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Re: [Amps] PIC processor inside HF amp

To: jeff millar <jeff@wa1hco.net>, ka1mwp@comcast.net
Subject: Re: [Amps] PIC processor inside HF amp
From: Robin Midgett <K4IDC@comcast.net>
Date: Thu, 31 Jan 2008 08:46:55 -0600
List-post: <mailto:amps@contesting.com>
Pieter Iblings has some knowledge of this...N4IP.


At 09:07 PM 1/30/2008, jeff millar wrote:
>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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