Newsgroups: rec.radio.amateur.misc

In rec.radio.amateur.misc, afpauls@lims01.lerc.nasa.gov (PAUL SCHWARTZ) writes:

    I have an application where the keying of hand held radio (160 MHZ) is 
    causing an unacceptable disturbance in the signal coming from a position 
    transducer.  The position transducer has a signal conditioner mounted 
    remotely from the sensor head.  The interferance is picked up at the signal 
    conditioner.  If feel that the best way to solve this problem would be to
    mount the signal conditioner in an RFI proof enclosure.  Here are my 
    questions.

    Would an RFI proof enclosure work?


First, you should determine where the interferring signal is entering into
your system.  As you describe it, the posssiblities are:
	1) The signal is coupling into the transducer
	2) The signal is coupling into the line between the tranducer and
	   the signal conditioner
	3) The signal is coupling into the signal conditioner
You mention that the signal is coupling into the signal conditioner.  You
should be sure of this assumption (an easy way to check would be to 
disconnect the transducer, terminate the input of the signal condtioner,
and see of the interference remains).  

*If* the signal is only coupling into the signal conditioner, then the
following things might help:
	1) Using good layout practices in the signal conditioner. E.g.,
		* solid ground planes
		* low impedance power paths (possibly even power planes)
		* rf decoupling caps (.01uF) with short leads from the
		  power supplies to ground at various places throughout 
		  the circuit
		* careful placement of key components, e.g., the front-end
		  amplifier
	2) Shielding the signal condtioner.  A good shield around the entire
	   signal condtioner may help.  Since the dimensions of the thing
	   are large as you describe them, you might even consider using
	   wire mesh (chicken wire??).  At 160MHz, a wavelength is ~1.9
	   meters. As long as you keep the "holes" in your shield well
	   below a tenth of a wavelength (.19m = 7.5 in), the shield will
	   work well.  You must take care about how the shield is grounded
	   and about putting gaps in the shield, however.

	   It may be possible to just shield a portion of the signal
	   conditioner.  For example, maybe just the front-end amplifier ---
	   especially if the amp has a fairly high gain. 

	3) Limit the bandwidth of your system.  It may be possible to 
	   put enough filering in the signal path to "bleed off" the
	   rf interference signal.  Presumably, the tranducer output is
	   at audio frequencies or below.  You should be able to place
	   low pass filtering in the signal path, e.g., small shunt caps.
	   Make sure that the capacitors have short lead length and that
	   they are tied to a good ground plane. Otherwise, you run the
	   risk that the caps will resonate as rf frequencies and prove
	   useless as low pass filtering.
	   
If the interferring signal is entering into other parts of the circuit,
these things may help:
	1) Shield the line between the transducer and the signal conditioner.
	   I would suggest a twisted-wire pair with a shield (assuming the
	   transducer outputs a signal differential signal).  
	2) Shield the tranducer.

Chris