From: Don Pearce
Subject: Re: How does a mixer work?
Date: Thu, 17 Oct 2002 08:47:48 +0430
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On 16 Oct 2002 20:56:30 -0700, email@example.com (Asa Cannell) wrote:
>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?
In normal operation, the local oscillator port of a mixer is
overdriven hard, resulting in a square wave drive. So you can consider
the local oscillator as switching between plus and minus 1 with the LO
signal. The output at the IF port is indeed the RF signal multiplied
by the plus and minus 1 LO signal. The resulting output is a pair of
signals at the sum and difference frequencies of the RF and LO
You can also use a mixer as an attenuator by putting DC on the LO
port. With zero volts, there is no output (provided everything is
balanced perfectly). As you increase the volts, the output will rise.
Drop the volts below zero, and the output will rise again, but this
time with the phase reversed.
What you don't get is a multiplication product greater than 1 - it is
strictly an attenuating process, and once the signal at the LO port
hits 1, it limits.