JPH10135743A - Fm detection circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To secure a combined use between a loop gain setting filter and a carrier elimination filter of a PLL circuit by preparing a series circuit of a capacitor and a 1st resistance where the current output of a phase comparator is applied to one of both ends with the reference potential applied to the other end and a 2nd resistance having its one of both ends connected to one of both ends of the series circuit with other end connected to the control terminal of a VCO and then fetching the signal the undergone FM detection from the middle point of connection between the 1st resistor and the capacitor. SOLUTION: The oscillation output signal of a VCO 10 is compared with the sound signal FM modulated by a phase comparator 12, and the output signal that undergone the FM detection is produced at the collector of a transistor 18. A series circuit 14 of a 1st resistor 15 and a capacitor 16 functions as a carrier component elimination LPF, and the circuit 14 and a 2nd resistor 17 function as a filter that sets the loop gain of a PLL circuit which contains the VCO 10 and the comparator 12. Then the desired loop gain is set when the time constant of the circuit 14 is varied.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an FM detection circuit for demodulating an audio signal of a TV receiver, and more particularly to an FM detection circuit which can be used as a filter and is suitable for use in an IC.

[0002]

2. Description of the Related Art An audio signal from a TV receiver is FM-modulated and transmitted. The carrier frequency of the audio signal varies depending on the broadcasting system and ranges from 4.5 MHz to 6.5 MHz. Conventionally, such FM signals have been detected by quadrature detection. In quadrature detection, each time the carrier frequency is different, an oscillator that matches that frequency is required.
Not suitable for conversion to C.

[0003] Therefore, an FM detection circuit which does not require an oscillator and can detect a plurality of audio signals with one detection circuit has been used. FIG. 2 shows such an F
FIG. 4 is a block diagram showing an M detection circuit, where VCO (1) is 4.5 MHz.
The oscillation frequency changes from 6.5 MHz to 6.5 MHz. The VCO
The oscillation output signal of (1) is applied to the phase comparator (2) and compared in phase with the FM-modulated audio signal from the input terminal (3). By the phase comparison, FM detection is performed, and a detected output signal is generated at the terminal (4).

The signal at the terminal (4) is smoothed by the LPF (5) and applied to the VCO (1) as a control voltage that determines the oscillation frequency of the VCO (1). LPF (5) is a VCO
(1) The loop gain of the PLL circuit composed of the phase comparator (2) and the LPF (5) is set. P
Due to the operation of the LL circuit, VCO (1) is 4.5Mhz to 6.5Mhz
The oscillation frequency changes over HZ, and multiple audio signals can be detected.

By applying the signal from the terminal (4) to the carrier filter (6) to remove the carrier component, an audio signal is obtained at the terminal (7).

[0006]

However, the circuit block shown in FIG. 2 has a problem that two filters, a carrier filter (6) and an LPF (5), are required to obtain an audio signal. Filters are not suitable for use in ICs, and their reduction is required.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and has a VCO that oscillates at a frequency corresponding to the frequency of an input FM signal, and a VCO that oscillates between an oscillation output signal of the VCO and an input FM signal. A phase comparator that performs a phase comparison and generates a phase comparison result as a current output, a series circuit of a first resistor and a capacitor having one end to which the current output of the phase comparator is applied and the other end to which a reference potential is applied, A second terminal having one end connected to one end of the series circuit and the other end connected to a control terminal of the VCO;
A resistor is provided, and a signal subjected to FM detection is extracted from a connection point between the first resistor and the capacitor.

[0008]

FIG. 1 shows an FM detection circuit according to the present invention. A transistor (10) oscillates at a frequency corresponding to the frequency of an input FM signal and operates as an operation current source for determining an oscillation frequency. A VCO having a multivibrator configuration having (11), and (12) a phase for comparing a phase of an oscillation output signal of the VCO (10) with an input FM signal from an input terminal (13) and generating a phase comparison result as a current output. A comparator (14) is a series circuit of a first resistor (15) and a capacitor (16) having one end to which the current output of the phase comparator (12) is applied and the other end grounded, and (17) having one end having the aforementioned A second resistor is connected to one end of the series circuit (14) and the other end is connected to the emitter of the transistor (11).

The oscillation frequency of the VCO (10) is determined by the current flowing through the collector-emitter path of the transistor (11). The collector-emitter current of the transistor (11) is equal to the VCO (1
The operation current becomes 0), and the oscillation frequency of the multivibrator changes by changing the value of the operation current. VCO (1
0) has a wide variable frequency range without the need for an oscillator by adopting a multivibrator configuration, from 4.5 MHz to 6.
Can be changed over 5MHZ.

[0010] The oscillation output signal of the VCO (10) is applied to a phase comparator (12) and compared with the phase of an FM-modulated audio signal from an input terminal (13). By the phase comparison, FM detection is performed, and a detected output signal is generated at the collector of the transistor (18). Since the output signal is generated from the collector, it is generated at the terminal (19) in the form of a current.

The signal at the terminal (19) is supplied to the first resistor (15)
And a capacitor (16) is applied to a filter comprising a series circuit (14) and a second resistor (17). Terminal (19)
When looking between the terminal and the terminal (20), the series circuit (14) of the first resistor (15) and the capacitor (16) operates as an LPF for removing a carrier component. Also, terminal (19)
When looking between the terminal and the terminal (21), the series circuit (14) and the second resistor (17) include the VCO (10) and the phase comparator (1).
This is a filter for setting the loop gain of the PLL circuit having the function 2).

The relationship between the input signal frequency and the loop gain of the PLL circuit shown in FIG. 1 is affected by the above-described filter for setting the loop gain, and is as shown by the solid line in FIG. In the solid line of FIG. 3, the carrier component of 4.5 MHz is attenuated, and the VCO (10) does not swing due to the carrier component. First resistor (15) and capacitor (16) of FIG.
Is changed, the characteristic curve becomes, for example, as shown by a dotted line in FIG. For example, when the value of the first resistor (15) is increased, the value of the current flowing from the terminal (19) to the capacitor (16) is reduced, and the influence of the capacitor is reduced. Therefore, the cutoff frequency of the filter is increased, and the characteristic shown by the dotted line in FIG. Direction. Conversely, when the value of the first resistor (15) is reduced, the value of the current flowing from the terminal (19) to the capacitor (16) increases, and the influence of the capacitor increases, so that the cutoff frequency of the filter decreases and the solid line in FIG. It becomes the characteristic direction.

Thus, a desired loop gain can be set by changing the time constant of the series circuit (14) of the first resistor (15) and the capacitor (16). Capacitor (1
6) is also used as a carry filter, so that it can also be used as a capacitor, that is, as a filter. The value of the first resistor (15) is limited as a carry filter, but the loop gain can be freely set by the presence of the second resistor (17). That is, when the value of the second resistor (17) is changed, the solid line and the dotted line in FIG.

Therefore, the filter composed of the series circuit (14) of the first resistor (15) and the capacitor (16) and the second resistor (17) can share two filter functions.
The signal at the terminal (21) is applied to the emitter of the transistor (11) constituting a part of the VCO (10).
The point of application to the emitter is also a feature of the present invention. By applying a control signal to the emitter of the low impedance transistor, a stable control current can be generated at the collector of the transistor (11). The same current change as that of the terminal (21) occurs at the collector of the transistor (11).

[0015]

As described above, according to the present invention, the PL
The filter for setting the loop gain of the L circuit and the filter for removing the carrier can be shared, and the F
An M detection circuit is obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an FM detection circuit according to the present invention.

FIG. 2 is a block diagram showing a conventional FM detection circuit.

FIG. 3 is a characteristic diagram for describing an FM detection circuit according to the present invention;

[Explanation of symbols]

 (10) VCO (11) Transistor (12) Phase comparator (14) Series circuit (15) First resistor (16) Capacitor (17) Second resistor

Claims (2)

[Claims] 1. A VCO that oscillates at a frequency corresponding to a frequency of an input FM signal, a phase comparator that compares a phase of an oscillation output signal of the VCO with an input FM signal, and generates a phase comparison result as a current output, A series circuit of a first resistor and a capacitor having one end to which the current output of the phase comparator is applied and the other end to which a reference potential is applied; and one end connected to one end of the series circuit and the other end connected to the VCO.
And a second resistor connected to the control terminal of (1), wherein an FM-detected signal is taken out from a connection point between the first resistor and the capacitor. 2. A multivibrator VCO having a transistor that oscillates at a frequency corresponding to the frequency of an input FM signal and operates as an operating current source that determines an oscillation frequency.
A phase comparator that compares the phase of the oscillation output signal of the VCO with the input FM signal and generates a phase comparison result as a current output, and a current output of the phase comparator is applied to one end and a reference potential is applied to the other end. A series circuit of a first resistor and a capacitor being applied, and a second resistor having one end connected to one end of the series circuit and the other end connected to the emitter of the transistor, and connecting the first resistor and the capacitor. An FM detection circuit wherein a signal detected by FM detection is extracted from a middle point.