A newbie asks for an explanation of a circuit

tomcat66

A newbie asks for an explanation of a circuit [Copy link]

This circuit is a screenshot, and other peripheral circuits have been abbreviated as comments. This circuit is in a foreign device, and its purpose is to check the strength of the audio signal in the TV channel (of course it is not a digital TV). Since I am not very familiar with modulation and demodulation, I would like to ask you experts to help. There is no need to describe the function of 4066, I already know it, but it is difficult to understand that this circuit does not look like a demodulation circuit. The general block diagram description: The received RF TV signal is down-converted by a mixing circuit, and A and V are separated. The audio signal is sent to this circuit. The signal processed by this circuit is detected and then sent to the meter to indicate the signal strength.

西门

I haven't seen your diagram, but based on experience, after the mixing circuit comes out, it should first be amplified in the intermediate frequency (because after mixing, there is also "frequency conversion", the intermediate frequency signal comes out, but the amplitude may not be enough and needs to be increased). The signal released from the intermediate frequency circuit has enough amplitude before it can be sent to the detection circuit. The detected signal can drive the U/V meter.

tomcat66

In theory, I know a little bit. What is the 4066 analog switch used for? The meter does not indicate the amplitude of the accompanying sound, that is, it does not change with the volume of the program, but the intensity of the modulation wave. It is definitely not the demodulated sound.

wangjiafu1985

You can look at the information related to the phase-locked amplifier. This circuit is a kind of phase-sensitive detection, which implements multiplication.....

wangjiafu1985

You can download the e-book in the post and take a look. The last chapter is about the lock-in amplifier. You will gain a lot after reading it. https://bbs.eeworld.com.cn/thread-89382-1-5.html

lixiaohai8211

:)

tomcat66

Thanks, I'll research it first.

xiaoxif

Originally posted by tomcat66 on 2010-2-14 17:58 This circuit is intercepted, and other peripheral circuits have been abbreviated as comments. This circuit is in a foreign device, and is used to check the strength of the accompanying audio signal in the TV channel (of course it is not a digital TV). Since I am not very familiar with modulation and demodulation, I would like to ask the masters for help. There is no need to describe the function of 4066, ...

I noticed the following sentence in your introduction: "Its purpose is to check the strength of the accompanying audio signal in the TV channel". I think you seem to have overcomplicated the issue. This is just a selection switch, used to select from multiple accompanying audio intermediate frequencies, or to sample the accompanying audio intermediate frequency at a certain moment (to do time-sharing detection and monitoring with other signals). If this is not the case, could you provide more detailed information, at least the frequency and waveform of F2 (probably a switching signal with a relatively low frequency), and F1 should be the accompanying audio intermediate frequency of 6.5MHz.

wangjiafu1985

Originally posted by xiaoxif on 2010-2-16 00:12 I noticed this sentence in your introduction: "It is used to check the strength of the accompanying audio signal in the TV channel". I think you seem to have complicated the problem. This is just a selection switch, which is used to select from multiple accompanying audio intermediate frequencies, or to sample the accompanying audio intermediate frequency at a certain moment (with other signals...

Personally, I feel that it should be a signal with the same frequency as the intermediate frequency signal, and it is very likely to be in the same phase, so that the amplitude of the audio signal can be obtained... If this is the case, it is equivalent to a half-wave rectifier circuit.

xiaoxif

Just to discuss the explanation above: The above said: "It should be a signal with the same frequency as the intermediate frequency signal, and it is very likely to be in the same phase, ...", then, 1. For FM signals, the same frequency and phase, isn't it the input signal? 2. FM signals, here specifically refers to the TV sound intermediate frequency, is a constant amplitude signal, and the frequency deviation of the intermediate frequency carries the audio information. Even if it is a rectifier circuit as the above said, the rectification---this kind of amplitude detection cannot obtain audio information, and only the DC proportional to the intermediate frequency carrier amplitude can be obtained. 3. Besides, the frequency detector circuit for demodulating audio signals is very common and cheap, so why do we need to use 4xxx to make the synchronous detection circuit for processing AM signals? [ This post was last edited by xiaoxif on 2010-2-17 22:57 ]

tomcat66

Finally, I went to work and carefully studied the drawings and product manual. After the TV audio signal is processed, the 4.25MHz intermediate frequency FM audio signal (without demodulation) is input to one end of the 4066 switch. The control end frequency of 4066 is 4.35MHz. The waveform of the control end is as shown below. According to the manual, the output end generates a base frequency of 0.1MHz audio (should be a carrier). Then it is filtered, "0.1MHzaccompanimentbyIC4A,[size=10.5pt IC5, D9 and IC18A amplify and detect, and this signal is used for the indication of the carrier of the accompanying sound. What I don't understand is why the frequency has to be reduced again and again, first from hundreds of megahertz to about 10M (accompaniment 4.25), and then from 4.25 to 100K frequency, and 4066 is used to reduce the frequency. The circuit is very complicated, is it necessary?

wangjiafu1985

Originally posted by xiaoxif on 2010-2-17 22:51 Just to discuss the explanation above: The above said: "It should be a signal with the same frequency as the intermediate frequency signal, and it is very likely to be in the same phase,..." Then, 1. For FM signals, the same frequency and phase, isn't it the input signal? 2. FM signal, here specifically refers to the TV sound intermediate frequency...

I have never studied TV circuits, but I just saw that it is the same as the phase-sensitive detection in the phase-locked amplifier, and the original poster only gave a picture of an analog switch, nothing else, just a guess...

xiaoxif

I have a short lifespan. This is the first time I have seen a TV audio intermediate frequency signal with a value of 4.25MHz. Before this, I only knew about the (second) audio intermediate frequencies of 5.5 MHz, 6.0 MHz, 6.5 MHz, and 4.5 MHz, and the subcarrier (i.e., the second intermediate frequency) of the DB color difference signal (BY) with a value of 4.25MHz in SECAM. Since: you have carefully studied the drawings and product manuals. Can you show this drawing (at least the model name of the product) to everyone so that they can learn more about it? Don't keep everyone guessing.

tiankai001

I learned a lot from Xiaoif. Thank you.