Topband: A/B testing K6se etc.
jkaufmann at alum.mit.edu
Tue Jul 1 21:55:36 EDT 2003
> If there is actually no 3rd harmonic being radiated by the BC station, I
> assume that you're saying that the fundamental (at 610 kHz) is causing
> the receivers under test to somehow generate a pseudo-3rd harmonic.
It seems to me there is a simple test that can be performed to determine if
a third harmonic is being generated internally (in the receiver) or
externally (at the BC transmitter). It just requires a step attenuator
inserted in series with the antenna. Since most receivers have built-in
attenuators, they are probably adequate, as long as they are ahead of the
first active stage in the radio.
The main point is that an internally generated spurious third harmonic
should respond nonlinearly to changes in input signal strength. An
third-order nonlinearity that is producing a third harmonic response should
theoretically cause the third harmonic generation to drop 3 dB for every 1
dB of input attenuation. If 10 dB of attenuation is inserted, then the
third harmonic should drop 30 dB if the receiver is generating the harmonic
itself. If the harmonic is being produced externally, then 10 dB of
attenuation will result in a linear 10 dB drop in the harmonic. It should
be easy to distinguish whether the harmonic has dropped 10 dB or 30 dB.
73, John W1FV
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