Re: [SI-LIST] : FW: signal isolation, RF board

Fred Townsend ([email protected])
Fri, 30 Jan 1998 11:08:26 -0800

Anthony Tse wrote:
> Hello all,
> I am in the process of designing a RF board (160MHz) and I need to isolate a low > level inputs from the output of a high gain amplifier. I re-read all the article in > the archive on guard-banding and shielding and didn't really get a good feel on how > to go about doing this.

You probably will not have a good feel until you have done it a few

> Here are some questions:
> Realistically, how much isolation can I get? The traces in question are about 2" > > long and 4" apart.

This sounds like plenty for electric fields which are much easier to
shield. Magnetic will depend on the strength of the fields. The
strength of magnetic field will depend on the ac current in the source
(field) trace.

> The stacking is 2 & 4 ground, layer 3 for RF signal, and layer 1 for video signal. > Since there are very few traces on layer 3, should I solder pour the whole layer and > stitch the 3 ground planes together around all the sensitive signal traces? Or am I > better of using fat guard traces and stitch them to the ground plane.

Can you do both?

> Does it make sense to separate the two signals in two different layers? Say one of > the signals in layer 2 with 1 & 3 as the ground while the other signal is in layer 3 > with 2 & 4 as ground. Or should I bit the bullet and go to a 6 layer board, with > 1,3,4,6 as ground and 2 & 5 as signal (50 ohm trace width may be a problem with a 6 > layer board).

I fail to understand your statement about 50 ohms since impedance tends
to converge near 50 ohms. I would understand you were in deep yogurt if
you had said 300 ohms.

You are in great danger of throwing out the baby with the bath water.
Video and RF are spectrally separated from each other so they do not
normally interfere with each other. Keeping the signal currents low and
matching impedances will do a lot to reduce coupling as does
separation. You have failed to mention anything about power here. Are
these circuits sharing power supplies or grounds? You will have major
problems with power and ground bounce if either video or RF have
transient conditions. (Video is almost by definition non-periodic and
therefore transient.) Better to go to 6 or even 8 layers and avoid
sharing any power or grounds.

Fred Townsend
DC to Light Consulting Services
San Jose, CA
(408) 263-8768