In a superheterodyne receiver, after selecting the frequency of the signal, how does the receiver know what the frequency is in Hz?

深圳小花

In a superheterodyne receiver, after selecting the frequency of the signal, how does the receiver know what the frequency is in Hz? [Copy link]

 

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The information about superheterodyne says:

The typical application of the superheterodyne principle is the superheterodyne receiver . The signal received from the antenna is amplified by a high-frequency amplifier (see tuned amplifier ), and added to the mixer for frequency conversion together with the signal generated by the local oscillator to obtain an intermediate frequency signal , which is then amplified , detected and amplified at a low frequency before being sent to the user. The operating frequency range of the receiver is often very wide. When receiving input signals of different frequencies, the local oscillator frequency f1 can be changed to keep the intermediate frequency fi after mixing at a fixed value, such as generating a 465KHZ.

How does the superheterodyne know the frequency of the signal received from the antenna after frequency selection? It must know the frequency first, and then it can determine the frequency of the local oscillator.

maychang

The superheterodyne receiver does not know the frequency of the external signal, but only changes the frequency of the local oscillator. There are many external signals, all mixed with the local oscillator signal, but only one external signal (there may be two or even more, which will be described later) is mixed with the same frequency as the center frequency of the intermediate frequency amplifier, which can be amplified by the intermediate frequency amplifier and demodulated to obtain the low-frequency signal we need.

maychang

The external signal we need is usually one intermediate frequency lower than the local oscillator signal, but if there is an external signal that is just one intermediate frequency higher than the local oscillator signal, the external signal will also generate an intermediate frequency after mixing, and this signal will be amplified by the intermediate frequency amplifier. This kind of external signal we don’t need is called the "image frequency" signal. It is usually filtered out before mixing, at least to reduce the amplitude of this unwanted signal to a level that is not harmful.

maychang

If the frequency of the signal to be received is known, then the local oscillator can generate a signal that is one intermediate frequency higher than the signal to be received. This is the case with digitally tuned radios.

深圳小花

maychang posted on 2021-8-5 12:26 The superheterodyne receiver does not know the frequency of the external signal, but only changes the frequency of the local oscillation. There are many external signals, all of which are mixed with the local oscillation signal, but...

Thank you very much. But according to what you said, this receiver can only listen to one channel? This is impossible, can't I tune in and choose which channel to listen to?

深圳小花

maychang posted on 2021-8-5 12:31 If the frequency of the signal to be received is known, then let the local oscillator generate a signal that is one intermediate frequency higher than the signal to be received. Digital tuning radio...

How does a digital radio detect the frequency of the received signal?

maychang

Shenzhen Xiaohua published on 2021-8-5 13:16 How does a digital radio detect the frequency of the received signal?

『How does a digital radio detect the frequency of the received signal? 』

Each radio station has its own transmitting frequency, which is known through other means when the receiver starts working, and is not "detected" when the receiver is working.

An extreme example: the frequency of the signal transmitted by the satellite must be known in advance, and the receiver on the ground only needs to receive the signal of this frequency.

maychang

Shenzhen Xiaohua posted on 2021-8-5 13:14 Thank you very much. But according to what you said, this receiver can only listen to one channel? This is impossible. Can't I change the channel and choose which one...

"Then this receiver can only receive one channel?"

By changing the local oscillator frequency, you can receive stations with different transmission frequencies. Changing the local oscillator frequency is achieved by changing the capacitance or inductance in the LC loop.

Car radios were widely used in the 1930s and 1940s. Car radios usually have several buttons. When a button is pressed, the local oscillator generates a signal of a certain frequency. The oscillation frequency corresponding to a button is adjusted in advance to an intermediate frequency higher than the transmission frequency of a radio station. Pressing this button will receive this radio station, and pressing another button will receive another radio station. The button actually plays the role of switching different capacitors to change the local oscillator frequency.

chunyang

The local oscillator frequency is adjusted manually or automatically under manual control. You must have used a radio or a TV. The process of selecting a channel is to change the local oscillator frequency until you receive the station you want to listen to/watch.

chunyang

Shenzhen Xiaohua published on 2021-8-5 13:16 How does a digital radio detect the frequency of the received signal?

There are two types of digital radios. One type transmits modulated carrier signals through radio or CATV lines just like traditional radios. The signal receiving process of this type of digital radio is exactly the same as that of traditional analog radios. The other type of digital radio is completely different. There is no carrier wave, but it uses TCP/IP network communication protocol. This type of digital radio is usually called "Internet radio" and does not involve any frequency of receiving signals.

chunyang

Shenzhen Xiaohua posted on 2021-8-5 13:14 Thank you very much. But according to what you said, this receiver can only listen to one channel? This is impossible. Can't I change the channel and choose which one...

You think so, obviously you don't understand what Maychang said. I have reminded you many times that you are seriously lacking in basic knowledge and you should make up for it first. You must re-learn systematically. Scattered knowledge points are meaningless to you. You will not really understand, at least not deeply. Instead of doing useless work like this, it is better to spend time to make up for the basics first.

maychang

Shenzhen Xiaohua posted on 2021-8-5 13:14 Thank you very much. But according to what you said, this receiver can only listen to one channel? This is impossible. Can't I change the channel and choose which one...

"But according to what you said, this receiver can only receive one channel, right?"

What I said on the second floor is: there are many external signals, all mixed with the local oscillator signal, but only one external signal has the same frequency as the center frequency of the intermediate frequency amplifier after mixing, which can be amplified by the intermediate frequency amplifier and demodulated to obtain the low-frequency signal we need.

At a certain moment, the LO frequency is fixed, so of course only one station can be received. The signals of other stations cannot be amplified by the IF amplifier (to be precise, they are greatly attenuated), so they cannot be heard. If the LO frequency is changed, other stations can be received, but still only one station can be received.

深圳小花

chunyang posted on 2021-8-5 15:26 You think so, obviously you don't understand what maychang said. I have repeatedly reminded you many times that you are seriously lacking in basic knowledge and should first...

OK. Thank you, teachers. I have not studied the two courses of high-frequency circuits and non-linear circuits, nor the course of signals and systems. This is a big shortcoming.

深圳小花

maychang posted on 2021-8-5 15:39 "But according to what you said, then this receiver can only listen to one station?" What I said on the second floor is: there are many external signals, ...

Thank you so much.

chunyang

Shenzhen Xiaohua published on 2021-8-5 15:42 OK. Thanks, both teachers. I have never studied the two courses of high-frequency circuits and non-linear circuits, nor the course of signals and systems. This is a big...

The knowledge involved here is all in the basic simulation part of electronic technology, and is detailed enough.