How to make a super fast sampling head?

[John Larkin]

On Fri, 26 Apr 2019 06:03:20 -0700 (PDT), klaus.kragelund@gmail.com
wrote:

On Friday, 12 April 2019 08:21:31 UTC+2, klaus.k...@gmail.com wrote:
On Wednesday, 10 April 2019 21:13:54 UTC+2, Hul Tytus wrote:
Klaus - take a look at Electronic Design, Sept 18 2000. The article in
"Ideas for Design", "1-Ghz Sampling Oscilloscope Front end is easily
modified" shows a couble diode swithches setup as a sampler. A couple text
quotes if the titles don't work: "adjustable from 1 to 50 ns/div." and
"circuit by switching the two Schottky".

Hul


klaus.kragelund@gmail.com wrote:
On Thursday, 10 January 2008 17:57:44 UTC+1, Jan Panteltje wrote:
J Larkin was saying his super sampling head costs 750$.
I was thinking how can you make one yourself, what else is needed?

So, I am trying to see if we can make a sub ns S/H, for equivalent time sampling of TDR signals

I was looking at this thread, interesting stuff

I only need 8 bit resolution and 500ps. Could that be done with a mux and a small storage cap (I can sample in about 1us with the ADC after the S/H)


That is a very nice reference.

I had some trouble finding it, links were broken on Electronic Design, but found somebody that had saved it as a PDF:

http://www.redrok.com/Circuits_1GHz-samplig-Oscilloscope-Front-End.pdf

Then I found a lot of info on this page:

https://www.eevblog.com/forum/projects/diy-ghz-sampling-head-for-lt100mhz-scopes/

Misc info here from another guy:

https://www.dropbox.com/sh/kmdbbjrzofdxmkb/AADKVlHctfqbE8CNwntixXnza?dl=0%27


A 150ps pulse widht circuit, extremely simple, for TDR generation and sampling:

https://www.eevblog.com/forum/projects/pulse-generator-150ps-1vpp/

Explanation here:

https://www.adv-radio-sci.net/2/7/2004/ars-2-7-2004.pdf


Cheers

Klaus

So, I am plying a little with the sampler from this article i Pspice:

http://www.redrok.com/Circuits_1GHz-samplig-Oscilloscope-Front-End.pdf

It can track a signal resonable, but when the 1ns comparator turns the sample head on (first quad diodes), som charge spills through giving an incorrect amplitude

It's worse when the sampler turns off, on a 1V signal, the amplitude is 100mV off. I am using a small capacitor of 20pF to hold the signal, but the charge transfer from the 2pF shunt capacitance of the diode versus this 20pF cap is too high

If I increase the cap, it's nowhere near a 1GHz sampler, if I decrease it, it has very low hold time

I do not have the 15MOhm/1MOhm - 1pF/15pF (internal cap of TL082) combination, but perhaps I should??

Cheers

Klaus

That's a horrible circuit, but it would be improved with better
sampling diodes. There are 0.25 pF diodes around, like SMS7621 or
BAT15.

Classic samplers had blowby compensation to cancel signals leaking
through the diode off capacitance.

I never has much luck with full-bridge samplers. Half-bridge worked
for me, whit short impulses as the sampling gate. I did a
theroretically 40 ps 1bit sampler/TDR which I never finished testing,
but that's an easy way to go fast.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

On Friday, 26 April 2019 16:20:15 UTC+2, John Larkin wrote:

On Fri, 26 Apr 2019 06:03:20 -0700 (PDT), klaus.kragelund@gmail.com
wrote:

On Friday, 12 April 2019 08:21:31 UTC+2, klaus.k...@gmail.com wrote:
On Wednesday, 10 April 2019 21:13:54 UTC+2, Hul Tytus wrote:
Klaus - take a look at Electronic Design, Sept 18 2000. The article in
"Ideas for Design", "1-Ghz Sampling Oscilloscope Front end is easily
modified" shows a couble diode swithches setup as a sampler. A couple text
quotes if the titles don't work: "adjustable from 1 to 50 ns/div." and
"circuit by switching the two Schottky".

Hul


klaus.kragelund@gmail.com wrote:
On Thursday, 10 January 2008 17:57:44 UTC+1, Jan Panteltje wrote:
J Larkin was saying his super sampling head costs 750$.
I was thinking how can you make one yourself, what else is needed?

So, I am trying to see if we can make a sub ns S/H, for equivalent time sampling of TDR signals

I was looking at this thread, interesting stuff

I only need 8 bit resolution and 500ps. Could that be done with a mux and a small storage cap (I can sample in about 1us with the ADC after the S/H)


That is a very nice reference.

I had some trouble finding it, links were broken on Electronic Design, but found somebody that had saved it as a PDF:

http://www.redrok.com/Circuits_1GHz-samplig-Oscilloscope-Front-End.pdf

Then I found a lot of info on this page:

https://www.eevblog.com/forum/projects/diy-ghz-sampling-head-for-lt100mhz-scopes/

Misc info here from another guy:

https://www.dropbox.com/sh/kmdbbjrzofdxmkb/AADKVlHctfqbE8CNwntixXnza?dl=0%27


A 150ps pulse widht circuit, extremely simple, for TDR generation and sampling:

https://www.eevblog.com/forum/projects/pulse-generator-150ps-1vpp/

Explanation here:

https://www.adv-radio-sci.net/2/7/2004/ars-2-7-2004.pdf


Cheers

Klaus

So, I am plying a little with the sampler from this article i Pspice:

http://www.redrok.com/Circuits_1GHz-samplig-Oscilloscope-Front-End.pdf

It can track a signal resonable, but when the 1ns comparator turns the sample head on (first quad diodes), som charge spills through giving an incorrect amplitude

It's worse when the sampler turns off, on a 1V signal, the amplitude is 100mV off. I am using a small capacitor of 20pF to hold the signal, but the charge transfer from the 2pF shunt capacitance of the diode versus this 20pF cap is too high

If I increase the cap, it's nowhere near a 1GHz sampler, if I decrease it, it has very low hold time

I do not have the 15MOhm/1MOhm - 1pF/15pF (internal cap of TL082) combination, but perhaps I should??

Cheers

Klaus

That's a horrible circuit, but it would be improved with better
sampling diodes. There are 0.25 pF diodes around, like SMS7621 or
BAT15.

I am sorry, my brain is slow today, that should have been the first thing I looked at. For once I have an application that allows for more expensive components

Classic samplers had blowby compensation to cancel signals leaking
through the diode off capacitance.

I never has much luck with full-bridge samplers. Half-bridge worked
for me, whit short impulses as the sampling gate. I did a
theroretically 40 ps 1bit sampler/TDR which I never finished testing,
but that's an easy way to go fast.


I litterature they state that the 2 diode sampler is not optimal, the 4 diode is better. Do you have an alternative 2 diode sampler circuit?

Cheers

Klaus

 

[Piglet]

On 26/04/2019 14:03, klaus.kragelund@gmail.com wrote:

On Friday, 12 April 2019 08:21:31 UTC+2, klaus.k...@gmail.com wrote:
On Wednesday, 10 April 2019 21:13:54 UTC+2, Hul Tytus wrote:
Klaus - take a look at Electronic Design, Sept 18 2000. The article in
"Ideas for Design", "1-Ghz Sampling Oscilloscope Front end is easily
modified" shows a couble diode swithches setup as a sampler. A couple text
quotes if the titles don't work: "adjustable from 1 to 50 ns/div." and
"circuit by switching the two Schottky".

Hul


klaus.kragelund@gmail.com wrote:
On Thursday, 10 January 2008 17:57:44 UTC+1, Jan Panteltje wrote:
J Larkin was saying his super sampling head costs 750$.
I was thinking how can you make one yourself, what else is needed?

So, I am trying to see if we can make a sub ns S/H, for equivalent time sampling of TDR signals

I was looking at this thread, interesting stuff

I only need 8 bit resolution and 500ps. Could that be done with a mux and a small storage cap (I can sample in about 1us with the ADC after the S/H)


That is a very nice reference.

I had some trouble finding it, links were broken on Electronic Design, but found somebody that had saved it as a PDF:

http://www.redrok.com/Circuits_1GHz-samplig-Oscilloscope-Front-End.pdf

Then I found a lot of info on this page:

https://www.eevblog.com/forum/projects/diy-ghz-sampling-head-for-lt100mhz-scopes/

Misc info here from another guy:

https://www.dropbox.com/sh/kmdbbjrzofdxmkb/AADKVlHctfqbE8CNwntixXnza?dl=0%27


A 150ps pulse widht circuit, extremely simple, for TDR generation and sampling:

https://www.eevblog.com/forum/projects/pulse-generator-150ps-1vpp/

Explanation here:

https://www.adv-radio-sci.net/2/7/2004/ars-2-7-2004.pdf


Cheers

Klaus

So, I am plying a little with the sampler from this article i Pspice:

http://www.redrok.com/Circuits_1GHz-samplig-Oscilloscope-Front-End.pdf

It can track a signal resonable, but when the 1ns comparator turns the sample head on (first quad diodes), som charge spills through giving an incorrect amplitude

It's worse when the sampler turns off, on a 1V signal, the amplitude is 100mV off. I am using a small capacitor of 20pF to hold the signal, but the charge transfer from the 2pF shunt capacitance of the diode versus this 20pF cap is too high

If I increase the cap, it's nowhere near a 1GHz sampler, if I decrease it, it has very low hold time

I do not have the 15MOhm/1MOhm - 1pF/15pF (internal cap of TL082) combination, but perhaps I should??

Cheers

Klaus

Writing as an idle idiot bystander that has never made a sampler but am
interested. Just wondering if it could be a balance issue - like do you
need to trim one side of the bridge drive resistors 150 bzw 1k to null
imbalances?

piglet

 

[John Larkin]

On Fri, 26 Apr 2019 08:17:43 -0700 (PDT), klaus.kragelund@gmail.com
wrote:

On Friday, 26 April 2019 16:20:15 UTC+2, John Larkin wrote:
On Fri, 26 Apr 2019 06:03:20 -0700 (PDT), klaus.kragelund@gmail.com
wrote:

On Friday, 12 April 2019 08:21:31 UTC+2, klaus.k...@gmail.com wrote:
On Wednesday, 10 April 2019 21:13:54 UTC+2, Hul Tytus wrote:
Klaus - take a look at Electronic Design, Sept 18 2000. The article in
"Ideas for Design", "1-Ghz Sampling Oscilloscope Front end is easily
modified" shows a couble diode swithches setup as a sampler. A couple text
quotes if the titles don't work: "adjustable from 1 to 50 ns/div." and
"circuit by switching the two Schottky".

Hul


klaus.kragelund@gmail.com wrote:
On Thursday, 10 January 2008 17:57:44 UTC+1, Jan Panteltje wrote:
J Larkin was saying his super sampling head costs 750$.
I was thinking how can you make one yourself, what else is needed?

So, I am trying to see if we can make a sub ns S/H, for equivalent time sampling of TDR signals

I was looking at this thread, interesting stuff

I only need 8 bit resolution and 500ps. Could that be done with a mux and a small storage cap (I can sample in about 1us with the ADC after the S/H)


That is a very nice reference.

I had some trouble finding it, links were broken on Electronic Design, but found somebody that had saved it as a PDF:

http://www.redrok.com/Circuits_1GHz-samplig-Oscilloscope-Front-End.pdf

Then I found a lot of info on this page:

https://www.eevblog.com/forum/projects/diy-ghz-sampling-head-for-lt100mhz-scopes/

Misc info here from another guy:

https://www.dropbox.com/sh/kmdbbjrzofdxmkb/AADKVlHctfqbE8CNwntixXnza?dl=0%27


A 150ps pulse widht circuit, extremely simple, for TDR generation and sampling:

https://www.eevblog.com/forum/projects/pulse-generator-150ps-1vpp/

Explanation here:

https://www.adv-radio-sci.net/2/7/2004/ars-2-7-2004.pdf


Cheers

Klaus

So, I am plying a little with the sampler from this article i Pspice:

http://www.redrok.com/Circuits_1GHz-samplig-Oscilloscope-Front-End.pdf

It can track a signal resonable, but when the 1ns comparator turns the sample head on (first quad diodes), som charge spills through giving an incorrect amplitude

It's worse when the sampler turns off, on a 1V signal, the amplitude is 100mV off. I am using a small capacitor of 20pF to hold the signal, but the charge transfer from the 2pF shunt capacitance of the diode versus this 20pF cap is too high

If I increase the cap, it's nowhere near a 1GHz sampler, if I decrease it, it has very low hold time

I do not have the 15MOhm/1MOhm - 1pF/15pF (internal cap of TL082) combination, but perhaps I should??

Cheers

Klaus

That's a horrible circuit, but it would be improved with better
sampling diodes. There are 0.25 pF diodes around, like SMS7621 or
BAT15.

I am sorry, my brain is slow today, that should have been the first thing I looked at. For once I have an application that allows for more expensive components

Classic samplers had blowby compensation to cancel signals leaking
through the diode off capacitance.

I never has much luck with full-bridge samplers. Half-bridge worked
for me, whit short impulses as the sampling gate. I did a
theroretically 40 ps 1bit sampler/TDR which I never finished testing,
but that's an easy way to go fast.


I litterature they state that the 2 diode sampler is not optimal, the 4 diode is better. Do you have an alternative 2 diode sampler circuit?

Cheers

Klaus

In classic times, only slow (1 GHz range) samplers were full-bridge,
and faster stuff was 2-diode. The bridge samplers were usually pulsed
by an avalanche transistor or a step-recovery diode, usually through a
balun.

A 4-diode sampler would be OK, but the sampler time constant, using a
rectangular gate, limits bandwidth. A short-impulse sampler is fast,
but gain ("Sampling efficiency") is low so usually has an integrator
and bias feedback loop.

Gotta get going now. I'll see if I have stuff later.

The old Tek sampler manuals are online, and one of the Tek concept
series books is about sampling.

I have a deconvolution program that takes an ugly sampler and makes it
into a pretty one. It could increase the bandwidth of a slow-gate
sampler but wouldn't fix blowby.

https://www.dropbox.com/s/kgr784ls963uun2/TDR_Decon_demo.jpg?dl=0




--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Cursitor Doom]

On Fri, 26 Apr 2019 17:37:39 +0100, Piglet wrote:

> Writing as an idle idiot bystander

No one in their right mind could ever accuse you of all people of being
any such thing, Erich. Don't be so self-deprecatory!



--
This message may be freely reproduced without limit or charge only via
the Usenet protocol. Reproduction in whole or part through other
protocols, whether for profit or not, is conditional upon a charge of
GBP10.00 per reproduction. Publication in this manner via non-Usenet
protocols constitutes acceptance of this condition.

 

[Piglet]

On 26/04/2019 18:04, Cursitor Doom wrote:

On Fri, 26 Apr 2019 17:37:39 +0100, Piglet wrote:

Writing as an idle idiot bystander

No one in their right mind could ever accuse you of all people of being
any such thing, Erich. Don't be so self-deprecatory!



Very kind! Just regretted possible waste of bandwidth on a silly idea.

piglet

 

On Friday, 26 April 2019 18:12:05 UTC+2, John Larkin wrote:

On Fri, 26 Apr 2019 08:17:43 -0700 (PDT), klaus.kragelund@gmail.com
wrote:

On Friday, 26 April 2019 16:20:15 UTC+2, John Larkin wrote:
On Fri, 26 Apr 2019 06:03:20 -0700 (PDT), klaus.kragelund@gmail.com
wrote:

On Friday, 12 April 2019 08:21:31 UTC+2, klaus.k...@gmail.com wrote:
On Wednesday, 10 April 2019 21:13:54 UTC+2, Hul Tytus wrote:
Klaus - take a look at Electronic Design, Sept 18 2000. The article in
"Ideas for Design", "1-Ghz Sampling Oscilloscope Front end is easily
modified" shows a couble diode swithches setup as a sampler. A couple text
quotes if the titles don't work: "adjustable from 1 to 50 ns/div." and
"circuit by switching the two Schottky".

Hul


klaus.kragelund@gmail.com wrote:
On Thursday, 10 January 2008 17:57:44 UTC+1, Jan Panteltje wrote:
J Larkin was saying his super sampling head costs 750$.
I was thinking how can you make one yourself, what else is needed?

So, I am trying to see if we can make a sub ns S/H, for equivalent time sampling of TDR signals

I was looking at this thread, interesting stuff

I only need 8 bit resolution and 500ps. Could that be done with a mux and a small storage cap (I can sample in about 1us with the ADC after the S/H)


That is a very nice reference.

I had some trouble finding it, links were broken on Electronic Design, but found somebody that had saved it as a PDF:

http://www.redrok.com/Circuits_1GHz-samplig-Oscilloscope-Front-End.pdf

Then I found a lot of info on this page:

https://www.eevblog.com/forum/projects/diy-ghz-sampling-head-for-lt100mhz-scopes/

Misc info here from another guy:

https://www.dropbox.com/sh/kmdbbjrzofdxmkb/AADKVlHctfqbE8CNwntixXnza?dl=0%27


A 150ps pulse widht circuit, extremely simple, for TDR generation and sampling:

https://www.eevblog.com/forum/projects/pulse-generator-150ps-1vpp/

Explanation here:

https://www.adv-radio-sci.net/2/7/2004/ars-2-7-2004.pdf


Cheers

Klaus

So, I am plying a little with the sampler from this article i Pspice:

http://www.redrok.com/Circuits_1GHz-samplig-Oscilloscope-Front-End.pdf

It can track a signal resonable, but when the 1ns comparator turns the sample head on (first quad diodes), som charge spills through giving an incorrect amplitude

It's worse when the sampler turns off, on a 1V signal, the amplitude is 100mV off. I am using a small capacitor of 20pF to hold the signal, but the charge transfer from the 2pF shunt capacitance of the diode versus this 20pF cap is too high

If I increase the cap, it's nowhere near a 1GHz sampler, if I decrease it, it has very low hold time

I do not have the 15MOhm/1MOhm - 1pF/15pF (internal cap of TL082) combination, but perhaps I should??

Cheers

Klaus

That's a horrible circuit, but it would be improved with better
sampling diodes. There are 0.25 pF diodes around, like SMS7621 or
BAT15.

I am sorry, my brain is slow today, that should have been the first thing I looked at. For once I have an application that allows for more expensive components

Classic samplers had blowby compensation to cancel signals leaking
through the diode off capacitance.

I never has much luck with full-bridge samplers. Half-bridge worked
for me, whit short impulses as the sampling gate. I did a
theroretically 40 ps 1bit sampler/TDR which I never finished testing,
but that's an easy way to go fast.


I litterature they state that the 2 diode sampler is not optimal, the 4 diode is better. Do you have an alternative 2 diode sampler circuit?

Cheers

Klaus

In classic times, only slow (1 GHz range) samplers were full-bridge,
and faster stuff was 2-diode. The bridge samplers were usually pulsed
by an avalanche transistor or a step-recovery diode, usually through a
balun.

A 4-diode sampler would be OK, but the sampler time constant, using a
rectangular gate, limits bandwidth. A short-impulse sampler is fast,
but gain ("Sampling efficiency") is low so usually has an integrator
and bias feedback loop.

Gotta get going now. I'll see if I have stuff later.

The old Tek sampler manuals are online, and one of the Tek concept
series books is about sampling.

I have a deconvolution program that takes an ugly sampler and makes it
into a pretty one. It could increase the bandwidth of a slow-gate
sampler but wouldn't fix blowby.

https://www.dropbox.com/s/kgr784ls963uun2/TDR_Decon_demo.jpg?dl=0

I compiled the two 4 diodes sampler circuits I tried out (turned out the low capacitance diode was not much better than I had first off)

http://www.electronicsdesign.dk/tmp/Sampler.pdf

First circuit is identical to the one from the link I presented first

It does not match the input voltage (could possible just be calibrated out, allthough calibration from get go is maybe asking for trouble)

Second circuit is an ac coupled version. It does not sample to the correct value at first sample, but since I am doing repetitive sampling (periodic), that is tolerable. It matches the input very well after 3 samples

Cheers

Klaus