tee trace...

[Simon S Aysdie]

On Monday, December 13, 2021 at 7:38:35 AM UTC-8, jla...@highlandsniptechnology.com wrote:

Suppose one had a microstrip trace that was higher impedance than
ideal, but couldn\'t be made wider. One could reduce the distance to
the ground plane, but that might have capacitance consequences
elsewhere.

I wonder if one could add a vertical conductor, sort of a shark fin,
to make the conductor have a t-shaped cross section.

I guess ATLC could analyze that. I might try it after I actually wake
up.

One could also glue a chunk of dielectric to the top of the trace,
with optional copper on top. Make it into a sort of stripline.

W/o the copper, I\'d call that \"dielectric loading.\" Not sure if it is a rigorous term. It is a good compromise when it is impractical to get the other copper layer in place, for whatever reason.

 

[server]

On Tue, 14 Dec 2021 09:39:39 -0800 (PST), Lasse Langwadt Christensen
<langwadt@fonz.dk> wrote:

tirsdag den 14. december 2021 kl. 05.27.11 UTC+1 skrev jla...@highlandsniptechnology.com:
On Mon, 13 Dec 2021 18:19:30 -0800 (PST), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

tirsdag den 14. december 2021 kl. 02.36.19 UTC+1 skrev John Larkin:
On Mon, 13 Dec 2021 14:03:55 -0800 (PST), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

mandag den 13. december 2021 kl. 16.38.35 UTC+1 skrev jla...@highlandsniptechnology.com:
Suppose one had a microstrip trace that was higher impedance than
ideal, but couldn\'t be made wider. One could reduce the distance to
the ground plane, but that might have capacitance consequences
elsewhere.

so add an extra layer for a ground plane close to the microstrip,
but leave it blank where you need low capacitance

standard pcb expect for the special stackup and no extra production steps
It\'s already a 6 layer board. Here\'s the trace, running from the IC to
C28. I want it to be 25 ohms but can\'t make it wide enough. Adding the
coplanar waveguide grounds only helped a little.

https://www.dropbox.com/s/97zkiqidwszo9le/T502_25r_Trace.jpg?raw=1

It might be OK... it\'s pretty short. But if not, maybe I can add
something to pull the impedance down.

I could TDR a microstrip and stick something on top, like a piece of
FR4 or maybe even just some epoxy or something. Yes, I\'ll try that
just to calibrate my expectations.

is is very short and looks you could easily make it shorter
That trace is about 0.12\" long, roughly 20 ps, but I\'ve made it about
as short as I can. There are all sorts of other parts around.

just move C28 to the left, right up against the IC ?

That makes the path to the right longer!



--

I yam what I yam - Popeye

 

[server]

On Tue, 14 Dec 2021 13:27:45 -0800 (PST), whit3rd <whit3rd@gmail.com>
wrote:

On Monday, December 13, 2021 at 8:27:11 PM UTC-8, jla...@highlandsniptechnology.com wrote:

That trace is about 0.12\" long, roughly 20 ps, but I\'ve made it about
as short as I can. There are all sorts of other parts around.

The RF boys measure flatness in dBs but we need PPMs. This is for a
laser modulator for people with extreme expectations.

Perhaps you should connect the current source in an expedient way, with two
transmission lines; one to the laser, one that is a \'stub\' onto which you can
hang loads, or etch to length. A field solver can guide the crafting of that stub.
Artistically crafted metal on a PCB is a powerful solution technique, at 20 ps.

I\'m driving a Mach-Zehnder modulator, a 50 ohm load. The actual laser
is lots of kilojoules.



--

I yam what I yam - Popeye

 

[Chris Jones]

On 14/12/2021 12:36, John Larkin wrote:

On Mon, 13 Dec 2021 14:03:55 -0800 (PST), Lasse Langwadt Christensen
langwadt@fonz.dk> wrote:

mandag den 13. december 2021 kl. 16.38.35 UTC+1 skrev jla...@highlandsniptechnology.com:
Suppose one had a microstrip trace that was higher impedance than
ideal, but couldn\'t be made wider. One could reduce the distance to
the ground plane, but that might have capacitance consequences
elsewhere.

so add an extra layer for a ground plane close to the microstrip,
but leave it blank where you need low capacitance

standard pcb expect for the special stackup and no extra production steps

It\'s already a 6 layer board. Here\'s the trace, running from the IC to
C28. I want it to be 25 ohms but can\'t make it wide enough. Adding the
coplanar waveguide grounds only helped a little.

https://www.dropbox.com/s/97zkiqidwszo9le/T502_25r_Trace.jpg?raw=1

It might be OK... it\'s pretty short. But if not, maybe I can add
something to pull the impedance down.

I could TDR a microstrip and stick something on top, like a piece of
FR4 or maybe even just some epoxy or something. Yes, I\'ll try that
just to calibrate my expectations.

Why is the track to the right of C28 so narrow, if you care about 25
Ohms impedance to the left of C28, or is that bit just a placeholder and
not done yet?

Is this a high-volume or cost-sensitive product? If not, and if you have
competent technicians who can solder, I\'d probably solder a piece of
semi-rigid micro-coax onto the board instead of using a microstrip, if I
had to achieve 25 Ohms. Then you can make it much longer if you like.
You can get it in very small diameters, and various impedances. I expect
you could order it pre-cut and stripped ready to hand-place onto the
board much like a SMT component, or thread it through a grounded
through-hole pad if you like, etc.

For example you could buy UT-034-25-SP from

https://www.microstock-inc.com/newpage2.php

 

[server]

On Wed, 15 Dec 2021 21:52:02 +1100, Chris Jones
<lugnut808@spam.yahoo.com> wrote:

On 14/12/2021 12:36, John Larkin wrote:
On Mon, 13 Dec 2021 14:03:55 -0800 (PST), Lasse Langwadt Christensen
langwadt@fonz.dk> wrote:

mandag den 13. december 2021 kl. 16.38.35 UTC+1 skrev jla...@highlandsniptechnology.com:
Suppose one had a microstrip trace that was higher impedance than
ideal, but couldn\'t be made wider. One could reduce the distance to
the ground plane, but that might have capacitance consequences
elsewhere.

so add an extra layer for a ground plane close to the microstrip,
but leave it blank where you need low capacitance

standard pcb expect for the special stackup and no extra production steps

It\'s already a 6 layer board. Here\'s the trace, running from the IC to
C28. I want it to be 25 ohms but can\'t make it wide enough. Adding the
coplanar waveguide grounds only helped a little.

https://www.dropbox.com/s/97zkiqidwszo9le/T502_25r_Trace.jpg?raw=1

It might be OK... it\'s pretty short. But if not, maybe I can add
something to pull the impedance down.

I could TDR a microstrip and stick something on top, like a piece of
FR4 or maybe even just some epoxy or something. Yes, I\'ll try that
just to calibrate my expectations.


Why is the track to the right of C28 so narrow, if you care about 25
Ohms impedance to the left of C28, or is that bit just a placeholder and
not done yet?

This is about it:

https://www.dropbox.com/s/apq219ofizpvugr/T502_Fiducial.jpg?raw=1

We want to inject a test pulse into an amplifier input sometimes. The
laser driver chip is dynamite but has a 25 ohm source impedance and
tiny wimpy pins. This resembles Leo Bodmar\'s neat step generator
gadgets. It\'s worth buying one of his boards just to study the layout.

I want a 25 ohm run to span the distance from the driver to the cap
and schottky, but then all I have is really tiny parts with minimal
spacings.

The entire fast part of this circuit board is on a little wing, about
a square inch.

https://www.dropbox.com/s/bb16beiwymbu28a/T502A.jpg?raw=1

An earlier version used a PIN diode instead of the schottky, but it
had carrier lifetime problems. Pity, it looked so good: 2 ohms on,
0.14 pF off. The schottky is more like 10 and 0.2



--

I yam what I yam - Popeye

 

[Simon S Aysdie]

On Wednesday, December 15, 2021 at 8:20:35 AM UTC-8, jla...@highlandsniptechnology.com wrote:

On Wed, 15 Dec 2021 21:52:02 +1100, Chris Jones
lugn...@spam.yahoo.com> wrote:

On 14/12/2021 12:36, John Larkin wrote:
On Mon, 13 Dec 2021 14:03:55 -0800 (PST), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

mandag den 13. december 2021 kl. 16.38.35 UTC+1 skrev jla...@highlandsniptechnology.com:
Suppose one had a microstrip trace that was higher impedance than
ideal, but couldn\'t be made wider. One could reduce the distance to
the ground plane, but that might have capacitance consequences
elsewhere.

so add an extra layer for a ground plane close to the microstrip,
but leave it blank where you need low capacitance

standard pcb expect for the special stackup and no extra production steps

It\'s already a 6 layer board. Here\'s the trace, running from the IC to
C28. I want it to be 25 ohms but can\'t make it wide enough. Adding the
coplanar waveguide grounds only helped a little.

https://www.dropbox.com/s/97zkiqidwszo9le/T502_25r_Trace.jpg?raw=1

It might be OK... it\'s pretty short. But if not, maybe I can add
something to pull the impedance down.

I could TDR a microstrip and stick something on top, like a piece of
FR4 or maybe even just some epoxy or something. Yes, I\'ll try that
just to calibrate my expectations.


Why is the track to the right of C28 so narrow, if you care about 25
Ohms impedance to the left of C28, or is that bit just a placeholder and
not done yet?
This is about it:

https://www.dropbox.com/s/apq219ofizpvugr/T502_Fiducial.jpg?raw=1

We want to inject a test pulse into an amplifier input sometimes. The
laser driver chip is dynamite but has a 25 ohm source impedance and
tiny wimpy pins. This resembles Leo Bodmar\'s neat step generator
gadgets. It\'s worth buying one of his boards just to study the layout.

I want a 25 ohm run to span the distance from the driver to the cap
and schottky, but then all I have is really tiny parts with minimal
spacings.

The entire fast part of this circuit board is on a little wing, about
a square inch.

https://www.dropbox.com/s/bb16beiwymbu28a/T502A.jpg?raw=1

An earlier version used a PIN diode instead of the schottky, but it
had carrier lifetime problems. Pity, it looked so good: 2 ohms on,
0.14 pF off. The schottky is more like 10 and 0.2

4 mil Meg6 ends up w/ about 22mil wide (µstrip, not GCPW). It\'ll have to neck down into the IC pad no matter what. Mask will slightly lower Z but is unpredicatable, as the thickness is not well controlled. I usually clear it because I can\'t count on it.

 

[server]

On Wed, 15 Dec 2021 10:28:38 -0800 (PST), Simon S Aysdie
<gwhite@ti.com> wrote:

On Wednesday, December 15, 2021 at 8:20:35 AM UTC-8, jla...@highlandsniptechnology.com wrote:
On Wed, 15 Dec 2021 21:52:02 +1100, Chris Jones
lugn...@spam.yahoo.com> wrote:

On 14/12/2021 12:36, John Larkin wrote:
On Mon, 13 Dec 2021 14:03:55 -0800 (PST), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

mandag den 13. december 2021 kl. 16.38.35 UTC+1 skrev jla...@highlandsniptechnology.com:
Suppose one had a microstrip trace that was higher impedance than
ideal, but couldn\'t be made wider. One could reduce the distance to
the ground plane, but that might have capacitance consequences
elsewhere.

so add an extra layer for a ground plane close to the microstrip,
but leave it blank where you need low capacitance

standard pcb expect for the special stackup and no extra production steps

It\'s already a 6 layer board. Here\'s the trace, running from the IC to
C28. I want it to be 25 ohms but can\'t make it wide enough. Adding the
coplanar waveguide grounds only helped a little.

https://www.dropbox.com/s/97zkiqidwszo9le/T502_25r_Trace.jpg?raw=1

It might be OK... it\'s pretty short. But if not, maybe I can add
something to pull the impedance down.

I could TDR a microstrip and stick something on top, like a piece of
FR4 or maybe even just some epoxy or something. Yes, I\'ll try that
just to calibrate my expectations.


Why is the track to the right of C28 so narrow, if you care about 25
Ohms impedance to the left of C28, or is that bit just a placeholder and
not done yet?
This is about it:

https://www.dropbox.com/s/apq219ofizpvugr/T502_Fiducial.jpg?raw=1

We want to inject a test pulse into an amplifier input sometimes. The
laser driver chip is dynamite but has a 25 ohm source impedance and
tiny wimpy pins. This resembles Leo Bodmar\'s neat step generator
gadgets. It\'s worth buying one of his boards just to study the layout.

I want a 25 ohm run to span the distance from the driver to the cap
and schottky, but then all I have is really tiny parts with minimal
spacings.

The entire fast part of this circuit board is on a little wing, about
a square inch.

https://www.dropbox.com/s/bb16beiwymbu28a/T502A.jpg?raw=1

An earlier version used a PIN diode instead of the schottky, but it
had carrier lifetime problems. Pity, it looked so good: 2 ohms on,
0.14 pF off. The schottky is more like 10 and 0.2

4 mil Meg6 ends up w/ about 22mil wide (µstrip, not GCPW). It\'ll have to neck down into the IC pad no matter what. Mask will slightly lower Z but is unpredicatable, as the thickness is not well controlled. I usually clear it because I can\'t count on it.

I want low pad capacitances elsewhere on the board, so there\'s a
compromise on dielectric thickness. I did one test board with an Isola
microwave lam, and it\'s didn\'t seem to be any better than FR4, given
that I keep everything short. The CPW grounds on my fat trace only
help a tiny bit, not really worth it.

I\'ve tried scraping away solder mask on microstrips and couldn\'t see
any difference on 30 ps TDR.

I have another board coming up, with a long TDR test trace. I can add
some official mask keepouts and see what happens.



--

I yam what I yam - Popeye

 

[Edward Hernandez]

The John Doe troll stated the following in message-id
<sdhn7c$pkp$4@dont-email.me>:

> The troll doesn\'t even know how to format a USENET post...

And the John Doe troll stated the following in message-id
<sg3kr7$qt5$1@dont-email.me>:

The reason Bozo cannot figure out how to get Google to keep from
breaking its lines in inappropriate places is because Bozo is
CLUELESS...

And yet, the clueless John Doe troll has itself posted yet another
incorrectly formatted USENET posting on Wed, 15 Dec 2021 18:56:13 -0000
(UTC) in message-id <spddoc$g7q$1@dont-email.me>.

SaEcO4pH/7TI

 

[Anthony William Sloman]

On Thursday, December 16, 2021 at 5:56:20 AM UTC+11, John Doe wrote:

Anthony William Sloman <bill....@ieee.org> wrote:
On Wednesday, December 15, 2021 at 1:31:14 AM UTC+11, jla...@highlandsniptechnology.com wrote:
On Tue, 14 Dec 2021 10:29:29 +0100, Gerhard Hoffmann <dk...@arcor.de
wrote:
Am 14.12.21 um 05:27 schrieb jla...@highlandsniptechnology.com:
On Mon, 13 Dec 2021 18:19:30 -0800 (PST), Lasse Langwadt Christensen <lang...@fonz.dk> wrote: tirsdag den 14. december 2021 kl. 02.36.19 UTC+1 skrev John Larkin:
On Mon, 13 Dec 2021 14:03:55 -0800 (PST), Lasse Langwadt Christensen <lang...@fonz.dk> wrote:
mandag den 13. december 2021 kl. 16.38.35 UTC+1 skrev jla...@highla ndsniptechnology.com:

snip

is is very short and looks you could easily make it shorter

That trace is about 0.12\" long, roughly 20 ps, but I\'ve made it about as short as I can. There are all sorts of other parts around.

The RF boys measure flatness in dBs but we need PPMs. This is for a laser modulator for people with extreme expectations.

You could use 2 parallel coplanar wave guides on different layers or load the top one with additional vias, or use a higher eps-r material on the top level. Vias are death at these speeds.

Except at the source and the termination.

Most microwave laminates are lower Er than FR4, and are exotic and expensive.

And a whole lot better behaved that FR4. The glass fibres that the FR4 epoxy resin ties together don\'t have a uniform density, so you get fluctuations in trace impedance along the trace.

If you want to push 11GHz through your board, epoxy resin-bonded glass fibre is a remarkably poor choice.

We didn\'t have any trouble getting stuff made on Rogers microwave substrates back in the late 1980s in the UK. They maybe somewhat exotic and a bit more expensive, but they are a whole lot more likely to give you a well behaved assembly. This ought to matter to you

Eval boards for exotic amps are usually very skinny coplanar waveguide on very thin microwave laminates, and assume external bias tees and DC blocks and manually tweaked power and bias supplies.

So they know where to spend their money, even if you don\'t.

snip

Unfortunately, we don\'t have device models for the critical parts, and c
ertainly not nonlinear Spice models. So it\'s faster to etch boards and futz with them than buy and learn and run and fight licensing issues on expensive simulators.

It might help if you etched boards that used a more suitable substrate than FR4.

We\'re doing large-signal high precision pulses with RF parts.

That\'s what you have been asked to, and may be what you are trying to do, but FR4 epoxy-glass substrates won\'t help it you need 11HGz bandwidth

The RF-specs are only suggestive and often deceptive.

Particularly when you don\'t read them carefully.

<snipped John Doe\'s usual drivel - he presumably didn\'t like my post, but lacks the wit to be able to post a specific objection to it>

--
Bill Sloman, Sydney

 

[Edward Hernandez]

The John Doe troll stated the following in message-id
<sdhn7c$pkp$4@dont-email.me>:

> The troll doesn\'t even know how to format a USENET post...

And the John Doe troll stated the following in message-id
<sg3kr7$qt5$1@dont-email.me>:

The reason Bozo cannot figure out how to get Google to keep from
breaking its lines in inappropriate places is because Bozo is
CLUELESS...

And yet, the clueless John Doe troll has itself posted yet another
incorrectly formatted USENET posting on Thu, 16 Dec 2021 03:13:28 -0000
(UTC) in message-id <speasn$61i$1@dont-email.me>.

This posting is a public service announcement for any google groups
readers who happen by to point out that the John Doe troll does not even
follow it\'s own rules that it uses to troll other posters.

q13rtDDuJTy8

 

[Anthony William Sloman]

On Thursday, December 16, 2021 at 2:13:33 PM UTC+11, John Doe wrote:
> NOBODY likes this troll\'s contentless spam. After being spanked in the electronics repair group, it wants to annoy everybody.

Nobody likes John Doe\'s contentless spam. For him to complain about somebody else going after him in the same way that he stalks others does have a certain ironic content.

<snipped the rest of the bitching. John Doe is responsible for a lot of wasted bandwidth, and it would be irresponsible to waste any more than I feel I have to.>

--
Bill Sloman, Sydney