# Propagation delay in PCB traces

Discussion in 'Electronic Design' started by [email protected], Aug 18, 2008.

1. ### Guest

Hey gang,
Time for another difficult question.
How can I figure out a tolerance across different PCBs for a given
electrical length?
Suppose I tuned a trace to have +200 ps, how will this 200ps vary
across
temperature, process variation, time, one batch to the next?
Anyone know where to start? I've asked around and all I got was
"measure it".
There has to be some sort of theoretical base to start from.
TIA

2. ### Guest

Thanks John. It is on a Rogers material. I am looking at Appcad now,
it is quite nice. Never heard of it before.
A well kept secret of pros, I guess.
I will read up on all relevant parameters of Rogers and see where this

3. ### TT_ManGuest

And you're going to put 75A down a PCB track?

5. ### Tom BruhnsGuest

RFSim99 is another freeware program that's useful for playing with
things like this; it has a built-in microstip and stripline
calculator.

You didn't elaborate on just why you care, but I'd suggest that for
most things, it's not going to matter much at all, and if you want
things to be seriously stable, you should probably in any event
arrange to have board effects cancel out in some way:
autocalibration, use of matched traces so it's the differential that
matters, not the absolute propagation, or things like that.

But at the same time, the physics helps out some: propagation
velocity depends on the effective permittivity that the fields are
traveling through, and propagation velocity varies as the square root
of that. Not only that, but the dielectric for microstrip is
partially air, which doesn't vary much. So for example, nominally 50
ohm microstrip on er=5.0 substrate yields about 1.56e8 m/sec
propagation velocity; er=4.5 makes that 1.64e8, and 4.0 makes it
1.73e8. (Different calculators will yield slightly different answers,
but the variation as a percentage should be very close.) So a 20%
reduction in er made for about a 10.9% increase in velocity. That was
for a trace width that stayed constant; if the trace width is adjusted
to keep impedance constant, the percentage in propagation velocity is
slightly less. For stripline (embedded in a constant er), the
velocity variation over the same er range is about 11.8%.

Note also that there is some variation in velocity as a function of
frequency...it's not huge, but it's there.

It will help a lot if you specify a particular substrate material.
"FR4" doesn't cut it; "Isola 370HR" (for example) will get you much
more consistent results, and you can search for substrate materials
which are particularly stable and whose manufacture is well

Cheers,
Tom

6. ### Guest

Thanks, in any case the widest swing seems to come from the
variability of the Er of the material, which is 3.36 +- 0.05, about +-
1.6% ( I think).
I don't know what causes the +-0.05 change though. Things like thermal
expansion of the board seem to have a negligible impact, if my home-
brewed spreadsheet is to be believed.

7. ### Tom BruhnsGuest

Yes, that should be the case. If the transmission is embedded
entirely in a dielectric of relative permittivity, Er, and the
magnetic permeability is the same as freespace, and the line is run in
TEM mode, then the propagation velocity is 1/sqrt(Er) times the
freespace propagation velocity. It doesn't matter what the dimensions
are (so thermal expansion has no direct effect), or what the line
impedance is. For microstrip, the fields aren't completely inside the
Er of the substrate; some are in the air above the board. Thus the
propagation velocity is slightly faster, the mode isn't true TEM, and
there's some dispersion. See, for example,
http://www.microwaves101.com/encyclopedia/dispersion.cfm, for more
info on this.

I'd expect Er to vary with temperature, with frequency, with absorbed
moisture, ... (Note: Er of air is slightly greater than 1, and
changes with pressure, temperature, humidity, ..., enough to be
noticeable if you are monitoring part per million changes in
capacitance of air-dielectric capacitors.)

Cheers,
Tom

8. ### Ken S. TuckerGuest

One tough application I ran into is doing PCB's
speeders use to detect cop radar.
In place of 90 degree track turns is a curved
wavelength track length needs to be accounted
for too, because of phase.

sim, and see how it works. You can build in provision
for slight tweaking to refine it.
Ken

9. ### Tom BruhnsGuest

On Aug 20, 9:40 am, "Joel Koltner" <>
wrote:
....
Do you really think it's going to be practical to tune the equivalent
of an inch or two of propagation with something like a "fat blob"
that's small enough to not mess things up at a few GHz?

How about some line sections in a binary sequence of lengths that
could be patched in or out, depending on where shorts were placed?

Or maybe just design things so the effect can be calibrated out...

Cheers,
Tom

10. ### Ken S. TuckerGuest

Hi Joel,
Judging from your post, betcha you're a sharp engineer
who knows his stuff, including Maxwell's Eqs, but I
won't go to math unless you want.

Ok. Agreed the 90 can be considered as an LC.
If the LC (Inductance capitance) causes a loss
of power apart from a resistive component that
power is assumed to be radiated, IMHO.
A rose by any other name.
Well my guess is a solder blob would set-up a
reflection (is that what you mean by "return loss",
I'll assume so.)

Another method may be to put jumpers over the trace,
two jumpers with approapriate spacing can null the
impedance and reflection but put in a propagation delay,
(I'm certain you know the theory of ferrite beads).

So that would LC store the energy and then reradiate
it along the trace, does that sound good?
Off hand I think 1/4 wavelength spacing would work??
Ken
PS:I got started on this stuff by installing TV antenna's
in the late 60's. There was a procedure to splice that
old flat 300 Ohm lead wire, that required each side of
the line to be spliced (pig tailed, NOT soldered) a few
inches apart to reduce ghosting and reflection.

11. ### Tom BruhnsGuest

OK, well, go and simulate the effect of what you're imagining as a
blob. But note that the width of a microstrip trace has much greater
effect on its impedance than on its velocity factor. Do you really
want to keep reflections low??

If you really want to significantly affect the propagation delay, you
should probably be milling out dielectric so it's replaced with air,
and adjusting the trace width (widening it) to maintain constant
impedance. It honestly doesn't sound very practical to me. I suppose
if you want to electrically lengthen a trace, you could do it by
milling out under it, narrowing the trace as well, and replace the
board dielectric with something with a higher dielectric constant.
(I'd be laughed out of here if I even hinted at putting something like
that into production!)

Let's say you change the net er around a section of microstrip from
2.5 to 8 (something like alumina...). In the er=2.5 section,
propagation is at 5.27psec/mm, and in the er=8 section, it's 9.43psec/
mm. So to adjust over a 200psec range, you need a 48mm length of
line. Ouch. I just can't imagine doing that with a Dremel tool.

12. ### Tom BruhnsGuest

Hmmmm...well, "us guys" here have always been into things like
continuous autocalibration that happens in the background while
measurements are running, and even very significantly improves the
measurement beyond the calibration effects. ;-)
I was thinking er=10 alumina, and WAGging that it would give a net
around er=8. If you're going to all that trouble, you _could_ pile
alumina over the top of the trace, too, so it's embedded
microstrip... (Eeeechhhh.) But you're right -- the effective er over
er=10 alumina is only about 6.5. For plain microstrip over er=2.1
solid Teflon, ATLC tells me that a 50 ohm line will be about 3.1 times
as wide as the substrate thickness and have net er=1.79 and 4.46ps/mm,
and 50 ohms over er=10 alumina will be about .95 as wide as the
substrate thickness, net er=6.52, and 8.52ps/mm. So to get 20ps
adjustment and keep the impedance constant, you'd have to considerably
narrow a 5mm length and completely fill under it with different
dielectric. There's gotta be a lot easier way... And if you don't
narrow the trace but leave it the same width that gives 50 ohms over
er=2.1 substrate, you have a section of 25 ohm line...

And this doesn't all sound terribly impractical to you??? ;-)

Cheers,
Tom

13. ### Ken S. TuckerGuest

Hi Joel.

Let me agree with you, cuz we're close to splitting
hairs, with the caveat that reflection can be
constructive or destructive, such as in an antenna,