Transmission line-like resonances at ~30MHz

[Joel Kolstad]

I have a design on an FR-4 PCB that has a few long traces on it, about 12"
each. When I examine those trace's responses on a network analyzer, they
have significant (>15dB) dips at 30MHz, then 90MHz, 150MHz, etc... this led
me to suspect that the traces were behaving like quarter wave transformers
(the traces have various components connected to them, but they're all
supposed to be pretty high impedances), but if I assume an effective
dielectric constant of ~2.35, I compute lambda/4 as ~60 inches. Indeed, the
trace lengths were originally set such that I would (hopefully) be able to
assume the system was behaving as a lumped network up to at least 30MHz!

Anyone have suggestions on what else might cause periodic resonances such as
the ones described?

Thanks,
---Joel Kolstad

 

[Joerg]

Hello Joel,

Lots of capacitive loads hooked up to them? An innocent CMOS input can
add 10pF easily. Each.

If these are clock distribution lines you need to terminated them with
their respective Z, as calculated from trace width, layer respectively
prepreg thickness and the dielectric constant of the material (probably
FR4). Hoping there is a solid ground plane underneath, that is.

Regards, Joerg

 

[Fred Bartoli]

I have a design on an FR-4 PCB that has a few long traces on it, about 12"
each. When I examine those trace's responses on a network analyzer, they
have significant (>15dB) dips at 30MHz, then 90MHz, 150MHz, etc... this led
me to suspect that the traces were behaving like quarter wave transformers
(the traces have various components connected to them, but they're all
supposed to be pretty high impedances), but if I assume an effective
dielectric constant of ~2.35, I compute lambda/4 as ~60 inches. Indeed, the
trace lengths were originally set such that I would (hopefully) be able to
assume the system was behaving as a lumped network up to at least 30MHz!

Anyone have suggestions on what else might cause periodic resonances such as
the ones described?

Additional capacitive loading along the trace?

What is your trace capacitance per unit lenght?
At 50p/m and 0.3m length you need 360p additional "distributed" capacitance
to get there.

 

[Joel Kolstad]

Thanks for the ideas, Fred and Joerg.

"Fred Bartoli"

Additional capacitive loading along the trace?

Hmm... so... thinking about this a little... I was thinking a bunch of
little lumped capacitors along a transmission line wouldn't electricially
shorten it, but would rather just add a regular RC-like roll-off to the
response. From what you and Joerg are saying, it sounds as though all the
little caps electrically shorten the line... period.

What is your trace capacitance per unit lenght?

They're supposedly 50 ohm lines, although I haven't verified this and
personally suspect they might be somewhat lower. The traces are 12 mils
wide and... if I were at home I could get you the trace capacitance from
this, but unfortunately I'm not so I can't.

The are numerous (~50) PN diodes hanging off of them (Agilent HSMP-4820);
the traces carry RF and the PN diodes are used to switches to route the RF
to one of numerous outputs. The 30/90/150/etc. MHz dips occur when none of
the diodes are turned on (the dips shift around when the diodes ARE on,
which is what I'd expect).

The HSMP-4820 is spec'd as being 0.75pF typically... hmm... ok... it sure
does sound as though this must be the problem. Grrr...

Thanks again,
---Joel


---Joel

 

[Joel Kolstad]

Looking at a Smith Chart, it's clear to me now that small lumped
capacitances along a transmission line will just make the line look
electrically longer than it physically is. Hmmmph. I have a few ideas
about how to fix the problem; namely by terminating the line and then adding
amplifiers.

I shudder to think what the frequency response of some of the digital
circuitry I've designed over the years has been... I wouldn't have guessed
that 12" traces could already exhibit significant distributed behavior at
30MHz in FR4.

---Joel

 

[Fred Bartoli]

Thanks for the ideas, Fred and Joerg.

"Fred Bartoli"


Hmm... so... thinking about this a little... I was thinking a bunch of
little lumped capacitors along a transmission line wouldn't electricially
shorten it, but would rather just add a regular RC-like roll-off to the
response. From what you and Joerg are saying, it sounds as though all the
little caps electrically shorten the line... period.

Yes, don't forget your small bit of line still has its inductance. At low
frequency, (this is LF compared to your bit of line lenght) you'll have a
bunch of L-C cells where the C isn't the one you think from just the line.

They're supposedly 50 ohm lines,

Not anymore : sqrt(L/(C+Cpar)) is lower due to Cpar

although I haven't verified this and
personally suspect they might be somewhat lower. The traces are 12 mils
wide and... if I were at home I could get you the trace capacitance from
this, but unfortunately I'm not so I can't.

The are numerous (~50) PN diodes hanging off of them (Agilent HSMP-4820);
the traces carry RF and the PN diodes are used to switches to route the RF
to one of numerous outputs. The 30/90/150/etc. MHz dips occur when none of
the diodes are turned on (the dips shift around when the diodes ARE on,
which is what I'd expect).

The HSMP-4820 is spec'd as being 0.75pF typically... hmm... ok... it sure
does sound as though this must be the problem. Grrr...

Also don't forget the stubs that connects the diodes to the line.

For example you can build a slow line by just adding a truck load of stubs
along the line, thus increasing its distributed capacitance without touching
the inductance.

BTW, if you've calculated your line to be 50R or whatever, the additional
parasitics will lower it, but I've already said that.

 

[Joerg]

Hello Joel,

Hmm... so... thinking about this a little... I was thinking a bunch of
little lumped capacitors along a transmission line wouldn't electricially
shorten it, but would rather just add a regular RC-like roll-off to the
response. From what you and Joerg are saying, it sounds as though all the
little caps electrically shorten the line... period.

They can make resonant circuits out of it.

The HSMP-4820 is spec'd as being 0.75pF typically... hmm... ok... it sure
does sound as though this must be the problem. Grrr...

The capacitance will be higher, with SMT lands, traces and all things
considered. Maybe you could post a schematic so people can toss some
ideas as to what could be done. You might want to try some termination
at the end but it is unlikely to give enough of an improvement. You will
also need a pretty stiff driver with this many diodes as a load. What
resistance is in series with each diode?

Regards, Joerg

 

[Joerg]

Hello Joel,

Looking at a Smith Chart, it's clear to me now that small lumped
capacitances along a transmission line will just make the line look
electrically longer than it physically is. Hmmmph. I have a few ideas
about how to fix the problem; namely by terminating the line and then adding
amplifiers.

Termination is always a good idea. Then look at the load points. How
much do these diodes draw from the line when turned on? Maybe a little
transistor buffer for each could help.

I shudder to think what the frequency response of some of the digital
circuitry I've designed over the years has been... I wouldn't have guessed
that 12" traces could already exhibit significant distributed behavior at
30MHz in FR4.

They can work very well up to a GHz and more, provided they are nicely
terminated and Z is the same along the whole trace. No sharp turns, no
big loads along the way.

Regards, Joerg

 

[John Larkin]

Looking at a Smith Chart, it's clear to me now that small lumped
capacitances along a transmission line will just make the line look
electrically longer than it physically is. Hmmmph. I have a few ideas
about how to fix the problem; namely by terminating the line and then adding
amplifiers.

I shudder to think what the frequency response of some of the digital
circuitry I've designed over the years has been... I wouldn't have guessed
that 12" traces could already exhibit significant distributed behavior at
30MHz in FR4.

---Joel

A transmission line that's periodically (or worse, non-periodically)
loaded by capacitors (or worse, lossy capacitors, like a CMOS input)
looks lower in impedance and slower in prop delay than the unloaded
trace. If there are lots of small loads spaced close, you can just
pretend the C per unit length has gone up, and recalc the trace
impedance. If the loads are lumpier compared to the signal risetime,
you enter reflection hell. Your skinny traces probably have much lower
native c/length than the loading, so the line will be a lot different
than the raw trace. Heavily loaded lines like this tend to be lossy,
which is sometimes a good thing. Risetimes tend to go to hell.

Extreme cases, like backplanes, can be awful.

I just did a board that has 14 schottky diodes spaced uniformly along
a 4" long trace, with the trace widths fudged down to give a 50 ohm
line when loaded by diode capacitance. Each diode has the option to
inject a 200 ps pulse into the line. Pretty much works, although some
of the waveforms along the line are kinda strange... seeems to be a
bit of bouncing going on.

John

 

[Chris Carlen]

I have a design on an FR-4 PCB that has a few long traces on it, about 12"
each. When I examine those trace's responses on a network analyzer, they
have significant (>15dB) dips at 30MHz, then 90MHz, 150MHz, etc... this led
me to suspect that the traces were behaving like quarter wave transformers
(the traces have various components connected to them, but they're all
supposed to be pretty high impedances), but if I assume an effective
dielectric constant of ~2.35,

I think FR-4 has mu_r = 4.7



--
_______________________________________________________________________
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA
[email protected] -- NOTE: Remove "BOGUS" from email address to reply.

 

[Joel Kolstad]

Joel Kolstad wrote:
I think FR-4 has mu_r = 4.7

The microstrip in question was on an outside layer, so as a rough estimate I
used 4.7/2 as the effective relative permittivity.

---Joel

 

[John Larkin]

I think FR-4 has mu_r = 4.7

Programs like Txline and Appcad will compute effective dielectric
constant. The classic 50-ohm microstrip on 0.062 FR4 has Eeff of about
3.4. Skinnier traces tend to be lower.

John

 

[Chris Carlen]

Programs like Txline and Appcad will compute effective dielectric
constant. The classic 50-ohm microstrip on 0.062 FR4 has Eeff of about
3.4. Skinnier traces tend to be lower.

John

Yes, I have a spreadsheet based on Howard Johnson's "...Black Magic"
text, which also computes an effective permeability. Probably very
similar formulas, since lo and behold it's about 3.4 for a 17 mil trace
10 mils over a ground plane which gives about 50 ohms.


Good day!


--
_______________________________________________________________________
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA
[email protected] -- NOTE: Remove "BOGUS" from email address to reply.

 

[John Larkin]

Yes, I have a spreadsheet based on Howard Johnson's "...Black Magic"
text, which also computes an effective permeability. Probably very
similar formulas, since lo and behold it's about 3.4 for a 17 mil trace
10 mils over a ground plane which gives about 50 ohms.

Just be careful about any simple formula. Most work over a small range
of geometries. The classic "Motorola ecl" equation cruises clear
through zero ohms and goes negative as the trace gets wider. I like
Appcad and Txline, which are smarter. Both are free.

John

 

[Joerg]

Hello John,

Just be careful about any simple formula. Most work over a small range
of geometries. The classic "Motorola ecl" equation cruises clear
through zero ohms and goes negative as the trace gets wider. I like
Appcad and Txline, which are smarter. Both are free.

But Motorola did a good thing with their MECL design book. They made
unsuspecting engineers aware that when digital stuff gets faster things
ain't totally digital anymore.

Another really good source with formulas and all is Fairchild's 1977 ECL
data book. I began using that one after my trusty old Motorola book
literally fell apart. It didn't like California summers.

Regards, Joerg

 

[Chris Carlen]

Just be careful about any simple formula. Most work over a small range
of geometries. The classic "Motorola ecl" equation cruises clear
through zero ohms and goes negative as the trace gets wider. I like
Appcad and Txline, which are smarter. Both are free.

John

Thanks for the links. I could use some software to do this, as
Johnson's formulas (actually from I. J. Bahl and Ramesh Garg, "Simple
and accurate formulas for microstrip with finite strip thickness", Proc.
IEEE, 65, 1977, pp. 1611-1612.) are not very simple, with various
conditionals, functions of functions, and so forth. Once in a
spreadsheet they are a cinch, but I don't want to have to type in the
several foot long formulas for every geometry.

Good day!



--
_______________________________________________________________________
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA
[email protected] -- NOTE: Remove "BOGUS" from email address to reply.

 

[John Larkin]

Thanks for the links. I could use some software to do this, as
Johnson's formulas (actually from I. J. Bahl and Ramesh Garg, "Simple
and accurate formulas for microstrip with finite strip thickness", Proc.
IEEE, 65, 1977, pp. 1611-1612.) are not very simple, with various
conditionals, functions of functions, and so forth. Once in a
spreadsheet they are a cinch, but I don't want to have to type in the
several foot long formulas for every geometry.

Good day!

The ultimate free tx line analyzer is ATLC, which will calculate the
impedance of a cowbell-shaped line inside a star-shaped tube with
mixed dielectrics. It's just an immense pain to use.

John

 

[Roy McCammon]

Looking at a Smith Chart, it's clear to me now that small lumped
capacitances along a transmission line will just make the line look
electrically longer than it physically is. Hmmmph. I have a few ideas
about how to fix the problem; namely by terminating the line and then adding
amplifiers.

after doing several of these, I hit on the following strategy.

Calculate the minimum and maximum impedences over all the
loading conditions. Usually that means no loads and maximum loads.
Then series terminate the drivers with the minimum impedance,
and parallel terminate the end with the maximum impedance. Its
not perfect, but one end or the other is usually close enough
to damp out resonances.

 

[Mark]

Goggle

salphasic clock


for some interesting reading.

Mark

 

[John Larkin]

Goggle

salphasic clock


for some interesting reading.

Mark

"It is estimated that the switching currents will peak at 1000 A
unless tightly controlled clock skews are utilized to avoid
simultaneous switching."

http://www.engalco.co.uk/new_page_16.htm

That's beyond interesting. That's terrifying.

John