitor 9 as an AF output. An operation point detection circuit 15 outputs an operation point to be the operation center voltage of the AF signal as DC and a differential amplifier 7 outputs a difference between the operation point voltage and the voltage of a reference voltage source 6 to drive the capacitor 9.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an FM detector, and more particularly to an FM detector for detecting an intermediate frequency (IF) signal into an audio frequency (AF) signal in an FM broadcast receiver.

0002

2. Description of the Related Art In general, an FM detector uses frequency modulation (frequency modulation) using various frequencies transmitted from radio broadcasting stations as carrier waves.
FM) signals, selects one of them, mixes it with a local oscillator in the receiver, and converts it into an FM signal with a fixed intermediate frequency as a carrier wave. This is the IF signal, and the IF signal is then compared with a unique frequency determined inside the detector (hereinafter referred to as internal frequency), and the frequency difference is extracted as a voltage, thereby outputting the AF signal.

In such an FM detector, if the internal frequency deviates from the frequency of the carrier wave, the distortion rate of the AF signal deteriorates, so it is necessary to make the internal frequency coincide with the frequency of the carrier wave.

FIG. 3 is a circuit diagram of a conventional FM detector. The variable inductance coil 13 is connected to the capacitor 1
4 constitutes a parallel resonant circuit, and the oscillator 10 oscillates at an internal frequency determined by this resonant circuit. By comparing the IF signal input from the IF input terminal 1 and the internal frequency output from the oscillator 10 in the FM detection circuit 2,
Outputs the AF signal to the F output terminal 3.

[0005] In this case, by adjusting the inductance of the coil 13, the AF signal can be set to the optimum point by making the internal frequency match the frequency of the carrier wave.

[0006]

[Problems to be Solved by the Invention] In the conventional FM detector described above, in order to set the AF signal at the optimum point, it is necessary to
It is necessary to adjust the variable coil 13 for each detector, which increases the number of adjustment steps during set assembly and reduces efficiency.Also, even if the optimum point is set in this way, subsequent fluctuations in the frequency of the local oscillator 10 occur. There is also a problem in that the distortion rate of the AF output deteriorates due to variations in the frequency of the carrier wave caused by such factors.

An object of the present invention is to provide an FM detector that solves these problems, eliminates the need for a variable coil, reduces the number of adjustment steps, and improves distortion by responding to frequency fluctuations in the internal frequency. It is in.

[0008]

[Means for Solving the Problems] The configuration of the FM detector of the present invention includes a resonant circuit in which a coil and a variable capacitor are connected in parallel, an oscillation circuit that oscillates at the resonant frequency of this resonant circuit, and an output of this oscillation circuit. An FM detector including an FM detection circuit that receives a signal and a frequency-modulated intermediate frequency signal and outputs a frequency difference between the two signals as an audible frequency signal,
an operating point detection circuit that extracts an operating point that is a center voltage of the operation of the audio frequency signal as a DC voltage; a reference voltage source that outputs a reference voltage that is a reference for the operating point; and a differential amplifier that outputs a difference signal corresponding to the difference between these voltages and supplies it to the variable capacitor, and the variable capacitor is characterized in that its capacitance value is changed by external control. shall be.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a block diagram of an embodiment of the present invention. In the figure, 4 is an operating point detection terminal, 5 is a large capacitor, 6 is a reference voltage source, 7 is a differential amplifier, 8 is a coil,
9 is a variable capacitor, 11 is a buffer, and 15 is an operating point detection circuit. Assuming that the output voltage of the differential amplifier 7 is V,
Assuming that this is input to the capacitor 9, the value of the capacitor 9 is expressed as a function of V as C=C(V), and if the inductance of the coil 8 is L, then the resonant frequency f0 of the parallel resonant circuit is expressed as the following equation (1).

[0010] f0 =1/2π{LC(V)}1/2 (1)
In addition, the FM detection circuit 2 has an IF for the resonant frequency f0.
It is assumed that a positive voltage is output when the signal frequency is high and a negative voltage is output when the signal frequency is low, and the capacitor 9 has a relationship with the output voltage V of the differential amplifier 7 as shown in the following equation (2). Here, A and B are constants.

C(V)=A×V-B...
...(2) Then, equation (1) becomes the following equation (3).

[0012] f0 =X・VB/2
......(3) However, X=1/2π(LA)1/
2 Here, if the frequency of the carrier is higher than the resonance frequency f0 of equation (3), the operating point will be higher than the reference voltage. As a result, the output of the differential amplifier 7 also becomes high, so according to equation (3), the resonant frequency f0 also becomes high, and the operating point moves toward the reference voltage. In this way, f0 changes automatically so that it always operates at the optimum point.

FIG. 2 is a circuit diagram of a second embodiment of the present invention, in which the externally controlled capacitor 9 in FIG. 1 is replaced with a voltage controlled capacitor 12. This voltage-controlled capacitor 12 is a capacitor that utilizes the junction capacitance of a semiconductor PN junction, and has characteristics as shown in equation (2) with respect to the control voltage.

[0014]

Effects of the Invention As explained above, the present invention automatically adjusts the resonant frequency using a voltage obtained by smoothing an FM detected signal, thereby eliminating the need for adjusting the variable coil and This has the effect of being able to detect fluctuations without deteriorating the distortion rate.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of an embodiment of the present invention.

FIG. 2 is a circuit diagram of a second embodiment of the present invention.

FIG. 3 is a block diagram of an example of a conventional FM detector.

[Explanation of symbols]

1 IF input terminal 2 FM detection circuit 3 AF output terminal 4 Operating point detection terminal 5 Large capacity capacitor 6 Reference voltage source 7 Differential amplifier 8 Coil 9 Externally controlled capacitor 10 Oscillator 11 Buffer 12 Voltage controlled capacitor 13 Variable coil 14 Capacitor 15 Operating point detection circuit

Claims (3)

[Claims] Claim 1: A resonant circuit in which a coil and a variable capacitor are connected in parallel, an oscillation circuit that oscillates at the resonant frequency of the resonant circuit, and an oscillation circuit that receives an output signal of the oscillation circuit and a frequency-modulated intermediate frequency signal. An FM detector that includes an FM detection circuit that outputs a frequency difference between signals as an audio frequency signal, an operating point detection circuit that extracts an operating point that is the center voltage of the operation of the audio frequency signal as a DC voltage; a reference voltage source that outputs a reference voltage as a reference;
a differential amplifier that receives the operating point voltage and the reference voltage, outputs a difference signal corresponding to the difference between these voltages, and supplies the signal to the variable capacitor; An FM detector characterized in that the FM detector changes. 2. The FM detector according to claim 1, wherein the variable capacitor is capable of varying its capacitance value depending on an externally input voltage, current, or physical factor. 3. The FM detector according to claim 1, wherein the reference voltage source has a voltage value that matches a voltage value at an optimal operating point where the electrical characteristics of the audio frequency signal are optimal.