Re: [PSES] Puzzle

Doug Powell Sat, 07 Dec 2013 10:14:18 -0800

Derek,

Have you stopped all data clocks? This may tell you is the noise is from an external source.

If you have done this and still have a high RE, I would suggest your entire setup has become a 1/4 stub antenna for something external It is apparently optimized for near 200 MHz or some multiple/submultiple of another frequency. You can try to de-tune this circuit and suppression tools are not always the answer.

It is possible for all the wires from the ground plane up to the table top to be part of this antenna. Ferrite sleeves will simply disappear since in this possible scenario.

The full RF circuit is the noise source, transmitted up the cable, radiated from the unintentional antenna, to the receive antenna, down that coax, to the receiver 50 Ohm terminator and back along common ground to the noise source (where ever that might be). Anything you can do to break into that circuit will give you a clue as to the location of the noise source and subsequently how to deal with it.

If possible, try a battery box to power the device, located on top of the table on short leads. If you eliminate the 200 MHZ, it's the power source. If not you have succeeded in eliminating the power supply from concern.

Try separating power and data cables into separate bundles and play with the separation between them. Remember, it may not solve the problem but at this point you simply want some kind of a response. Then you'll know if you are in the right area.

Are the cables from the product simply routed over top of the ground plane? If possible take them directly below ground plane or at least cover them with ground bonded foil to a point directly below the table. By the way make certain you are using heavy restaurant grade foil or at least a few layers.

Put the analyzer in the shield room or at least a VGA monitor so you can see the response, and then walk around. See where you can stand to cause a nice dip in the emissions, think about wavelengths, reflections and V/H polarization. Can you notice any affect any of these parameters?

Try touching the box, connector housings or cables with your hand. Keep in mind you are an additional element on the RF Circuit. Which way do the emissions go, up or down? Ask yourself why you get the responses you get when you change the shape and impedance match of the "antenna" with your whole body.

Finally, once you identify the source, do something to return the noise in a tight loop back to the source instead of allowing it to take the full circuit through the air.

My paraphrase of Henry Ott is, "EMC is not black magic, it is the scientific study of all the circuits the design engineer neglected to put on the schematic". This thought has helped me over the years to understand the parasitics and nuance of many products.

, - doug

Douglas Powell

http://www.linkedin.com/in/dougp01

Sent from my BlackBerry 10 smartphone on the Verizon Wireless 4G LTE network.

From: Derek Walton

Sent: Saturday, December 7, 2013 8:28 AM

To: [email protected]

Reply To: Derek Walton

Subject: [PSES] Puzzle

Good day folks,

I'm faced with a challenge that's confusing me a bit, and I'm concerned I'm overlooking something.

I have a small box, about 4" by 6" by 2" that contains some Ethernet circuitry. Normally there are cables that come and go from this box.

During RE testing, there are several very strong emissions ( above 200 MHz ) that exceed all other noise made by the box. As yet, we have not been able to link them with any of the anticipated frequencies one would expect. In chasing the source down I have taken a number of steps which are:

1) Disconnect all cables except the power. There was no change on the large spikes, other noise came down as anticipated.

2) Added several ferrites on the power cable. No change.

3) Added two feed through type caps at the power entry point. No change.

4) Wrapped the box completely in Aluminium foil. No change.

5) Checked Ambients... Nothing, quiet.

6) Replaced power source. No change.

7) Rotated box, very minor change in amplitude.

This box is a new version of an existing design that does not have the same issue and close field probing is very difficult inside the box. My suspicion is that a small daughter board is the source, but this cannot be removed from the box without destruction. A new box has been ordered build without that board. That may not arrive for a few weeks.

In the meantime, can anyone make suggestions about what may cause this noise, besides parasitic oscillations/faulty parts, and what radiating mechanisms can be uninfluenced with the protection measures I took.

Many thanks,

Derek Walton.

L F Research.

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