fast AC coupling

I have a fast (10s of ps edges) pulse that I want to AC couple. The
bias on the source side will be 15 volts, and the load is 50 ohms to
ground. I need at least 1 uF to minimized droop, preferably more. The
50 ohm trace is 45 mils wide.

https://www.dropbox.com/s/002r5ias7yveya4/Fast_AC_Coupling_1.jpg?raw=1

The 0204 fits the trace nicely but doesn't have enough capacitance at
15 volts. The 0603 is too small too, and has more inductance than I
want. 0306 doesn't have enough capacitance either.

A 10 uF 16V 0508 kinda works. It will have roughly 1.5 uF left at 15
volts. It does bulge out a bit from the trace width, which adds
capacitance and maybe a little inductance. I guess I could teardrop
the traces and then cut away a little ground plane to correct for the
extra capacitance. That's hard to calculate.

Any suggestions on how to do fast-edge-long-tau AC coupling on
microstrip?



--

John Larkin Highland Technology, Inc

The cork popped merrily, and Lord Peter rose to his feet.
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"

 

As coupling is good for low duty cycle pulse information. If the pulse rate is too high an intelligent discharging circuit may be required to keep the baseline ground from creeping up. Not really what you are asking...but that is my 2 cents on as coupling pulses

 

On Thu, 16 Jan 2020 08:53:49 -0800 (PST), bulegoge@columbus.rr.com
wrote:

>As coupling is good for low duty cycle pulse information. If the pulse rate is too high an intelligent discharging circuit may be required to keep the baseline ground from creeping up. Not really what you are asking...but that is my 2 cents on as coupling pulses

Luckily, my duty cycle will be low.

Intelligent discharging would be tricky at these speeds. Schottky
diodes are semi-intelligent, I guess, but I begrudge every tenth of a
pF.

https://www.dropbox.com/s/h02lnp2yu4ho8xy/Fast_AC_Coupling_2.JPG?raw=1

Actually, there could be 20 volts across the cap. I wouldn't worry
about blowing out a 16v ceramic cap, but it will lose even more
capacitance as the voltage increases.

4.7 uF at 25v is available in 0508. That would have about the same
capacitance at bias as the 10 uF 16v part.




--

John Larkin Highland Technology, Inc

The cork popped merrily, and Lord Peter rose to his feet.
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"

 

[Rick C]

On Thursday, January 16, 2020 at 12:26:53 PM UTC-5, jla...@highlandsniptechnology.com wrote:

On Thu, 16 Jan 2020 08:53:49 -0800 (PST), bulegoge@columbus.rr.com
wrote:

As coupling is good for low duty cycle pulse information. If the pulse rate is too high an intelligent discharging circuit may be required to keep the baseline ground from creeping up. Not really what you are asking...but that is my 2 cents on as coupling pulses

Luckily, my duty cycle will be low.

Intelligent discharging would be tricky at these speeds. Schottky
diodes are semi-intelligent, I guess, but I begrudge every tenth of a
pF.

https://www.dropbox.com/s/h02lnp2yu4ho8xy/Fast_AC_Coupling_2.JPG?raw=1

Actually, there could be 20 volts across the cap. I wouldn't worry
about blowing out a 16v ceramic cap, but it will lose even more
capacitance as the voltage increases.

4.7 uF at 25v is available in 0508. That would have about the same
capacitance at bias as the 10 uF 16v part.

I hope you have this info from the data sheet or you've tested it. Often the V-C curve is the same for different voltage ratings rather than being related. Without hard evidence I wouldn't count on any assumptions.

--

Rick C.

- Get 1,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209

 

[whit3rd]

On Thursday, January 16, 2020 at 9:26:53 AM UTC-8, jla...@highlandsniptechnology.com wrote:

Actually, there could be 20 volts across the cap. I wouldn't worry
about blowing out a 16v ceramic cap, but it will lose even more
capacitance as the voltage increases.

Capacitance loss with voltage is characteristic of nonlinear
materials; those are piezoelectric, so will ALSO ring like a bell
from your fast edge; the long tail won't be quiet.

 

[Winfield Hill]

jlarkin@highlandsniptechnology.com wrote...

I have a fast (10s of ps edges) pulse that I want
to AC couple. The bias on the source side will be
15 volts, and the load is 50 ohms to ground. ...

You're talking quite low frequencies for the droop
issue, why not create two pathways, one for GHz and
high NHz, the other for below that. Bias-T style.


--
Thanks,
- Win

 

[John Larkin]

On 16 Jan 2020 12:26:30 -0800, Winfield Hill <winfieldhill@yahoo.com>
wrote:

jlarkin@highlandsniptechnology.com wrote...

I have a fast (10s of ps edges) pulse that I want
to AC couple. The bias on the source side will be
15 volts, and the load is 50 ohms to ground. ...

You're talking quite low frequencies for the droop
issue, why not create two pathways, one for GHz and
high NHz, the other for below that. Bias-T style.

The widest pulse will be about 200 ns. But every time we design a
really spiffy fast box, some potential customer calls and asks for a
slower version!

1 uF and 50 ohms is 50 us tau, which will droop 0.1% every 50 ns. If
we ride on the Vpi flat top of the eom transfer curve, the optical
droop will be much less.

Two pathways sounds difficult.

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Chris Jones]

On 17/01/2020 03:11, jlarkin@highlandsniptechnology.com wrote:

I have a fast (10s of ps edges) pulse that I want to AC couple. The
bias on the source side will be 15 volts, and the load is 50 ohms to
ground. I need at least 1 uF to minimized droop, preferably more. The
50 ohm trace is 45 mils wide.

https://www.dropbox.com/s/002r5ias7yveya4/Fast_AC_Coupling_1.jpg?raw=1

The 0204 fits the trace nicely but doesn't have enough capacitance at
15 volts. The 0603 is too small too, and has more inductance than I
want. 0306 doesn't have enough capacitance either.

A 10 uF 16V 0508 kinda works. It will have roughly 1.5 uF left at 15
volts. It does bulge out a bit from the trace width, which adds
capacitance and maybe a little inductance. I guess I could teardrop
the traces and then cut away a little ground plane to correct for the
extra capacitance. That's hard to calculate.

Any suggestions on how to do fast-edge-long-tau AC coupling on
microstrip?

I'd probably try to move the ground plane down to a lower layer for that
part of the board, with a row of vias to stitch the groundplanes
together where the groundplane steps down. That way you could use e.g. a
0.1" trace for the 50 Ohm microstrip line, and fit much more capacitance
across the gap in it. Some SMA edge launch connectors also work better
with a wider microstrip line on the PCB, so it might be unnecessary to
go back to a narrow trace on the other side.

Optimising the part where the microstrip trace gets wider and the
groundplane gets lower would be easier with a field solver, but as you
have TDR equipment, you could lay out a couple of dozen random options
and for not much money get them made and pick the best one.

 

[John Larkin]

On Fri, 17 Jan 2020 09:09:18 +1100, Chris Jones
<lugnut808@spam.yahoo.com> wrote:

On 17/01/2020 03:11, jlarkin@highlandsniptechnology.com wrote:
I have a fast (10s of ps edges) pulse that I want to AC couple. The
bias on the source side will be 15 volts, and the load is 50 ohms to
ground. I need at least 1 uF to minimized droop, preferably more. The
50 ohm trace is 45 mils wide.

https://www.dropbox.com/s/002r5ias7yveya4/Fast_AC_Coupling_1.jpg?raw=1

The 0204 fits the trace nicely but doesn't have enough capacitance at
15 volts. The 0603 is too small too, and has more inductance than I
want. 0306 doesn't have enough capacitance either.

A 10 uF 16V 0508 kinda works. It will have roughly 1.5 uF left at 15
volts. It does bulge out a bit from the trace width, which adds
capacitance and maybe a little inductance. I guess I could teardrop
the traces and then cut away a little ground plane to correct for the
extra capacitance. That's hard to calculate.

Any suggestions on how to do fast-edge-long-tau AC coupling on
microstrip?




I'd probably try to move the ground plane down to a lower layer for that
part of the board, with a row of vias to stitch the groundplanes
together where the groundplane steps down. That way you could use e.g. a
0.1" trace for the 50 Ohm microstrip line, and fit much more capacitance
across the gap in it. Some SMA edge launch connectors also work better
with a wider microstrip line on the PCB, so it might be unnecessary to
go back to a narrow trace on the other side.

Optimising the part where the microstrip trace gets wider and the
groundplane gets lower would be easier with a field solver, but as you
have TDR equipment, you could lay out a couple of dozen random options
and for not much money get them made and pick the best one.

It's a 4-layer board, with, tenatively, 25 mil outer dielectrics. So
the ground plane is 25 mils below the microstrip. The amp and
microwave SMA connector have a tiny pins so a 45 mil strip isn't bad.

One idea was to use a bigger cap and teardrop into it, and punch a
hole in the ground and power planes below to make up for the lump of
capacitance. But that's hard to simulate (Sonnet Lite maybe?) and I'm
not eager to do a dozen experiments.

One of my guys may still have access to a student version of a serious
em simulator.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

Chris Jones <lugnut808@spam.yahoo.com> wrote in
news:la5UF.59602$5X1.52879@fx06.am4:

On 17/01/2020 03:11, jlarkin@highlandsniptechnology.com wrote:
I have a fast (10s of ps edges) pulse that I want to AC couple.
The bias on the source side will be 15 volts, and the load is 50
ohms to ground. I need at least 1 uF to minimized droop,
preferably more. The 50 ohm trace is 45 mils wide.

https://www.dropbox.com/s/002r5ias7yveya4/Fast_AC_Coupling_1.jpg?r
aw=1

The 0204 fits the trace nicely but doesn't have enough
capacitance at 15 volts. The 0603 is too small too, and has more
inductance than I want. 0306 doesn't have enough capacitance
either.

A 10 uF 16V 0508 kinda works. It will have roughly 1.5 uF left at
15 volts. It does bulge out a bit from the trace width, which
adds capacitance and maybe a little inductance. I guess I could
teardrop the traces and then cut away a little ground plane to
correct for the extra capacitance. That's hard to calculate.

Any suggestions on how to do fast-edge-long-tau AC coupling on
microstrip?




I'd probably try to move the ground plane down to a lower layer
for that part of the board, with a row of vias to stitch the
groundplanes together where the groundplane steps down. That way
you could use e.g. a 0.1" trace for the 50 Ohm microstrip line,
and fit much more capacitance across the gap in it. Some SMA edge
launch connectors also work better with a wider microstrip line on
the PCB, so it might be unnecessary to go back to a narrow trace
on the other side.

Optimising the part where the microstrip trace gets wider and the
groundplane gets lower would be easier with a field solver, but as
you have TDR equipment, you could lay out a couple of dozen random
options and for not much money get them made and pick the best
one.

The tried and true method.

 

[Winfield Hill]

John Larkin wrote...

Winfield Hill wrote:

You're talking quite low frequencies for the droop
issue, why not create two pathways, one for GHz and
high NHz, the other for below that. Bias-T style.

Two pathways sounds difficult.

If you're using "normal" PCB thicknesses, with their
fairly-wide 50-ohm traces, you could diverge a thin
trace, that won't siphon off much of the GHz signal,
and yet providing a "DC signal" that you can route
along-side the 50-ohm GHz signal. After its LF AC
coupling job, you can merge it back in the same way,
Maybe add some resistos to further the GHz isolation.
You'd need to have final high-Z loading at low freqs.


--
Thanks,
- Win

 

[John Larkin]

On 16 Jan 2020 18:27:24 -0800, Winfield Hill <winfieldhill@yahoo.com>
wrote:

John Larkin wrote...

Winfield Hill wrote:

You're talking quite low frequencies for the droop
issue, why not create two pathways, one for GHz and
high NHz, the other for below that. Bias-T style.

Two pathways sounds difficult.

If you're using "normal" PCB thicknesses, with their
fairly-wide 50-ohm traces, you could diverge a thin
trace, that won't siphon off much of the GHz signal,
and yet providing a "DC signal" that you can route
along-side the 50-ohm GHz signal. After its LF AC
coupling job, you can merge it back in the same way,
Maybe add some resistos to further the GHz isolation.
You'd need to have final high-Z loading at low freqs.

A small, fast cap that spans the trace could be connected to a big
ugly cap, maybe through a couple of small inductors. I might Spice
that.

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

On Thursday, January 16, 2020 at 11:11:33 AM UTC-5, jla...@highlandsniptechnology.com wrote:

I have a fast (10s of ps edges) pulse that I want to AC couple. The
bias on the source side will be 15 volts, and the load is 50 ohms to
ground. I need at least 1 uF to minimized droop, preferably more. The
50 ohm trace is 45 mils wide.

https://www.dropbox.com/s/002r5ias7yveya4/Fast_AC_Coupling_1.jpg?raw=1

The 0204 fits the trace nicely but doesn't have enough capacitance at
15 volts. The 0603 is too small too, and has more inductance than I
want. 0306 doesn't have enough capacitance either.

A 10 uF 16V 0508 kinda works. It will have roughly 1.5 uF left at 15
volts. It does bulge out a bit from the trace width, which adds
capacitance and maybe a little inductance. I guess I could teardrop
the traces and then cut away a little ground plane to correct for the
extra capacitance. That's hard to calculate.

Any suggestions on how to do fast-edge-long-tau AC coupling on
microstrip?

So what did you do? Get the board made (and populated) before you realized this was a problem?

And what are the consequences of too much droop? If it's a minimum amplitude at the load deal, then there are things you can do like add some pre-bias in series with coupling capacitor to maintain amplitude during the pulse.

You may have to butcher the board and write it off as a proto to a finished working product.

--

John Larkin Highland Technology, Inc

The cork popped merrily, and Lord Peter rose to his feet.
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"

 

[John Larkin]

On Fri, 17 Jan 2020 12:19:00 -0800 (PST),
bloggs.fredbloggs.fred@gmail.com wrote:

On Thursday, January 16, 2020 at 11:11:33 AM UTC-5, jla...@highlandsniptechnology.com wrote:
I have a fast (10s of ps edges) pulse that I want to AC couple. The
bias on the source side will be 15 volts, and the load is 50 ohms to
ground. I need at least 1 uF to minimized droop, preferably more. The
50 ohm trace is 45 mils wide.

https://www.dropbox.com/s/002r5ias7yveya4/Fast_AC_Coupling_1.jpg?raw=1

The 0204 fits the trace nicely but doesn't have enough capacitance at
15 volts. The 0603 is too small too, and has more inductance than I
want. 0306 doesn't have enough capacitance either.

A 10 uF 16V 0508 kinda works. It will have roughly 1.5 uF left at 15
volts. It does bulge out a bit from the trace width, which adds
capacitance and maybe a little inductance. I guess I could teardrop
the traces and then cut away a little ground plane to correct for the
extra capacitance. That's hard to calculate.

Any suggestions on how to do fast-edge-long-tau AC coupling on
microstrip?

So what did you do? Get the board made (and populated) before you realized this was a problem?

We're entering the hand-drawn schematic into PADS now. Then reviews,
and then layout.

And what are the consequences of too much droop? If it's a minimum amplitude at the load deal, then there are things you can do like add some pre-bias in series with coupling capacitor to maintain amplitude during the pulse.

It's a probabilities thing. The better it works, the more likely that
people will buy them.

You may have to butcher the board and write it off as a proto to a finished working product.

We don't prototype products. We design and release rev A, and
manufacturing builds some first articles that we test, and we prefer
that rev A be a sellable product, which is usually it. This one is
scarier than most.

How do you develop products?

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Bill Sloman]

On Friday, January 17, 2020 at 3:11:33 AM UTC+11, jla...@highlandsniptechnology.com wrote:

I have a fast (10s of ps edges) pulse that I want to AC couple. The
bias on the source side will be 15 volts, and the load is 50 ohms to
ground. I need at least 1 uF to minimized droop, preferably more. The
50 ohm trace is 45 mils wide.

https://www.dropbox.com/s/002r5ias7yveya4/Fast_AC_Coupling_1.jpg?raw=1

The 0204 fits the trace nicely but doesn't have enough capacitance at
15 volts. The 0603 is too small too, and has more inductance than I
want. 0306 doesn't have enough capacitance either.

A 10 uF 16V 0508 kinda works. It will have roughly 1.5 uF left at 15
volts. It does bulge out a bit from the trace width, which adds
capacitance and maybe a little inductance. I guess I could teardrop
the traces and then cut away a little ground plane to correct for the
extra capacitance. That's hard to calculate.

Any suggestions on how to do fast-edge-long-tau AC coupling on
microstrip?

There's always the transmission line transformer approach. You can get a lot of inductance by wrapping a long length of twisted pair around a decent sized pot core.

For 10psec edges you might have to go for sub-minature coax, and wrap it around a decent-sized toroid.

The practicalities depend on the details that you haven't specified.

--
Bill Sloman, Sydney

 

[Uwe Bonnes]

jlarkin@highlandsniptechnology.com wrote:

I have a fast (10s of ps edges) pulse that I want to AC couple. The
bias on the source side will be 15 volts, and the load is 50 ohms to
ground. I need at least 1 uF to minimized droop, preferably more. The
50 ohm trace is 45 mils wide.

https://www.dropbox.com/s/002r5ias7yveya4/Fast_AC_Coupling_1.jpg?raw=1

The 0204 fits the trace nicely but doesn't have enough capacitance at
15 volts. The 0603 is too small too, and has more inductance than I
want. 0306 doesn't have enough capacitance either.

A 10 uF 16V 0508 kinda works. It will have roughly 1.5 uF left at 15
volts. It does bulge out a bit from the trace width, which adds
capacitance and maybe a little inductance. I guess I could teardrop
the traces and then cut away a little ground plane to correct for the
extra capacitance. That's hard to calculate.

Any suggestions on how to do fast-edge-long-tau AC coupling on
microstrip?

Is this a design for rough handling? Otherwise, what about a manual
solders inverse pyramid of a small HF capacitor on the transmission
line and higher values, largers size capacitors with increasing size
on top ?

Cheers
--
Uwe Bonnes bon@elektron.ikp.physik.tu-darmstadt.de

Institut fuer Kernphysik Schlossgartenstrasse 9 64289 Darmstadt
--------- Tel. 06151 1623569 ------- Fax. 06151 1623305 ---------