[Elecraft] P3 generates noise on 144MHz

[Dave New, N8SBE]

The point is that splitting the ground planes in an attempt to isolate
them, often makes the situation the PCB designer thought they were
fixing much worse.  It's a case of knowing where the actual return paths
are, and minimizing the length of those return paths.  The other half of
the EMC equation is proper loading of signal lines, to shape the edges,
and suppress ringing.  It is far more effective to put series 'ballast'
resistors in the signal lines, rather than try to suppress EMC with
capacitors across the lines to ground.

Please refer to the posted EMC tutorial links I posted below for
graphics/explanations.

Designing for EMC is not a easy chore, but starting with a bad PCB
design will make it almost impossible to suppress systems issues that
may crop up after the layout is 'frozen' and the unit is headed for
production.  At that point, trying to fix the issues with shielding
and/or ferrite cores is very expensive, and often doesn't produce the
desired results.  Nothing short of re-laying out the offending PCB will
suffice in the majority of cases.

73,

-- Dave, N8SBE

> -------- Original Message --------
> Subject: Re: [Elecraft] P3 generates noise on 144MHz
> From: "hawley, charles j jr" <c-hawley at illinois.edu>
> Date: Mon, February 18, 2013 1:08 pm
> To: "Dave New, N8SBE" <n8sbe at arrl.net>
> Cc: Roger Crofts <roger at monitorsensors.com>,        elecraft reflector
> <elecraft at mailman.qth.net>
>
>
> What does the "zig zag" gap look like and what is the issue with it? Or...how does it contribute to the noise?
> Thanks,
>
> Sent from my iPad
> Chuck, KE9UW
> (Jack for BMW motorcycles)
>
> On Feb 18, 2013, at 11:42 AM, "Dave New, N8SBE" <n8sbe at arrl.net> wrote:
>
> > 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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