On Wed, 17 Nov 2004 09:06:27 -0500, Pete Smith wrote:
>This raises a question I have wondered about. Years ago, I had
>interference to a stereo that originated with the speaker leads. Not
>knowing any better, I went to Radio Shack and got a couple of RF chokes and
>a couple of disk ceramic caps, and put them in series and shunt. Worked great.
>
>If the sensors on this alarm system are DC, wouldn't a similar fix be the
>best way to go about cleaning up the sensor lines?
It depends on how the RF is getting in. If it is a pin 1 problem, it probably
would not fix it.
If it was differential mode, it probably would.
To understand why, I'll review what "the pin 1 problem" is. A cable shield, if
there is one,
should go directly to the shielding enclosure, not to the circuit board. If it
goes to the
circuit board first, the path from that shield to "ground" along the PCB and
other wiring
will have inductance, and the received antenna current flowing through that
inductance
will put an IZ drop across various points on the PCB depending on the board
layout.
Thus any RF current on the cable shield is injected into the circuitry, where
it is likely to
be detected.
There are two practical fixes for this problem. One is to move the shield to
where it
belongs -- the shielding enclosure, or, in the case of an unshielded enclosure,
the star
ground point. The other is to choke off the current (or reduce its strength to
the point
where detection either no longer occurs or is no longer audible/causing data
errors.
Now, consider an unshielded cable going to an unbalanced input, with the signal
return
going to "circuit common" on the circuit board. Now you have the same problem,
except
that it is common mode current on the signal pair, not the shield. The solution
here is to
first terminate the circuit return to the shielding enclosure. By virtue of the
capacitance
between the conductors, this will reduce the common mode RF at the input. The
second
part of this solution (without changing the equipment) is to use twisted pair.
This
reduces the loop area, and thus the differential mode RF, and it also makes the
RF
more common mode. And the common mode has been taken to the shielding
enclosure. Again, choking the common mode current is a good thing.
There is another mechanism at play with shielded twisted pair cables. It is
called shield-
current-induced noise (SCIN), and it converts RF shield current into a
differential-mode
voltage on the signal pair. It happens when the shield current couples
asymetrically to
the signal pair -- that is, more to the red than the black. That most typically
happens with
foil/drain shields, and it occurs because the drain wire is usually twisted at
the same
rate (lay) as the signal pair, and much closer to one signal conductor than the
other.
SCIN is a MAJOR contributor to RFI between about 100 kHz and 10 MHz. Above that
frequency, shield current symmetry tends not to be disturbed by the drain wire,
flowing
more uniformly in the foil instead. I use the word "contributor" because the
victim
equipment must contribute its inadequate filtering of the input or output to
which that
cable is connected. A simple RC or LC filter (like what you did) at the
equipment would
fix this, assuming that it didn't also degrade the signal. But choking the
shield current
also fixes SCIN, and it can't degrade the signal.
I've published some research on both of these mechanisms in four AES papers,
and
they can be downloaded from the AES website.
Jim Brown K9YC
_______________________________________________
RFI mailing list
RFI@contesting.com
http://lists.contesting.com/mailman/listinfo/rfi
|