[Elecraft] P3 generates noise on 144MHz

[Dave New, N8SBE]

Roger saith:

"When I had the I/O printed circuit board out of the case, I noticed
that the earth planes for the power supply section and RS232 section are
separated by a zig zag gap."

Ouch! That indicates that whoever laid out the board had no idea that
'gapping' the board is a BAD IDEA(tm).

Please, whoever was responsible for laying out the I/O board for the P3,
go to this site, and learn what REALLY has to be done to properly design
PC boards for EMC:

http://www.learnemc.com/index.html

I attended a whirlwind 4-hour course on automotive EMC at last year's
Freescale Technology Forum that was a very compressed version of the
2-day courses shown on the LearnEMC home page, and it was a real
eye-opener.  If there is anything critical as a take away from that
experience, it was that the PCB layout will make-or-break your EMC
issues, and is the best and most cost-effective way to stop EMC issues,
rather than trying to use ferrites, etc. after the fact, all of which
will never be as effective.

If you can't do the two-day course (highly recommended) there are free
extensive tutorials to be found at the "EMC Tutorials" link on the main
page:

http://www.learnemc.com/EMC-Tutorials.html

It's all good stuff, and should be mandatory learning for anyone laying
out PC boards these days.

73,

-- Dave, N8SBE

> -------- Original Message --------
> Subject: Re: [Elecraft] P3 generates noise on 144MHz
> From: "Roger Crofts" <roger at monitorsensors.com>
> Date: Tue, February 12, 2013 6:02 pm
> To: "elecraft reflector" <elecraft at mailman.qth.net>
>
>
> Last night I cleaned all mating metal surfaces in the P3 but, alas, this made no difference. The noise was still at S6.
> When I had the I/O printed circuit board out of the case, I noticed that the earth planes for the power supply section and RS232 section are separated by a zig zag gap. They are commoned together towards the inner edge. This has the effect of lengthening the route of RF currents from the earthy side of C500 to the metal case. My thoughts are that C500 will do a great job of removing RF differential mode voltages across the power cord. It will not do such a good job of removing common mode RF voltages from the power cord. I believe it is the common mode RF voltages that are the problem. This was confirmed when I replaced the power cord with a coaxial one (as suggested by John, G4ZTR). This made no difference to the noise.
> I found a ferrite ring which was large enough to pass the power plug through it. In fact I passed the plug through it three times to form two tight turns and I moved this choke as close as possible to the P3 power socket. The result was that the noise dropped to S4. Almost there!  I think it would be better if the filter was totally inside the P3 case. I will try that next.
>
> Roger Crofts, VK4YB
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