Yes, there are many ways for RFI to get into circuitry.

But skin effect means if you shield all circuitry with a faraday shield
and put ferrite beads in contact with wires going
in and out of that circuitry most possibly
you can limit signals of like 1Mhz or more
making entry into the device.

BALUNS as I understand, keep outside and inside
shielded signals from mingling.

The low frequency of seismic signals means they travel
deep within the wires and not in the surface only.

I have seen boxes built to isolate wires
but you need filters in their own cavities
for every wire going in or out of the circuitry
this means you build shielding like those
Russian Dolls of one nested inside of another.

Superbeads at microwave frequencies must be
a tight fit around a wire and epoxied into place
right against the metal box itself.
Or it has little effectiveness.

The other way is to use fiber optics
I/O that does not pick up EM waves
like an antenna,
or so I understand.

This all means very expensive results to get an RFI free
circuitry.

I feel like that robot on the tv commercial
which hangs it head in dealing with rfi.

cheers;
geoff

BTW, I just time tested the Locked GPS/A-D WinSDR setup purchased from
Larry and it passed with flying colors.
As did the system I had before Larry's.
But Larry's A/D is superior to the 12 bit I was using.
And, everything is much easier to put together.

Unless you are a glutton for punishment,
Larry's system is the way to go.

I recommend you position the GPS receiver
at some level above general life, maybe
7 or more feet above the floor with no metal
on your roof.
I attached it to an AC ground plane on top of a PVC pipe
inside the kitchen and it seems to work OK.
But still worry about radio stuff which can
realistically do nothing about.

Darn, Darn, to the mole power,
No one builds for RFI today they just limit
power levels to limit radiation OUT of a box.
No one cares about INTO unless the emergency people
happen to be around with their powerful
transmitters.








-----Original Message----- 
From: Jim Santee
Sent: Monday, November 01, 2010 8:31 PM
To: psnlist@..............
Subject: Re: GPS RECEIVER


Radio interference by transmitters is quite common in a number of other applications. There are several factors that come to mind:
1. The power output of the transmitter leaks into the circuit due to proximity i.e. next door to a high power station.
2. The wiring of the circuit suffering the interference be close to the wavelength of the transmitting frequency.
3. The lack of shielding in the circuit being interfered with.
4. The harmonics - i.e. multiples or submultiples is a simple term for this -  from the transmitter leaking into the circuit.

You can find yourself chasing your tail on this. Here is a suggestion:
1. Shielded cable - but watch out - your shield can act as a receiving antenna as well. If you are still suffering try and lift the 
ground connection on one end of the shield. You could have a ground loop arrangement. Sometimes you get better results with the 
ground be attached to one end of the circuit. There are decoupling devices to prevent ground loops.
2. Ferrite beads are a great help in eliminating interference. You place a number of small beads on each wire and the beads act as a 
filter. If you notice on some computer wiring that you will see a drum like device on the ends of cables, this is a large ferrite 
bead. You can also try this method.

The worst offender for interference is cheap fluorescent lighting from home centers. These buggers can really mess things up. You 
can try grounding the fixture to see what happens but you still notice some buzzing. You shop some surplus outlets and might find 
some Navy surplus shielded fluorescent light fixtures and these might help. You can also try placing metal mesh over the light to 
see if this also helps out.

Jim Santee 

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