>Mixers are always non-linear devices, if they aren't they won't
>"mix". Why doesn't the mixer in your receiver create close-in IMD
>or cross modulate?
Mixers do generate intermod products. However, we have to define what is
an "acceptable" level of intermodulation distortion. A mixer driven at
proper levels with a proper LO applied will produce negligible
distortion. If IM performance improvements are desired, a mixer with a
higher LO drive capability can be used.
The mixer in your receiver WILL generate IM in the presense of strong
signals (such as in a contest weekend!). That's why on almost all rigs
you have a front end attenuation setting. On the fancier rigs like the
FT-1000 series, you have a preamp that you can switch in and out and
several attenuation levels you can switch in and out as well.
As to Tom's comment about 1/2 cycle single stage amps producing more
distortion than a push-pull amp, I agree with him. Distortion (IM
products) are dependent upon bias conditions (conduction angle) not how
many tubes you use. If that were the case, you could build a class C
linear amplifier with many tubes! Two tubes each running in opposite
half cycles will produce just as much IM products as a single tube.
Perhaps this myth got started from the fact that perhaps at one time
someone's exciter when driven hard produced a lot of IM. So, the guy
built an amp with two tubes, cut his drive level in half an viola!
reduced his IM. But that had everything to do with a crappy exciter and
not the amp.
73,
Jon
KE9NA
--------------------------------------------------------------------------
The Second Amendment is NOT about duck hunting!
Jon Ogden
jono@enteract.com
www.qsl.net/ke9na
"A life lived in fear is a life half lived."
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