Re: [amsat-bb] Conductive paint

[Franklin Antonio <antonio@xxxxxxxxxxxx>]

At 08:21 AM 2/14/2000 -0600, Dave Metz wrote:
>Feritte devices had to be avoided in audio applications.  The beads and
>chokes I had available then would magnetically saturate when used on
>speaker (and even some input leads) leads.  This caused the audio to
>be distorted.

The trick is to run BOTH speaker leads thru the same ferrite toroid (or 
wrap them together around the same ferrite rod, or whatever).  That way, 
the (differential mode) field due to the audio signal cancels to zero, and 
never causes the core to saturate.  RF pickup is almost always common 
mode.  I've seen this work well for home stereos picking up CB or ham 
transmissions.

>For some unknown reason some amplifiers could demodulate FM! Demodulation 
>by rectification of AM signals made sense, but FM???

That's a new one for me!  I can offer a theory.  The circuit that picked up 
the FM modulated RF signal did not have a flat frequency response at the RF 
frequency.  Therefore, the FM modulated signal had an AM component.  The AM 
was then rectified.  You might think gee, the FM deviation was really 
small, how could it be enough to make enough AM to cause 
rectification?  Perhaps the answer was your proximity to the FM 
transmitter.  The amount of RF picked up was large, so even this small 
effect was enough.


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