chuck wrote:
>
>
> I'm thinking of putting a PIC inside my amp and having it run a LCD display
> out front. I've seen it done, but am curious about
>
> The types of interference I'd end up with. I have my enclosure separated PS
> on left side and RF on right, but I bet when its running
>
Let's cover four kind of interference
* Radiated emission from the amplifier
o High E or B fields inducing currents on the PIC board
* Conducted emissions from the amplifier
o Signals entering the PIC board via it's connections to the
amplifier's sense points
* Radiated emissions from the PIC board
o The logic noise might get into the Receiver
* Conducted emissions from the PIC board
o noise on the I/O from the PIC that couples to the Rx coax
center conductor.
The conducted emissions probably create more problems than radiated
because the HV probe line, Plate and Grid current lines, etc. have a lot
of RF voltage on them and they get connected to the PIC. Imagine the
currents flowing into the PIC board on one pin and out on some other
pins...this can definitely mess up the A/D's and even logic on the PIC.
For radiated emission to create a problem, the radiated wave has to
generate significant voltage across a wire or circuit trace and the
advantage of the PIC comes from the very short circuits, located close
to a ground plane. Its circuit traces make really bad antennas.
The PIC may cause Receiver problems by conducting noise from the PIC
through it's sensor cables to RF circuits close to the antenna relays.
That provides a much lower loss path for noise than radiating up to the
HF antenna on top of the tower.
So, how to deal with this.
* Put the PIC in a box and filter the bejezzus out of all the lines
going in and out.
* Use .001 uFd feed through capacitors on each sense line and power
line.
* Ensure that noise voltages flowing on sense and power gets
bypassed to the enclosure
* Put about 47 uH inductors on each signal between case wall and PCB
(1000 Ohms at 3.5 MHz)
* Add a second 0.1 uFd bypass from signal to PCB ground plane on the
PIC board.
* Ground the digital logic enclosure to the RF ground of the
amplifier, not just the front panel
* Once the filtering is in place, there's little need for shielding
on the sense lines.
The digital bus between processor and display creates a problem because
ribbon shielding isn't that good. Maybe put the LCD and the PIC in the
same box?
jeff, wa1hco
> At a good clip, there will be lots of noise. I'm guessing lots of rfi
> filtering in a shielded box and maybe shielded ribbon out to the
>
> Front panel? Any others do similar? Looking to monitor plate and grid
> volts/current perhaps as a start..
>
>
>
> Later on I'd use it to hold off for heater warmup and such..
>
> Perhaps the Tetrode board(g3sek) is easy to interface to a pic/lcd?
>
>
>
>
>
> Chuck -KA1MWP
>
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