On 10/2/2018 12:10 PM, Stuart Rohre wrote:
Another idea for the cause of the noise clicks is a bad ground in
interconnecting cable for this circuit.
Yes, but please don't call it "ground" -- it's NOT! Rather it's a RETURN
for the current in the center conductor, making it a transmission line.
When it's a transmission line, all the return current flows in the
shield (or the other conductor if it's parallel or twisted pair). If the
return is missing, it's not a transmission line, so return current flows
on whatever path the rest of the wiring provides, like the chassis. This
causes a large inductive loop to be formed by the center conductor of
the coax and the large return path, and that loop couples the magnetic
field from other currents in the radio.
Try a clip lead end to end of the patch cable.
That's a start, because if it closely follows the path of the coax, it
reduces the size of the loop. But it's still a loop.
Good Luck! Had a similar noise when a custom builder we were using left
off a second (ground) conductor for a cross chassis path for a digital
clock signal, TTL btw. We wadded a second grounded condcutor wrapped as
twisted pair, and cured our noise.
Yes but it's NOT a ground. You simply formed a transmission line.
BTW -- for those who don't know, twisted pair forms a transmission line,
and if it's really good twisted pair, it's VERY good at resisting noise
and other crosstalk. CAT5/6/7 cables are four very good twisted pairs,
with the added (and very important) technique of twisting each at a
different rate.
Elecraft uses a short length of twisted pair to tie the input SO239
connector of their 500W amp to the circuit board.
A very important design flaw of TenTec's 229-238 series of antenna
tuners is that it uses the chassis as a return. Palstar made the same
mistake in the high power tuner I looked at in their Dayton booth, and
when I pointed that out to the design engineer in the booth, he didn't
understand why that was a problem!
Ten Tec did NOT make that mistake in their 425 Titan amp -- coax is used
for 100% of the input and output RF path.
73, Jim K9YC
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