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From: blackm00@cam.org (Michael Black)
Newsgroups: sci.electronics.design
Subject: Re: How does a mixer work?
Date: 17 Oct 2002 21:35:10 -0700
Organization: http://groups.google.com/
Message-ID: <6447bcd3.0210172035.21a591c0@posting.google.com>
References: <51ca721c.0210161956.73a25483@posting.google.com> <74fd7cec.0210170455.79758cd0@posting.google.com>
NNTP-Posting-Host: 204.19.188.18
NNTP-Posting-Date: 18 Oct 2002 04:35:10 GMT
robin.pain2@bewator.co.uk (Robin) wrote in message news:<74fd7cec.0210170455.79758cd0@posting.google.com>...
> You could create an AM modulated carrier like this:-
>
> modulated carrier op
> |
> |
> |
> carrier(-+V)------\/\/\/\/\/\-------.
> |
> |
> -----
> ---
> -
>
> i.e. by swinging the (linear) pot's wiper up and down periodically
> (and symetrically about its midpoint).
>
> 1) When the wiper is at ground there will be no output.
> 2) When midway the output will swing -V/2 and +V/2.
> 3) When full up the output will swing -V and +V.
>
> The ratio between 2) and 1) is infinity.
> The ratio between 3) and 2) is 2:1
>
> i.e. non-linear.
>
> cheers
>
> Robin
>
>
I saw your second post, but when I first read the above, my
reaction was that you will end up with a carrier of varying strength,
not an AM signal. I think this is worth going into.
Mixing/modulating is about translating frequencies. If you have
an AM transmitter, say at 10MHz, and you feed in a 1KHz signal,
the output will have three parts. The audio translated to 10.001MHz
(the sum), the audio translated to 9.999MHz (the difference), and
the 10MHz carrier. The sum and difference will vary in amplitude
as you vary the audio level, but the carrier will remain constant.
The sum and difference, also known as the sidebands, carry the
information. They are the audio translated to radio frequency.
The carrier is just incidental to the fact that the modulator
is unbalanced, and so the 10MHz signal is passed through to
the output. The 1KHz audio signal would probably be at the output
too, except that the component values chosen for the 10MHz freqency
would tend to shunt out the audio.
If you put a meter on the output of the transmitter, of course
the amplitude varies. You are measuring the whole output, and
since the sidebands will vary as the audio level varies, you
will see a varying amplitude signal. But if you put filtering
between the output and the meter, so you could pick off the
three distinct outputs, you would see the constant amplitude
carrier, and the two sidebands that varied with the audio level.
If you filtered out the carrier, you would still have an output.
Or if you used a fancier modulator/mixer, one that balanced out
the carrier, you'd still have the two sidebands at radio frequencies
at the output.
Whether the signal to the modulator is audio or some other
radio frequency is irrelevant to the discussion. If you changed
the 1KHz signal to 1MHz, the output of the modulator would have
a signal at 10MHz (again the carrier simply passing through
the unbalanced modulator or mixer), 11MHz (the sum) and 9MHz
(the difference). The terminology tends to be different when
talking of modulators and mixers, but again one could call the
sum and difference the sidebands. When talking in terms of
two RF signals, the lack of need for the carrier may be more
obvious, because when mixing RF signals, you don't really want
the input signals to be at the output of the mixer. In practice,
at least some of it is filtered out by the tuned circuits commonly
used.
Michael
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