That is absolutely a real effect. Back in the mid-70's I was a product
engineer responsible trying to sell tuning diodes to the TV tuner
industry. UHF tuners in the U.S. back then were still almost all
mechanical, and even though the rest of the world had pretty much
already converted to electronic tuning it was a tough sell. Those RF
engineers back then were VERY conservative but they really, really knew
their stuff. I remember a couple of them giving me a mini-primer on how
they designed a mechanical tuner, and it was fascinating. Coupling from
the oscillator to the mixer was partially affected by current flow in
the very heavy stamped chassis (i.e., ground) partition, and they
described how if they needed more coupling they just punched a hole here
and there to direct the current where it would have more effect.
It might be easy to dismiss such effects as being at VHF frequencies,
but that would be a mistake. Currents most certainly do not flow
uniformly in ground planes, and coupling effects can be real whenever
significant gain is involved.
73,
Dave AB7E
On 8/6/2013 1:46 PM, Jim Brown wrote:
Or perhaps even a circuit layout issue. On several occasions, I've
urged the Elecraft engineering team to attend one of Henry Ott's
excellent EMC workshops. Henry talks about keeping track of where the
current is flowing -- ALL of the current, not just the "intended"
current, and he talks about the invisible schematic hidden behind the
"ground" symbol, which is one of the most common ways we lose track of
the return current. If, for example, the current associated with that
12 kHz clock happens to share a return current path with an audio gain
stage, it gets added to the audio. It's the same sort of mechanism
that if it's at the junction of the box and the outside world we call
a "Pin One Problem."
73, Jim K9YC
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