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Listening to the ether

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ronv

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What kind of sound are you looking for at the output? A beep or a crackle or?
 

JimB

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OK; is this better?

carbonzit-albums-att-picture52986-hf-freq-shift-2.gif


(The bandpass filter would need to be adjustable to track the selected oscillator frequency.)

That will work quite well. It is a well established technique known as direct conversion.
Effectively it is a superhet receiver with an intermediate frequency of zero.

This on the other hand:

carbonzit-albums-att-picture52994-mixer.gif


will not work at all.

All you will get is the sum of the incoming voltages, the frequencies will not change.
If the input frequencies are F1 and F2, the output frequencies will also be F1 and F2.

A radio mixer as distinct from an audio mixer is a multiplicative thing where the frequencies add and subtract.
If the input frequencies are F1 and F2, at the output of the mixer you will get F1, F2, F1+F1 and F1-F2, and probably lots of others as well like 2xF1-F2 and 2F2-F1 etc.
It can get very messy, which is where you need good filters.

JimB
 
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crutschow

Well-Known Member
Most Helpful Member
Redundant post
 
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carbonzit

Active Member
This on the other hand:

carbonzit-albums-att-picture52994-mixer.gif


will not work at all.

All you will get is the sum of the incoming voltages, the frequencies will not change.
If the input frequencies are F1 and F2, the output frequencies will also be F1 and F2.

A radio mixer as distinct from an audio mixer is a multiplicative thing where the frequencies add and subtract.

OK, whoa. Why would RF be different from AF in this respect? Remember we're not dealing with modulated carrier waves here, but just with raw signals. It sounds as if you're saying this would work with audio, but not with RF (correct me if I'm wrong there); why so?

I'm thinking of what seems like an analogous situation with sound waves. If you play two tones of different frequencies together, you get the two tones plus their sum and difference frequencies, correct? I know, because I can hear this (try two sustained notes, say on a violin, to prove this). So this being so, why won't this work with a RF (VLF, actually) and a LO signal?
 

JimB

Super Moderator
Most Helpful Member
OK, whoa. Why would RF be different from AF in this respect?
You misunderstand what I am trying to say.
I am not saying that AF ans RF are different.
I am saying that the device which is referred to as a "mixer" by audio people is a linear device, and is not the same as the device which is referred to by RF people as a "mixer".
The RF mixer has some non-linear characteristics.

Mathematically:

Sin(f1) x Cos(f2) = (Sin(f1 + f2) + Sin(f1 - f2))/2

When two sine waves f1 and f2 (OK a sine and a cosine) are multiplied together, you get their sum and difference frequencies (f1+f2) and (f1-f2).

OK?

JimB
 

carbonzit

Active Member
Thanks. I see. A quick demo with Sound Forge has further convinced me.

So what I get using my "mixer" with, say, two audio waves of different frequency is an intermodulated result, not a truly "mixed" result, correct?

OK, it's off to find some mixer circuits out there in Web-land (and also in my books; the ARRL Handbook has a bunch). Will report back here after I am further enlightened.
 
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