RFI Problems

J

Thread Starter

James Sheldon

When a security radio is keyed close to a group of electronic RTD Love temperature switches, the switches trip. We have the hardwired I/O signal leads and power leads(125VDC)properly grounded at one end but the problem still persists - even when grounded at both ends! We even removed all wires but the shielded power leads, placed it on the bench, and the switch still tripped when the radio was keyed. Enclosing the switch in a grounded Hoffman box did not work either. Somehow the waves are getting through. Manufacturer says they have not seen such problems in the 10 year history of the device.

Security cannot avoid going into this area.

Does anyone know what is causing this device to trip and what can be done to correct it? Do I need a more RFI immune switch?

Thanks.

James
 
Hi,
we have had similar problems with all sorts of devices picking up noise, not just radios. However we have a switchroom where there are RTD input cards which can be caused to go into fault if you use a portable radio in the room. As a result, our policy is now not to use portables in the room, but only used the fixed radio system.

You could try removing the shield connection all together so it isnt earthed at all. I know it sounds silly and goes against all installation practices, but I've had experiences where we pick up too much noise on the earth itself and it causes peaks to be seen on the actual cores of the screened cable (can be seen on the C.R.O.), which depending on the input tolerance, can cause troubles.

Hope this helps.
Julian
[email protected]
 
Just one thing I did forget to mention in my first reply, you said you've tried a grounded enclosure. The radio waves can still get through the gaps in the door of the enclosure and any holes.

You can try sealing them up with ani RFI stick on material (RS Components in Australia sell the stuff) and stick it around the inside of your enclosure, and around the door.

Julian
 
B
Hi James,

You mentioned shielded power leads, have you also put inline filtering on those leads? Sometimes a simple clamp-on ferrite helps (TDK and Fair Rite, www.fair-rite.com, are manufacturers, they are available from most electronic supply houses). Looping the wires thru the ferrite increases the effectiveness of the ferrite. Use one ferrite and pass the supply wires thru it if they will fit.

You mentioned 125VDC for the supply on the switch, I do not know if there are manufacturers of inline power filters for that level of DC but that might also be an option to explore.

The grounded Hoffman box might also be someplace to revisit. An older engineer engineer I worked with w/ RF design experience used to say "Ground is not the same all the world around" (usually it was when I had a real problems w/ RF interference w/ instrumentation). Sometimes the metal boxes have a non conductive neoprene door gasket that effectively leaves a slot around the door opening. In RF terms, slots can be effective antennas at the right frequencies. The easier route would be to contact Hoffman to see if they have an enclosure developed specifically for EMI shielding applications that might have all the conductive gasketing in place.

If not then...

It might be effective to experiment with the door gasket and use some conductive copper tape to try to seal the door and see what effect it might have - remove any paint and ensure that the surfaces are clean before placing the tape. Also look at the door hinge, it is usually just secured by the hinge pin and not a real good electrical connection to the rest of the enclosure. If that is successful, companies like Laird Technologies sells conductive elastomers for door sealing gaskets that you might be able to use to seal the door (w/out sealing it shut like with the copper tape).

You might also want to look at this site, they have info on all types of RF related issues and you might be able to pick some additional info (altho' you might have to poke around there a bit).

http://www.rbitem.com/askexpert/default.asp

After all said and done the best protection from EM interference starts out at the component level (specifically the device) and then builds out to the system accordingly. Unfortunately for you it looks like you're stuck w/ the switch you have. I hope this info helps!

Good luck!

Bob
EMC Test Technician
 
Many manufaturers say that they have not seen the problem. It is in the design. I had similar probems with different devices. In one I modified the power supply connections to each of the IC with two 22 ohms resistors in series with supply pins and two 10 nf capacitors on to the supply pins to ground. This solved the problem. But this may not be possible every time.
Try providing ferrite beads to the supply wires and ferrite ring to all control wires going in / out of the box. You can also try a low pass filter at the input signal terminals.
If you can try to replace the existing brand with another one.
BEst regards,
Sekar
 
M

Michael Griffin

On November 26, 2002 05:37 pm, James Sheldon wrote:
<clip>
> When a security radio is keyed close to a group of electronic RTD Love
> temperature switches, the switches trip. We have the hardwired I/O
> signal leads and power leads(125VDC)properly grounded at one end but
> the problem still persists - even when grounded at both ends! We even
> removed all wires but the shielded power leads, placed it on the
> bench, and the switch still tripped when the radio was keyed.
> Enclosing the switch in a grounded Hoffman box did not work either.
> Somehow the waves are getting through. Manufacturer says they have not
> seen such problems in the 10 year history of the device.
>
> Security cannot avoid going into this area.
>
> Does anyone know what is causing this device to trip and what can be
> done to correct it? Do I need a more RFI immune switch?
<clip>

Perhaps you can get your security guards to switch to a different type of radio. I am told that the older models of radio operated at power levels and frequencies which are much more prone to cause interference problems than newer ones are. The ones intended to meet CE compliance are supposedly much less likely to cause problems.
We went through a similar experience with some electronic controls, and all the shielding, grounding and screening we tried did no good at all. The biggest problem with simply trying to shield the problems you know about, is that this is such a nebulous phenomenon that you don't know if there are other problems which are manifesting themselves in ways which are not obvious. What about the devices which aren't "tripping" which should be?
The best solution is to address the problem at the source - the radio. Your radio supplier should be able to help you with this.


--

************************
Michael Griffin
London, Ont. Canada
************************
 
F

Fred Townsend

A quick course in fields and waves: RFI contains components of both electric and magnetic fields. Electric fields are easily shielded by the methods you describe. Magnetic fields are much more difficult to shield (look how well a cell phone works inside a car or airplane) and use very difficult (and expensive) techniques. Almost certainly you have a magnetic field problem.

Where shielding is not effective or practical you need to attack the components of the RFI receiver. Look for the component that is acting as an antenna first. Start with metal parts or wires that are approximately a quarter wavelength at the frequency of the security radio. Reduce wire lengths to less than 1/8 wavelength if possible. In some case cases it may be necessary to use ferrite beads (prayer beads). Then look for the detectors or entry points into your circuitry. Bypass signal lines with 50 to 100 pf caps at both ends. Bypass all power supply lines with .01uf caps (in addition to regular bypasses). All RFI bypasses should be ceramic. Capacitor leads should be no more than 1/8 inch.

Good luck,

Fred Townsend
DC to Light Consulting
 
R

Robert Dusza

James,

What frequency are the on? What is the power level (watts) of the RF source? It may be a problem with proximity of the RF source and how close they get. The length of the cable feeding the RTD might be the right length to act as an antenna. Check the 1/2 and 1/4 wavelength of the frequency and then look at odd multiples of the frequency.

I don't know if there are any filters that can be put on the leads for the RTD. If you are unsuccessful there are some RFI references that may help. I can send you the web site if you need it.

Bob


Robert J. Dusza, Jr.
Project & Technical Support Manager
(V) 1-860-647-3219
(F) 1-860-647-3150
E-mail - [email protected]
Manchester Water & Sewer Dept.
125 Spring St. P.O. Box 191
Manchester, CT 06045-0191
 
C

Curt Wuollet

Feedthrough capacitors are intended for this type of problem. Put the electronics in a box and bring all connections in through feedthrough capacitors. It needn't even be the signal lines that are picking up the RF. It could as well be power, etc.Caps should be sized for low Xc at the RF and minimal effect on intended signals. In extreme cases, a ferrite bead or two ahead of the feedthrough will help. More specific solutions are possible, but would require actually analyzing the problem. Google feedthrough caps.

Regards

cww
 
P

Peter Whalley

Hi All

Quite right about the gaps. Radio waves (particularly VHF and UHF) will slip through all sorts of cracks and be conducted via cables running through the walls of the enclosure. If you seriously want to shield the equipment then you may need a properly design shielded cabinet. Most of the major cabinet manufacturers have these(try Rital for instance). Their distributor may be able to advise on shielding of cable entries etc otherwise go to a specialist in radio frequency shielding.

BTW the reason cell phones work inside cars is that the windows are large in relation to the wavelength of the radio signals used so they just go straight through the holes.

Regards

Peter Whalley
Magenta Communications Pty Ltd
Melbourne, VIC, Australia
e-mail: peter*no-spam*@magentacomm.com.au
delete *no-spam* before sending
 
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