JPH0157523B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a circuit for detecting the presence or absence of an FM modulated wave.

[Prior art]

Conventionally, in order to easily determine the presence or absence of an FM modulated wave, a method has been adopted in which a pilot signal is simultaneously transmitted on the transmitting side.

In this case, since the pilot signal has a single frequency, it was relatively easy to detect this pilot signal on the receiving side. An example of this is FM stereo broadcasting. However, when the pilot signal is not transmitted and only the FM modulated wave is transmitted, it is difficult to reliably determine the presence or absence of the FM modulated wave.

[Summary of the invention]

The present invention has been made in view of the problems of the conventional devices as described above, and the free running frequency of the voltage controlled oscillator of the phase-locked loop is set to a certain appropriate value compared to the carrier wave frequency of the FM signal whose presence or absence of modulated waves is to be detected. By making the average DC voltage of the control voltage output from the phase-locked loop different between no signal and FM modulation, it is possible to detect the presence or absence of FM modulation waves that do not include the pilot signal. The purpose of this invention is to provide an FM modulated wave detection circuit that can detect reliably and inexpensively.

[Embodiments of the invention]

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

FIG. 1 shows an FM modulated wave detection circuit according to an embodiment of the present invention. In the figure, 1 is a PLL IC in which the main part of the phase-locked loop is integrated;
IC1 includes a phase comparator 11, a DC amplifier 12, a low-pass filter 13, and a voltage controlled oscillator (hereinafter referred to as VCO).
(referred to as ) consists of 10. Also, Co and Ro are VCO10
A timing capacitor and a timing resistor that determine the free running frequency of 2 is a control voltage averaging circuit, and in this circuit 2, a resistor R1 and a capacitor C1 are VCO1 output from PLL IC1.
It constitutes an integrating circuit for averaging the zero control voltage, and C2 is a capacitor. In addition, Ra~
Rd is a resistance, and Ca and Cin are capacitors.

Next, the operation will be explained.

Now, in order to detect the presence or absence of an FM modulated wave, the free running frequency of the VCO 10 is set by a capacitor Co and a resistor Ro so that it is slightly lower than the carrier frequency of the FM modulated wave to be determined.
By doing this, the input terminal of phase-locked loop 1
The relationship between the frequency of the FM signal applied to IN and the averaged DC voltage of the control voltage appearing at the output terminal OUT of the control voltage averaging circuit 2 is as shown in FIG. Here, p is the free running frequency of VCO 10, c is the carrier frequency of the FM modulated wave,
Vp is the average DC voltage of the control voltage when there is noise or when there is no FM modulation wave, and Vc is the average DC voltage of the control voltage when there is an FM modulation wave.

As is clear from FIG. 2, the averaged DC voltage of the control voltage changes between Vc and Vp with or without the FM modulation wave. And due to this change in DC voltage value, FM
The presence or absence of modulated waves can be easily detected.

The FM modulated wave detection circuit according to this embodiment having such a configuration and operation can easily determine the presence or absence of an FM modulated wave, and can also detect FM even when there is noise.
It is possible to very reliably distinguish between the modulation and the modulation.

〔Effect of the invention〕

As described above, according to the present invention, the free-running frequency of the voltage-controlled oscillator of the phase-locked loop is shifted by a certain appropriate value from the carrier frequency of the FM signal that does not include the pilot signal and is used to detect the presence or absence of a modulated wave. In addition, since the averaged DC voltage of the control voltage output from the phase-locked loop is different between no signal and FM modulation, the FM
This has the effect of providing an FM modulated wave detection circuit that can reliably and inexpensively detect the presence or absence of a modulated wave.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of an FM modulated wave detection circuit according to an embodiment of the present invention, and FIG. 2 is a relationship between the FM signal frequency input to the phase-locked loop of FIG. 1 and the output voltage of the control voltage averaging circuit. FIG. 1...PLL IC (phase locked loop), 10...voltage controlled oscillator, Ro...timing resistor, Co...timing capacitor, 2...control voltage averaging circuit.

Claims (1)

[Claims] 1. A phase-controlled oscillator that is intended to detect the presence or absence of a modulated wave and is provided with a voltage-controlled oscillator that receives an FM signal that does not include a pilot signal and has a free-running frequency that is shifted by a predetermined amount from the carrier frequency of the FM signal. It comprises a locked loop and a control voltage averaging circuit that averages the control voltage of the voltage controlled oscillator which is the output of the phase locked loop, and calculates the FM signal from the averaged DC voltage value from the control voltage averaging circuit. An FM modulated wave detection circuit characterized by detecting the presence or absence of a modulated wave.