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From: Don Pearce
Subject: Re: How does a mixer work?
Date: Thu, 17 Oct 2002 08:58:31 +0430
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On Wed, 16 Oct 2002 21:22:52 -0700, "Alfred Lorona"
>"Asa Cannell" wrote in message
>> How does a mixer work? I don't understand why it needs a non-linear
>> element. I think examples of why a linear element (resistor) wont work
>> would help illustrate why a non-linear element (diode) will. Also,
>> what does 'mutiply' mean in terms of mixing? What if two DC signals
>> are applied to a mixer, like 2V and 4V? Will the output be 8V?
>If you apply a 10 hertz and a 15 hertz signal to a resistor all you can
>expect as output is a 10 hertz signal and a 15 hertz signal. In a mixer, the
>idea is to obtain the difference between the two frequencies which is 5
>hertz and the sum of the two frequencies which is 25 hertz at the output.
>The output will also contain the two original frequencies of 10 and 15
Only as a spurious result of less than perfect balance. This is not
intrinsic to the process. I am talking about "real" double balanced
mixers, of course. For a single diode acting as a non-linear element,
you are right.
>The useful, or desired, output signal is either the sum or the
>difference frequency. All of the others are filtered out and only the
>desired frequency is allowed to pass through to the rest of the circuitry.
>You can only obtain the sum and difference frequencies in a non linear
>circuit element. That is why a resistor, being a linear circuit element,
>won't do the job.
>The correct technical term for the mixing process in a mixer is called
>'multiplication' but this sometimes confuses people such as you indicate
>with your example of the two DC voltages. Hi.
>Until you gain a more detailed idea of the mixing process, it may help to
>just think of a mixer as a 'distorting' device that generates frequencies at
>the output that are not present at the input . This is what distorting
>A diode is a non linear circuit element and that is why it makes a good
>mixer and is widely used in inexpensive AM radios.
>No, mixers do not work with DC input signals.
Oh yes they do. DC on the LO port works just fine. If the mixer is
transformer coupled, then obviously you can't use DC on the RF and IF
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