RE: [SI-LIST] : even-odd mode influence

WAUGH,RAY (ray_waugh@am.exch.hp.com)
Mon, 19 Jul 1999 09:27:17 -0600

As another contributor commented, we are drifting off the original topic
(which was line-to-line crosstalk).

However, I must comment in defense of FR4. I have made some very nicely
performing microwave mixers (frequency downconverters) operating at RF and
LO frequencies above 10 GHz on FR4. The board was single layer, 0.032"
thick, with half ounce copper cladding, forming a microstrip line (conductor
over a ground plane). Make the microstrip thinner and the losses get high.
Thicker, and you suffer radiation losses. However, FR4 is very decent
material for microwave circuits which are properly designed.

Ray

-----Original Message-----
From: Douglas McKean [mailto:dmckean@corp.auspex.com]
Sent: Friday, July 16, 1999 1:24 PM
To: si-list@silab.eng.sun.com
Subject: RE: [SI-LIST] : even-odd mode influence

Weber,

Lengthy but here goes ...

At a former company, rf designers were wrestling
with at times a 3dB difference between one side
of the board and the other. The company was a
CATV place. Waveforms out to the customer were
controlled by the FCC. This concern was crucial.

I picked up on this because I thought I "saw"
something with the problem. I ended up writing
a paper internal to the company whereby I
mathematically modelled by hand a simple trace
over a ground plane between an active source and
passive load with FR-4. What I ended up with was
a frequency response that was flat out to about
800 MHz where a resonance occurred then the
response fell off at about -40 dB/decade. This
roughly correlated with what I expected since
the basic model of the trace involved two energy
storage devices acting as a lo pass filter.
Namely the inductance and capacitance per length

There were of course a lot of variables to that
analysis so that's not to be taken as a hard
fast general rule.

This was a totally theoretical discussion between
me and the designers since they were moving onto
10 GHz ECL design using FR-4. I concluded that
just with the edge rates they were considering
(I forget exactly what they were) they had to
find other material and drop their beloved FR-4.
Besides that, mfring variability of the FR-4 for
the tighter constraints at the fab house was of
concern.

Here's why -

The board must be capable of maintaining a particular
bandwidth in order to support a particular edge rate.