[time-nuts] Basic regenerative-divider questions

John Miles jmiles at pop.net
Sat Sep 29 19:06:42 EDT 2007


> Am I missing something here?
>
> I always thought mixers were non linear by definition, and
> relying on that
> non linearity to function:-)

Sure, a mixer is nonlinear with respect to the multiplicative function it
applies to its two inputs to obtain the desired output.  It should, however,
behave linearly with respect to multiple frequency components that may be
present at any *one* input.  You don't want it to modify either input
signal, just multiply them together.

Think of a mixer with a perfect sine wave at its RF input and a square wave
at its LO input.  It's nonlinear with respect to the switching action caused
by the LO signal, but it had better be linear with respect to how it handles
the sine wave being switched on and off.  If it distorts the sine wave
input, it will generate harmonics that you probably didn't want.  And if you
apply two or more tones to the mixer's input at once, you want only those
same tones coming out, with the usual +/- translation by the LO frequency.
To the extent that the mixer allows the RF input tones to interact or
multiply with each other, it's nonlinear.

This wasn't such a big deal in the old days when your radio had a high-Q
tuned circuit in front of its first mixer, but modern designs work by
shovelling a large portion of the spectrum into the mixer at once.
Nonlinearity is a bad thing in that case.

-- john, KE5FX




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