How does a frequency converter works in a machine?

Answer 1

Frequency converters are common in radio transmitters and receivers, also in any

[analog] device that has to combine many phone-calls onto one cable, or separate

them into individuals on the other end of the cable.

When the frequency of a signal needs to be converted, the 'machine' that does it

has another signal source inside, and the two signals are fed together to a 'mixer'.

In principle, the 'mixer' can be any nonlinear device, because those always create

new frequencies. Non-linear things include diodes, and amplifiers that are being

pushed to the max. (The fuzzy 'audio distortion' you hear when you crank your

amp too high is actually the new frequencies that are created when the audio

frequencies mix on their way through it.)

Here's the idea: Let's say the frequency you have is 'A', and the frequency of the

source inside the machine is 'B'. Feed these two signals together into a non-linear

device and, believe it or not, here are the frequencies that come out of it:

• A
• B
• A+B
• A-B
• 2A+B
• 2A-B
• 2A+2B
• 2A-2B
• 3A+B
• 3A-B
• 3A+2B
• 3A-2B
• 3A+3B
• 3A-3B
• .
• .
• etc.

Each and every one of these 'mix products' is a copy of your signal 'A' ... carrying

the same modulation (information) with it, at a different frequency.

Look back toward the top of the list, where you see "A+B" and "A-B" ... the sum

and difference of your frequency and the one generated inside the 'machine'.

These are typically the strongest products that come out of the mixer, and they're

the ones that are most often used. The output of the mixer is simply 'filtered' to

get rid of all the unwanted mix products.

All you have to do is adjust the internal signal 'B' to whatever frequency it needs

to be, in order to place either the sum, A+B, or the difference, A-B, at the frequency

where you want your copy of 'A' to appear.

The most familiar example of this process is AM radio:

-- 'A' is the music or voice signal that comes from the microphone.

-- 'B' is the carrier frequency, generated inside the transmitter.

-- 'A+B' is a copy of the music/voice signal, near the carrier frequency.

This 'copy' is what gets broadcast, and what comes into your radio ...

along with many other signals at other frequencies.

-- You move the pointer on your radio dial to 'B'. In principle, that sets the

frequency of a source inside your radio to 'B'. Then that internal signal goes

to the mixer in the radio, along with all the stuff coming in from the antenna.

Remember that one of the signals from the antenna is at frequency A+B.

-- One of the frequencies coming out of the mixer is the difference of these two:

(A+B) - B

-- That's just 'A' ... the music and voice signal, which you only have to run to your

speaker or earphones, and the job is done.

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(A purist reading this will object to my simplification on the receiving end,

and will want to jump in and add the descriptions of the superheterodyne

system, and the conversion to IF before the detector. Please don't. The

question has been answered and an example given in principle. Correct it

if it's wrong, but we'll lose people if you 'improve' it.)