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

Stephen Zinck (zinck@cabletron.com)
Mon, 19 Jul 1999 12:39:25 -0400

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Ray,

How long was the transmission line on this board? Have you had any success with transmission lines on the order of 18" long (@ 10 GHz) using FR-4? Just curious.

Thanks,

Steve

"WAUGH,RAY (HP-SanJose,ex1)" wrote:

> 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.
> >From communications theory and depending upon which
> book one reads, we know this required bandwidth to be
> either BW = 1/(2*tr) or BW = 0.35/tr (tr = transition
> time or edge rate). Fourier expansion of an edge rate
> shows a multitude of frequencies in order to build
> the required edge rate. Let's use the worst case
> BW = 0.5/tr.
>
> Nanosecond edge rates (10^-9) requires ~ 500 MHz BW.
> 1/2 nanosecond edge rate requires ~ 1 GHz BW.
> ... and so on.
>
> Now I ask - take a trace, any trace no matter what it
> is and reduce it to it's lumped or non-lumped parameters
> of resistance, capacitance and inductance. Look at it
> as a "circuit" now and ask yourself if this "circuit"
> can actually support a flat BW out to ... whatever it is.
> Since board houses measure controlled impedances with
> only 1 MHz unless specifically told otherwise,
> my bet is no.
>
> A designer may want to argue by way of Shannon's
> Theorem that data rates with S/N ratios on the
> order of 30 dB or more need only 50 MHz or so BW.
> But, edge rates can easily swamp that out very
> quickly in terms of orders of magnitude. And with
> it, I submit, all of the above also.
>
> Sorry for the lengthy discussion but my point is ...
>
> Put that one circuit in parallel with others and the
> odd versus even impedance effects makes this situation
> all the worse by changing phase and group velocities.
> Thus one ends up with smeared or "slurry" edges. That
> in turn *could* effect setups, prop delay, metastability,
> and ultimately latency. I'm of course painting a
> really worst case scenario.
>
> And as you say, Weber, with the onset of faster edge
> rates and fine pitch constructions, we are stuck.
> And all we've got so far is trade offs.
>
> Regards, Doug McKean
>
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