On Wednesday, January 25, 2023 at 5:00:04 AM UTC-8, Phil Hobbs wrote:
> whit3rd wrote:
> > On Tuesday, January 24, 2023 at 5:36:47 PM UTC-8, Phil Hobbs wrote:
> >> whit3rd wrote:
> >>> On Tuesday, January 24, 2023 at 12:47:09 PM UTC-8, Phil Hobbs wrote:
> >>>
> >>>> It's already not that easy to get less than 0.1 pF to ground from a PCB
> >>>> pad, let alone a trace.
> >>>
> >>> Yeah, but the usual trim techniques still work; RG174 is 30 pF/foot,
> >>> so you can get your 0.1 pf by soldering a short length onto the board, and then
> >>> with flush nippers, cut it off.
> >> I invite you to try getting the fringing capacitance of an actual piece
> >> of RG-174, connected to a circuit, to be that low. Show your work. ;)
> >>
> >> (The capacitance per unit length only applies when the fringing
> >> capacitance is negligible.)
> >
> > But, because this is a trimming technique, there's an end correction BEFORE the
> > snip as well as after. At least, there is until the two ends of the cable coincide...

> However, John asked for a small absolute capacitance. A picofarad or so
> of end effect doesn't fit that bill.
>
> Plus when the coax gets too short, the approximation you're relying on
> starts getting inaccurate because (a) there are varying axial fields
> throughout the length of the coax, and (b) the fields at the two ends
> interact.

Not so; this (or any) trim isn't intended to hit only one absolute value, it's for
test-and-adjust finagling. If you want to hit near-zero, any adjustable
capacitor plus a negative offset circuit (negative capacitance) seems to
me to be a suitable solution. Having a suitable length to snip on,
does eliminate the concern with an invariant element, like the end correction.

On Wed, 25 Jan 2023 13:45:07 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>John Larkin wrote:
>> On Wed, 25 Jan 2023 07:59:52 -0500, Phil Hobbs
>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>
>>> whit3rd wrote:
>>>> On Tuesday, January 24, 2023 at 5:36:47 PM UTC-8, Phil Hobbs wrote:
>>>>> whit3rd wrote:
>>>>>> On Tuesday, January 24, 2023 at 12:47:09 PM UTC-8, Phil Hobbs wrote:
>>>>>>
>>>>>>> It's already not that easy to get less than 0.1 pF to ground from a PCB
>>>>>>> pad, let alone a trace.
>>>>>>
>>>>>> Yeah, but the usual trim techniques still work; RG174 is 30 pF/foot,
>>>>>> so you can get your 0.1 pf by soldering a short length onto the board, and then
>>>>>> with flush nippers, cut it off.
>>>>> I invite you to try getting the fringing capacitance of an actual piece
>>>>> of RG-174, connected to a circuit, to be that low. Show your work. ;)
>>>>>
>>>>> (The capacitance per unit length only applies when the fringing
>>>>> capacitance is negligible.)
>>>>
>>>> But, because this is a trimming technique, there's an end correction BEFORE the
>>>> snip as well as after. At least, there is until the two ends of the cable coincide...
>>>>
>>>> I'm not concerned with high frequencies much, so I might apply a negative capacitor
>>>> first, swamp it with the long-RG174 trimmer, then trim down to get near zero. Negative
>>>> impedance converter, you know...
>>>>
>>>> <https://wiki.analog.com/university/courses/electronics/text/chapter-4>
>>>>
>>>
>>> Yes and no. Given invariant fringing capacitance, changing the length
>>> incrementally does give a nice predictable delta-C. Trombone line is
>>> good that way down at low frequency, for instance.
>>>
>>> This is because, in a nice long piece of coax, the E field inside is
>>> purely radial almost everywhere, and any departure from the pure TEM
>>> mode at the ends dies off exponentially, roughly as exp(-2 pi L/r),
>>> where L is the distance from the open end and r is the radius. (That's
>>> a consequence of Laplace's equation, and is also why perforated metal
>>> makes good electrostatic shielding.)
>>>
>>> However, John asked for a small absolute capacitance. A picofarad or so
>>> of end effect doesn't fit that bill.
>>
>> And I want something that can be manufactured, not a hobby fiddle
>> thing.
>>
>> Something like this:
>>
>> https://www.dropbox.com/s/z4vevjyuflm732l/T578_LVDS.jpg?raw=1
>>
>> It needs extreme CMRR to work. If C1 is just a resistor parasitic, I
>> can make C2 a bit larger and the CMRR error becomes positive feedback,
>> which is OK in moderation. High voltage Schmitt trigger.
>>
>> I could just use a digital isolator to drive the GaN fet, but they are
>> slow. The modulated ones may add jitter, too.
>>
>> Of course, detonators probably don't care about a bit of jitter.
>
>Well, not repeatable jitter, anyway. ;)

Right. Measuring jitter requires a statistically valid number of
pulses.

We used to ship CAMAC systems that went down-hole near a nuke. They
shipped a bunch of time stamps up a long cable. Good replacement
market.

Just drilling the hole cost a million dollars.

John Larkin wrote:
> On Wed, 25 Jan 2023 07:59:52 -0500, Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>
>> whit3rd wrote:
>>> On Tuesday, January 24, 2023 at 5:36:47 PM UTC-8, Phil Hobbs wrote:
>>>> whit3rd wrote:
>>>>> On Tuesday, January 24, 2023 at 12:47:09 PM UTC-8, Phil Hobbs wrote:
>>>>>
>>>>>> It's already not that easy to get less than 0.1 pF to ground from a PCB
>>>>>> pad, let alone a trace.
>>>>>
>>>>> Yeah, but the usual trim techniques still work; RG174 is 30 pF/foot,
>>>>> so you can get your 0.1 pf by soldering a short length onto the board, and then
>>>>> with flush nippers, cut it off.
>>>> I invite you to try getting the fringing capacitance of an actual piece
>>>> of RG-174, connected to a circuit, to be that low. Show your work. ;)
>>>>
>>>> (The capacitance per unit length only applies when the fringing
>>>> capacitance is negligible.)
>>>
>>> But, because this is a trimming technique, there's an end correction BEFORE the
>>> snip as well as after. At least, there is until the two ends of the cable coincide...
>>>
>>> I'm not concerned with high frequencies much, so I might apply a negative capacitor
>>> first, swamp it with the long-RG174 trimmer, then trim down to get near zero. Negative
>>> impedance converter, you know...
>>>
>>> <https://wiki.analog.com/university/courses/electronics/text/chapter-4>
>>>
>>
>> Yes and no. Given invariant fringing capacitance, changing the length
>> incrementally does give a nice predictable delta-C. Trombone line is
>> good that way down at low frequency, for instance.
>>
>> This is because, in a nice long piece of coax, the E field inside is
>> purely radial almost everywhere, and any departure from the pure TEM
>> mode at the ends dies off exponentially, roughly as exp(-2 pi L/r),
>> where L is the distance from the open end and r is the radius. (That's
>> a consequence of Laplace's equation, and is also why perforated metal
>> makes good electrostatic shielding.)
>>
>> However, John asked for a small absolute capacitance. A picofarad or so
>> of end effect doesn't fit that bill.
>
> And I want something that can be manufactured, not a hobby fiddle
> thing.
>
> Something like this:
>
> https://www.dropbox.com/s/z4vevjyuflm732l/T578_LVDS.jpg?raw=1
>
> It needs extreme CMRR to work. If C1 is just a resistor parasitic, I
> can make C2 a bit larger and the CMRR error becomes positive feedback,
> which is OK in moderation. High voltage Schmitt trigger.

Okay, for something like that, just dorking it to one side a bit makes
sense, assuming that the worst-case RC time constant is small enough
that it doesn't distort the next transition.

I'm actually working on a proof-of-concept that needs two separate TIAs
to have very closely similar phase and amplitude response out to a
couple of hundred kilohertz, which requires excellent phase matching.
We're only making 10 or so boards, so I'm planning to put a couple of
DNP caps in parallel with the main one, to get the tolerance down from
~5% to ~0.5%. (Yes, this will require a bit of measurement and
selection on each board, but there aren't very many, and we have all the
stuff in stock.)

Maybe it's possible to use a dpot with a shunt cap on the wiper to
balance the two sides.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

On Wed, 25 Jan 2023 21:26:38 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>John Larkin wrote:
>> On Wed, 25 Jan 2023 07:59:52 -0500, Phil Hobbs
>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>
>>> whit3rd wrote:
>>>> On Tuesday, January 24, 2023 at 5:36:47 PM UTC-8, Phil Hobbs wrote:
>>>>> whit3rd wrote:
>>>>>> On Tuesday, January 24, 2023 at 12:47:09 PM UTC-8, Phil Hobbs wrote:
>>>>>>
>>>>>>> It's already not that easy to get less than 0.1 pF to ground from a PCB
>>>>>>> pad, let alone a trace.
>>>>>>
>>>>>> Yeah, but the usual trim techniques still work; RG174 is 30 pF/foot,
>>>>>> so you can get your 0.1 pf by soldering a short length onto the board, and then
>>>>>> with flush nippers, cut it off.
>>>>> I invite you to try getting the fringing capacitance of an actual piece
>>>>> of RG-174, connected to a circuit, to be that low. Show your work. ;)
>>>>>
>>>>> (The capacitance per unit length only applies when the fringing
>>>>> capacitance is negligible.)
>>>>
>>>> But, because this is a trimming technique, there's an end correction BEFORE the
>>>> snip as well as after. At least, there is until the two ends of the cable coincide...
>>>>
>>>> I'm not concerned with high frequencies much, so I might apply a negative capacitor
>>>> first, swamp it with the long-RG174 trimmer, then trim down to get near zero. Negative
>>>> impedance converter, you know...
>>>>
>>>> <https://wiki.analog.com/university/courses/electronics/text/chapter-4>
>>>>
>>>
>>> Yes and no. Given invariant fringing capacitance, changing the length
>>> incrementally does give a nice predictable delta-C. Trombone line is
>>> good that way down at low frequency, for instance.
>>>
>>> This is because, in a nice long piece of coax, the E field inside is
>>> purely radial almost everywhere, and any departure from the pure TEM
>>> mode at the ends dies off exponentially, roughly as exp(-2 pi L/r),
>>> where L is the distance from the open end and r is the radius. (That's
>>> a consequence of Laplace's equation, and is also why perforated metal
>>> makes good electrostatic shielding.)
>>>
>>> However, John asked for a small absolute capacitance. A picofarad or so
>>> of end effect doesn't fit that bill.
>>
>> And I want something that can be manufactured, not a hobby fiddle
>> thing.
>>
>> Something like this:
>>
>> https://www.dropbox.com/s/z4vevjyuflm732l/T578_LVDS.jpg?raw=1
>>
>> It needs extreme CMRR to work. If C1 is just a resistor parasitic, I
>> can make C2 a bit larger and the CMRR error becomes positive feedback,
>> which is OK in moderation. High voltage Schmitt trigger.
>
>Okay, for something like that, just dorking it to one side a bit makes
>sense, assuming that the worst-case RC time constant is small enough
>that it doesn't distort the next transition.
>
>I'm actually working on a proof-of-concept that needs two separate TIAs
>to have very closely similar phase and amplitude response out to a
>couple of hundred kilohertz, which requires excellent phase matching.
>We're only making 10 or so boards, so I'm planning to put a couple of
>DNP caps in parallel with the main one, to get the tolerance down from
>~5% to ~0.5%. (Yes, this will require a bit of measurement and
>selection on each board, but there aren't very many, and we have all the
>stuff in stock.)
>
>Maybe it's possible to use a dpot with a shunt cap on the wiper to
>balance the two sides.
>
>Cheers
>
>Phil Hobbs

How about a trimmer cap? Beats soldering.

If it's TIA feedback, a trimpot can tweak the effective C value.

DPOTs are usually slow with the slowness code-dependent. Real trimpots
are great and easy to program.

John Larkin wrote:
> On Wed, 25 Jan 2023 21:26:38 -0500, Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>
>> John Larkin wrote:
>>> On Wed, 25 Jan 2023 07:59:52 -0500, Phil Hobbs
>>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>>
>>>> whit3rd wrote:
>>>>> On Tuesday, January 24, 2023 at 5:36:47 PM UTC-8, Phil Hobbs wrote:
>>>>>> whit3rd wrote:
>>>>>>> On Tuesday, January 24, 2023 at 12:47:09 PM UTC-8, Phil Hobbs wrote:
>>>>>>>
>>>>>>>> It's already not that easy to get less than 0.1 pF to ground from a PCB
>>>>>>>> pad, let alone a trace.
>>>>>>>
>>>>>>> Yeah, but the usual trim techniques still work; RG174 is 30 pF/foot,
>>>>>>> so you can get your 0.1 pf by soldering a short length onto the board, and then
>>>>>>> with flush nippers, cut it off.
>>>>>> I invite you to try getting the fringing capacitance of an actual piece
>>>>>> of RG-174, connected to a circuit, to be that low. Show your work. ;)
>>>>>>
>>>>>> (The capacitance per unit length only applies when the fringing
>>>>>> capacitance is negligible.)
>>>>>
>>>>> But, because this is a trimming technique, there's an end correction BEFORE the
>>>>> snip as well as after. At least, there is until the two ends of the cable coincide...
>>>>>
>>>>> I'm not concerned with high frequencies much, so I might apply a negative capacitor
>>>>> first, swamp it with the long-RG174 trimmer, then trim down to get near zero. Negative
>>>>> impedance converter, you know...
>>>>>
>>>>> <https://wiki.analog.com/university/courses/electronics/text/chapter-4>
>>>>>
>>>>
>>>> Yes and no. Given invariant fringing capacitance, changing the length
>>>> incrementally does give a nice predictable delta-C. Trombone line is
>>>> good that way down at low frequency, for instance.
>>>>
>>>> This is because, in a nice long piece of coax, the E field inside is
>>>> purely radial almost everywhere, and any departure from the pure TEM
>>>> mode at the ends dies off exponentially, roughly as exp(-2 pi L/r),
>>>> where L is the distance from the open end and r is the radius. (That's
>>>> a consequence of Laplace's equation, and is also why perforated metal
>>>> makes good electrostatic shielding.)
>>>>
>>>> However, John asked for a small absolute capacitance. A picofarad or so
>>>> of end effect doesn't fit that bill.
>>>
>>> And I want something that can be manufactured, not a hobby fiddle
>>> thing.
>>>
>>> Something like this:
>>>
>>> https://www.dropbox.com/s/z4vevjyuflm732l/T578_LVDS.jpg?raw=1
>>>
>>> It needs extreme CMRR to work. If C1 is just a resistor parasitic, I
>>> can make C2 a bit larger and the CMRR error becomes positive feedback,
>>> which is OK in moderation. High voltage Schmitt trigger.
>>
>> Okay, for something like that, just dorking it to one side a bit makes
>> sense, assuming that the worst-case RC time constant is small enough
>> that it doesn't distort the next transition.
>>
>> I'm actually working on a proof-of-concept that needs two separate TIAs
>> to have very closely similar phase and amplitude response out to a
>> couple of hundred kilohertz, which requires excellent phase matching.
>> We're only making 10 or so boards, so I'm planning to put a couple of
>> DNP caps in parallel with the main one, to get the tolerance down from
>> ~5% to ~0.5%. (Yes, this will require a bit of measurement and
>> selection on each board, but there aren't very many, and we have all the
>> stuff in stock.)
>>
>> Maybe it's possible to use a dpot with a shunt cap on the wiper to
>> balance the two sides.
>>
>> Cheers
>>
>> Phil Hobbs
>
> How about a trimmer cap? Beats soldering.

If it can be adjusted, it can be mis-adjusted, leading to
hard-to-diagnose misbehavior and reasonable-looking wrong answers. (My
least favorite kind.)

>
> If it's TIA feedback, a trimpot can tweak the effective C value.
>
> DPOTs are usually slow with the slowness code-dependent. Real trimpots
> are great and easy to program.

Sure, if you can still get them. Those nice Murata PVA2 things are long
gone.

A dpot is a variable RC network, so by putting the cap at the wiper, you
change the capacitive loading of the ends. There's some built in to the
dpot as well, as you say, but it varies with code too.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
> I'm actually working on a proof-of-concept that needs two separate TIAs
> to have very closely similar phase and amplitude response out to a
> couple of hundred kilohertz, which requires excellent phase matching.
> We're only making 10 or so boards, so I'm planning to put a couple of
> DNP caps in parallel with the main one, to get the tolerance down from
> ~5% to ~0.5%. (Yes, this will require a bit of measurement and
> selection on each board, but there aren't very many, and we have all the
> stuff in stock.)
>

Did you look at parts similar to
https://www.mouser.de/c/passive-components/capacitors/silicon-rf-capacitors-thin-film/?q=0402%201pf

At least you do not start with 5 %.

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

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

On 26 Jan 2023 19:06:59 GMT, Uwe Bonnes
<bon@hertz.ikp.physik.tu-darmstadt.de> wrote:

>Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
>
>> I'm actually working on a proof-of-concept that needs two separate TIAs
>> to have very closely similar phase and amplitude response out to a
>> couple of hundred kilohertz, which requires excellent phase matching.
>> We're only making 10 or so boards, so I'm planning to put a couple of
>> DNP caps in parallel with the main one, to get the tolerance down from
>> ~5% to ~0.5%. (Yes, this will require a bit of measurement and
>> selection on each board, but there aren't very many, and we have all the
>> stuff in stock.)
>>
>
>Did you look at parts similar to
>https://www.mouser.de/c/passive-components/capacitors/silicon-rf-capacitors-thin-film/?q=0402%201pf
>
>At least you do not start with 5 %.
>
>Bye

Check out

PEREGRINE PE64907MLAA-Z

We use them for coarse-tuning an LC oscillator which is then
fine-tuned over a small range by a varicap.

It replaced a Maxim part that of course went EOL.

Uwe Bonnes wrote:
> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
>
>> I'm actually working on a proof-of-concept that needs two separate TIAs
>> to have very closely similar phase and amplitude response out to a
>> couple of hundred kilohertz, which requires excellent phase matching.
>> We're only making 10 or so boards, so I'm planning to put a couple of
>> DNP caps in parallel with the main one, to get the tolerance down from
>> ~5% to ~0.5%. (Yes, this will require a bit of measurement and
>> selection on each board, but there aren't very many, and we have all the
>> stuff in stock.)
>>
>
> Did you look at parts similar to
> https://www.mouser.de/c/passive-components/capacitors/silicon-rf-capacitors-thin-film/?q=0402%201pf
>
> At least you do not start with 5 %.
>
> Bye
>

Thanks, Uwe,

Very nice. I confess that I wasn't aware of the whole precision
picofarad capacitor niche--I can get 100 pF 1% NP0s for two cents in
hundreds, hurrah.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

Get your xacto knife & slice across a trace on the pc board.

Hul

John Larkin <jlarkin@highlandsnipmetechnology.com> wrote:
> I need a tiny cap.

> This one is interesting

> https://www.mouser.com/ProductDetail/Walsin/RF03N0R1B100CT?qs=sGAEpiMZZMvsSlwiRhF8qtsBU8Zhqm2Ra%2Fa5698GTEQZP2uSIBTS%2FQ%3D%3D

> Below 0.1 pF, maybe I can use a 10 meg resistor, which is around 0.04
> pF. Maybe a couple in series? Maybe a network?

On Fri, 27 Jan 2023 16:56:47 -0000 (UTC), Hul Tytus <ht@panix.com>
wrote:

>Get your xacto knife & slice across a trace on the pc board.
>
>Hul

That's not very manufacturable.

On Friday, January 27, 2023 at 9:26:34 AM UTC-8, John Larkin wrote:
> On Fri, 27 Jan 2023 16:56:47 -0000 (UTC), Hul Tytus <h...@panix.com>
> wrote:
> >Get your xacto knife & slice across a trace on the pc board.

> That's not very manufacturable.

Unknowable. Do you recall turret tuners, where each of a dozen channels'
LC time constants were adjusted, in several places per channel,
in mass production? That started out with handwork, but got
automated, and... was actually pretty efficient.

Someone got clever and put an array of computer-controlled pistons
to work squishing coils until they hit the target value.

The 'slice across' could be laser scan, or vibrating chisel, or
platinum-spark-plug electrode doing spark erosion, if you want
a manufacturable process. Maybe just a grinding wheel
and a jig to present the right edge to the right feed depth,
about as fast as applying an inspection stamp.

On Thursday, January 26, 2023 at 11:07:06 AM UTC-8, Uwe Bonnes wrote:
> Phil Hobbs <pcdhSpamM...@electrooptical.net> wrote:
>
> > I'm actually working on a proof-of-concept that needs two separate TIAs
> > to have very closely similar phase and amplitude response out to a
> > couple of hundred kilohertz, which requires excellent phase matching.
> > We're only making 10 or so boards, so I'm planning to put a couple of
> > DNP caps in parallel with the main one, to get the tolerance down from
> > ~5% to ~0.5%. (Yes, this will require a bit of measurement and
> > selection on each board, but there aren't very many, and we have all the
> > stuff in stock.)
> >
> Did you look at parts similar to
> https://www.mouser.de/c/passive-components/capacitors/silicon-rf-capacitors-thin-film/?q=0402%201pf
>
> At least you do not start with 5 %.

You beat me to it. It can be had with 10 fF tolerance. They are very repeatable. There is nothing quite like Accu-P. They can get expensive with the tight tolerance.

On Saturday, January 28, 2023 at 11:21:47 PM UTC-4, Simon S Aysdie wrote:
> On Thursday, January 26, 2023 at 11:07:06 AM UTC-8, Uwe Bonnes wrote:
> > Phil Hobbs <pcdhSpamM...@electrooptical.net> wrote:
> >
> > > I'm actually working on a proof-of-concept that needs two separate TIAs
> > > to have very closely similar phase and amplitude response out to a
> > > couple of hundred kilohertz, which requires excellent phase matching.
> > > We're only making 10 or so boards, so I'm planning to put a couple of
> > > DNP caps in parallel with the main one, to get the tolerance down from
> > > ~5% to ~0.5%. (Yes, this will require a bit of measurement and
> > > selection on each board, but there aren't very many, and we have all the
> > > stuff in stock.)
> > >
> > Did you look at parts similar to
> > https://www.mouser.de/c/passive-components/capacitors/silicon-rf-capacitors-thin-film/?q=0402%201pf
> >
> > At least you do not start with 5 %.
> You beat me to it. It can be had with 10 fF tolerance. They are very repeatable. There is nothing quite like Accu-P. They can get expensive with the tight tolerance.

That was a trip. Accessing the German Mouser site from Puerto Rico. My choices are German or Spanish... I think!

--

Rick C.

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