JPS61230536A - Frequency separation circuit - Google Patents

Abstract

PURPOSE:To obtain a frequency separation circuit with a simple constitution and low cost while the frequency selecting characteristic is attained by providing a parallel resonance circuit having a specific resonance frequency between the input sections of a differential amplifier in the frequency separation circuit. CONSTITUTION:When a multiplex signal is fed to an input terminal 2 and an input signal having frequency components f1, f2 to be detected is fed thereto, the impedance between the input terminals of the differential amplifier 23 shows the highest value at the resonance frequency f2. As a result, the attenuated frequency component of the frequency f2, that is, the frequency components including the frequency f1 as the major component is extracted. Further, since the component of the frequency f1 passes through a parallel resonance circuit 24, the frequency component having the frequency f2 mainly comprised of the difference component only between the signal in the fre quency f2 and the signal attenuated by the circuit 24 is extracted at the differential output of the differential amplifier 23 and the difference component of the frequency f1 is shut off. Thus, a specific component is extracted separately from an optional frequency component by the differential output side of the differential amplifier 23 and the input side of the differential amplifier 23 short-circuited by the parallel reso nance circuit 24.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a frequency separation circuit that separates a specific frequency component from a plurality of frequency components, and is used, for example, in broadcasting in FM tuners, cassette tape recorders, tape players, etc. The present invention relates to a frequency separation circuit that is suitable for use as a bandpass filter that is installed in an audio multiplex demodulation circuit that demodulates television audio multiplex broadcasts and separates a specific frequency component from arbitrary frequency components in a receiving device.

[Conventional technology]

Audio multiplex broadcasts in television broadcasting include dual audio broadcasting such as bilingual broadcasting in which main audio is broadcast in Japanese and sub audio in a foreign language, and stereo broadcasting using main audio and sub audio.

The frequency spectrum of the multiplex signal of such audio multiplex broadcasting is established as shown in Figure 3, where Sl is the main channel signal, S2° is the subcarrier, Sit is the subchannel signal for bilingual broadcasting, and S22 is the sub-channel signal during stereo broadcasting, S is the control channel signal, and S is the control channel signal. is the control signal subcarrier, and S,1,S3t are the control signal sidebands. In this case, in stereo broadcasting, the sum signal (L+R) of left and right signals and H is set as the main channel signal, and the difference signal (L-R) between them is set as the sub-channel signal. And the horizontal synchronization signal f)I of television broadcasting is approximately 15.75k
Hz, and the subcarrier Sho is 2 f H (=31.5
kHz), control signal subcarrier S3° is 3.5f,
(= 55.125kHz).

A receiving device that receives such audio multiplex broadcasting includes a fourth
An audio multiplex demodulation circuit as shown in the figure is installed.

The multiplex broadcast waves received by the antenna are converted to an intermediate frequency through channel selection and high frequency amplification at the front end, amplified by the intermediate frequency amplification section, and then detected by the FM detection circuit. The multiplexed signal obtained is applied to input terminal 2. Normally, in devices with a stereo demodulation circuit, there are two types: one that applies the multiplexed signal that has passed through the stereo demodulation circuit to input terminal 2, and the other that applies the multiplexed signal to input terminal 2 from its own route without passing through the stereo demodulation circuit. However, demodulation of multiplexed signals is possible in either case.

When this multiplexed signal is applied to a low pass filter (LPF) 4, a main channel signal (L+R) is extracted and applied to a matrix circuit 6. The multiplexed signal is also applied to a bandpass filter (BPF) 8, which separates a subcarrier S20 with a frequency of 31.5kHz and a control signal subcarrier S3° with a frequency of 55.125kHz, with the former subcarrier S2° being F
M demodulation circuit 10, the latter control signal subcarrier S
3° is applied to the AM demodulation circuit 12.

In this case, the F Mlz circuit 10 outputs the sub-channel signal (
L-R) is demodulated and added to the matrix circuit 6, and A
In the M demodulation circuit 12, the frequency in the multiplexed signal is 922.5H.
A control signal of either 982.5 Hz or 982.5 Hz is demodulated. If a 922.5 Hz control signal is detected,
The received broadcast is in dual audio mode, and if a 982.5 Hz control signal is detected, it is in stereo broadcast mode. These control signals are applied to a discrimination circuit 14, which discriminates whether it is a stereo broadcast or a dual audio broadcast based on the frequency of the control signal, and the discrimination output becomes a switching control input of a switch circuit 16. The switch circuit 16 generates a switch output according to the output for determining whether it is a stereo broadcast or a dual audio broadcast, and applies it to the matrix circuit 6.

The matrix circuit 6 is switched to the stereo demodulation mode or the dual audio demodulation mode according to the switch output from the switch circuit 16. For example, in the stereo demodulation mode, the main channel signal (L + R) and the sub channel signal ( LR), the left signal and right signal R are separated, and these signals R and R are output terminal 1.
8.19 will be taken out individually.

[Problem that the invention seeks to solve]

In this type of audio multiplexing demodulation circuit, although it varies depending on the size of the receiving device and the required performance, it is generally
It consists of a chip semiconductor integrated circuit. in this case,
The bandpass filter 8 is composed of a plurality of inductors and capacitors having frequency selection characteristics outside the main circuit integrated into a semiconductor circuit, and has the drawback of being very expensive.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a frequency separation circuit that can obtain excellent frequency selection characteristics with a simple configuration and can be realized at a low cost.

[Means for solving problems]

The present invention will be explained with reference to FIG. 1 corresponding to an embodiment. The frequency separation circuit 22 of the present invention includes a differential amplifier 23
A parallel resonant circuit 24 having a specific resonant frequency f2 is installed between the input parts of the resonant circuit 24. And its resonant frequency f
2 and another frequency f1 component is applied to the differential amplifier 23, and the frequency component excluding the resonant frequency component is extracted via the parallel resonant circuit 24,
from the differential output section of the differential amplifier 23 to the resonant frequency f
2 is the main component.

[For production]

Therefore, since the input parts of the differential amplifier 23 are short-circuited via the parallel resonant circuit 24 having a specific resonant frequency f2, when an input signal having a frequency component to be detected, , f, is applied, At the resonant frequency f2, the impedance between the input terminals of the differential amplifier 23 has the highest value, and the frequency component of the frequency f2 is attenuated, that is,
A frequency component mainly having the frequency f is extracted. Also, since the component of the frequency f passes through the parallel resonant circuit 24, the differential output side of the differential amplifier 23 has a parallel resonant circuit 24 with the frequency f. A frequency component consisting only of a difference component from the attenuated signal, that is, a frequency component mainly consisting of frequency f, is extracted, and a difference component of frequency f1 is cut off. Therefore, it is possible to separate and extract a specific frequency component from the differential output side of the differential amplifier 23 and the input side of the differential amplifier 23 short-circuited by the single parallel resonant circuit 24. can.

Further, the parallel resonant circuit 24 is composed of a pair of inductor 32 and capacitor 34 having a single resonant frequency,
The circuit configuration can be simplified compared to the conventional one.

〔Example〕

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 shows an embodiment of an audio multiplexing demodulation circuit using the frequency separation circuit of the present invention, and the same parts as those of the audio multiplexing demodulation circuit shown in FIG. 4 are given the same reference numerals.

As shown in FIG. 1, in the stereo mode, the multiplexed signal applied to the input terminal 2 is passed through a low-pass filter 4 to extract the main channel signal (L + R) and applied to the matrix circuit 6. High pass filter (HPF) 20
After the main channel signal (L+R) is removed via a buffer amplifier 21, it is applied to a frequency separation circuit 22 which is used as a bandpass filter 8. In this case, the multiplexed signal applied to the frequency separation circuit 22 is a composite signal consisting of a sub-channel signal (LR) and a control signal.

The frequency separation circuit 22 has first and second frequencies f,
, f2, and a parallel resonant circuit 24 is connected between the differential input sections of the differential amplifier 23. Differential amplifier 23 is transistor 26.2
8 and a constant current source 30, and the parallel resonant circuit 24 is composed of an inductor 32 and a capacitor 34. The parallel resonant circuit 24 is a pair of inductor 32 and capacitor 34 connected in parallel, and in this embodiment, the second frequency f2 is set to a resonant frequency (for example, 55.125
kHz).

Further, an active load as an output circuit for extracting the second frequency f2 component is installed on the collector side of each transistor 26.28 of the differential amplifier 23, and in this embodiment, the transistor 36.38. 40.42.44.
A current mirror circuit consisting of 46 is installed.

The component of the first frequency f is the transistor 28
is taken out from the base of the FM demodulator 10 via the buffer amplifier 48,
．． The second frequency f2 taken out from the collector side of 46
The component of
2, and the other configuration is the same as the audio multiplex demodulation circuit shown in FIG.

The operation of the above configuration will be explained.

The multiplexed signal applied to the input terminal 2 is applied to the low-pass filter 4 to extract the main channel signal (L'+R), and the main channel signal (L+R) is removed via the high-pass filter 20. Thereafter, the signal is applied from the buffer amplifier 21 to the frequency separation circuit 22.

In the frequency separation circuit 22, the parallel resonant circuit 24 resonates at the second frequency f 2 (=55.125kHz) and exhibits low impedance characteristics, so the differential amplifier 2
The impedance between the input terminals of No. 3 shows the highest value. Therefore, the audio subcarrier component is transmitted through this parallel resonant circuit 24.
A signal having the same level as the input multiplexed signal applied to the base of the transistor 26 is applied to the base of the transistor 28, and the second frequency f t (=55.12
The control signal subcarrier (5kHz) appears attenuated by several tens of dB at the base of transistor 28 and is applied to buffer amplifier 48.

The control signal subcarrier of the second frequency f z (=55.125kHz) is transmitted to the transistor 42 in an amplified form.
46 and applied to a buffer amplifier 50. In this case, the first frequency f.

The component passes through the parallel resonant circuit 24 and the differential amplifier 23
In the active load side output of , only the difference component from the attenuated signal in the parallel resonant circuit 24 having the second frequency f2 as the resonant frequency appears, and the difference component at the first frequency f does not appear.

Figure 2 shows the output level of the frequency separation circuit 22 in this case.
It shows the frequency characteristics, where A is the subcarrier output whose main component is the first frequency f1 extracted from the base side of the transistor 28, and B is the collector of the transistors 42 and 46 on the differential output side of the differential amplifier 23. This is a control signal subcarrier output whose main component is the second frequency f2 extracted from the side.

Then, in the FM demodulation circuit 10 from each output,
The sub-channel signal (LR) is demodulated and applied to the matrix circuit 6, the AM demodulation circuit 12 demodulates the control signal, and the discrimination circuit 14 discriminates whether it is stereo mode or dual audio mode based on the frequency of the demodulated signal. Based on the determined output, the switch circuit 16 sets the matrix circuit 6 to either the stereo mode or the dual audio mode.

As a result, a stereo signal or a dual audio signal is demodulated in the matrix circuit 6 and taken out from the output terminals 18, 19.

In the embodiment, the frequency separation circuit is used as a bandpass filter of the audio multiplexing and demodulating circuit, but the frequency separating circuit of the present invention can also be used in addition to the audio multiplexing and demodulating circuit as a means for extracting a plurality of frequency components.

〔Effect of the invention〕

As explained above, according to the present invention, it is possible to efficiently separate and extract a specific frequency component from a signal of multiple frequency components, and there are fewer external parts.
Since the circuit configuration is simple, it can be easily integrated into a semiconductor circuit, and can be used, for example, as a bandpass filter for extracting a specific frequency component from continuous or discontinuous frequency components.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the frequency separation circuit of the present invention, FIG. 2 is an explanatory diagram showing the output level-frequency characteristic of several e, and FIG. 3 is an explanatory diagram showing the frequency spectrum of a television multiplexed signal. FIG. 4 is a block diagram showing a conventional audio multiplexing and demodulating circuit. 22... Frequency separation circuit, 23... Differential amplifier, 2
4...Parallel resonant circuit. Figure 2 M 3 Figure-

Claims (2)

[Claims] (1) A parallel resonant circuit with a specific resonant frequency is installed between the input sections of a differential amplifier, and a signal consisting of the resonant frequency and other frequency components is applied, and the resonant frequency is transmitted through the parallel resonant circuit. What is claimed is: 1. A frequency separation circuit characterized in that a frequency component is extracted from which the frequency component is removed, and a frequency component having the resonance frequency as a main component is extracted from a differential output section of the differential amplifier. (2) The frequency separation circuit according to claim 1, wherein the parallel resonant circuit includes a pair of inductor and capacitor.