Author Topic: Generating a <3 pS rise time step  (Read 3797 times)

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Offline rhb

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Generating a <3 pS rise time step
« on: March 13, 2019, 10:47:35 pm »
I have a pair of Leo Bodnar's wonderful BNC pulsers, both the 40 pS step and a 100 pS impulse version.  However, I'm afraid that was a grave mistake as it led to my buying a Tek 11801 and a pair of SD-22 heads.

Rather obviously I cannot test 23 pS rise time heads with any of Leo's pulsers which use a laser diode driver with a 21 pS rise time.  So I am trying to design a faster step.  For a hobbyist a mercury wetted relay, good connectors and Rogers laminate seem the path of least difficulty.  The latter being entirely relative.

Pickering offers SMD mercury wetted reed relays specified with 4 pF between the closed contacts and the coil and were kind enough to send me a simulation which assumed slightly more capacitance across the relay and about the same from the output to ground.  That showed ~3 pS 20% - 80%.  Not as fast as I'd like for testing an SD-32 with 7 pS rise time, but not too bad for the SD-22s.

The minimum sampling rate for the 11801 is 200 Hz which is asking rather a lot of a relay.

Has anyone tried to do this?  Is there an alternate design I should consider?  How do modern laboratories produce sub pS steps?  Literature links especially appreciated.  I have not found google very helpful at exotic subjects like this.
 

Offline David Hess

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Re: Generating a <3 pS rise time step
« Reply #1 on: March 13, 2019, 11:32:14 pm »
Has anyone tried to do this?

Tektronix did it in their 108 and 109 pulse generators.  The relay operating life is limited and the repetition rate is a problem except for real time oscilloscopes.  A sampling oscilloscope will require a delay line which compromises the edge speed or random sampling like the 7T11.

Quote
Is there an alternate design I should consider?

Maybe a non-linear transmission line built inside of a transmission line structure would work?  I do not know if they can get below 10 picoseconds.

Tunnel diodes only got down to 10s of picoseconds as far as i know.
 

Offline rfeecs

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Re: Generating a <3 pS rise time step
« Reply #2 on: March 13, 2019, 11:55:02 pm »
Isn't 3pS about 100GHz bandwidth?

You could search for Nonlinear transmission line pulse generator, NLTL, Shock Line pulse generator.

If you amplify the output with a high slew rate amplifier, you can improve the rise/fall time.

Here is Keysight's "New world standard" sub 7pS pulse generator:
http://literature.cdn.keysight.com/litweb/pdf/5991-0311EN.pdf
« Last Edit: March 13, 2019, 11:57:14 pm by rfeecs »
 

Offline rhb

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Re: Generating a <3 pS rise time step
« Reply #3 on: March 14, 2019, 01:07:27 am »

Tektronix did it in their 108 and 109 pulse generators.  The relay operating life is limited and the repetition rate is a problem except for real time oscilloscopes.  A sampling oscilloscope will require a delay line which compromises the edge speed or random sampling like the 7T11.


Thanks.  Why the delay line?  That adds capacitance.  As I understand the problem, capacitance is the big issue.  Are you referring to the trigger delay?  That might be better handled by a clock chip with sub pS  jitter. I think those are fairly easy to find, at least relative to the raw physics of charging the transmission line capacitance.

Yes, this is insanely high BW.  Failure is the most likely outcome, but it seems worth taking a crack at it.  Learning to use a suitable  PCB design tool is far more trouble than the rest.

I'd never heard of "nonlinear shock line pulse generators" so I should have fun chasing that.
 

Online DaJMasta

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Re: Generating a <3 pS rise time step
« Reply #4 on: March 14, 2019, 04:14:55 am »
That bandwidth is going to be near impossible to achieve, the highest end Keysight UXR uses a pulser like what you're describing for its self calibration, and uses the many fast amplifiers on a hard edge approach.... and it's all custom silicon (and achieves sub 3.5ps rising and falling edges).  You may be able to get into the tens of GHz region with a self designed pulser, but you're going to need faster amplifier processes and smaller physical construction to get anywhere near 100GHz.  You're also going to need a faster rated connector, doing this on a regular 3.5mm is just not going to physically happen.

The UXR calibrator is this one: https://www.keysight.com/en/pd-2949589-pn-N2125A/infiniium-uxr-real-time-oscilloscope-calibration-module-100-mm-80-ghz-and-higher


FWIW, what I think LeCroy was using to demo very fast edges on its 100GHz scope was an optical signal into an extremely fast photodiode integrated into the connector that plugs into the 1mm channel input.
 

Offline Henrik_V

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Re: Generating a <3 pS rise time step
« Reply #5 on: March 14, 2019, 10:50:32 am »
There's a technique to use a photo conductive switch and femto-second laser pulses...
https://www.researchgate.net/publication/231119820_Optoelectronic_measurement_of_the_transfer_function_and_time_response_of_a_70_GHz_sampling_oscilloscope

migth have improved since then..
Greetings from Germany
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Offline rhb

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Re: Generating a <3 pS rise time step
« Reply #6 on: March 14, 2019, 02:08:01 pm »
Both the 108 and the 109 are much slower than Leo's unit.  However, they are a good reference for the design concept. I've got a description of the switch and transmission line concept somewhere, but don't recall what book it is in.

The SD-30/32 input is 2.4 mm.  So I have to live within the constraints of that connector. Ultimately, the capacitance of the mated connectors and the 50 ohm load determines the minimum rise time possible.

The photodiode and femto second laser pulse reminds me of how to make a small fortune on Wall Street. Start with a large fortune.

I just found Michael G. Case's 1993 UCSB dissertation on NLTLs.  That looks *very* promising.

Getting a researchgate account is proving rather irritating.  Big oil doesn't let contract scientists publish much.  It's  actually fairly painful to get permission on anything really interesting even for regular staff.
« Last Edit: March 14, 2019, 03:06:31 pm by rhb »
 

Offline David Hess

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Re: Generating a <3 pS rise time step
« Reply #7 on: March 15, 2019, 02:52:18 am »
Thanks.  Why the delay line?  That adds capacitance.  As I understand the problem, capacitance is the big issue.  Are you referring to the trigger delay?

The delay line is a transmission line and splitter.  The problem is that 50 nanoseconds of coaxial transmission line by itself limits bandwidth.

Sequential sampling oscilloscopes require a pretrigger which for the 11801 is 47.5 nanoseconds.  The Tektronix DL-11 delay line for the 11k series limits bandwidth to 5 GHz.  Some of the mainframes have the DL-11 built in.  Check out the Tektronix 113 for the old school way.

Quote
That might be better handled by a clock chip with sub pS  jitter. I think those are fairly easy to find, at least relative to the raw physics of charging the transmission line capacitance.

The main signal is the part which must be delayed.  If your clock chip provides a 5 picosecond edge on its output, then this whole discussion is irrelevant.

The problem with the mercury relay based pulse generator is that the delay must be generated after the pulse is generated.  Triggered pulse generators including tunnel diode, avalanche, and step-recovery can generate the pretrigger pulse before the pulse is produced.
 

Offline FriedLogic

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Re: Generating a <3 pS rise time step
« Reply #8 on: March 15, 2019, 10:50:30 pm »
Pickering offers SMD mercury wetted reed relays specified with 4 pF between the closed contacts and the coil and were kind enough to send me a simulation which assumed slightly more capacitance across the relay and about the same from the output to ground.  That showed ~3 pS 20% - 80%.
Is that the rise time across the relay, or just the time taken for the contact to be made?
 

Offline rhb

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Re: Generating a <3 pS rise time step
« Reply #9 on: March 15, 2019, 11:43:34 pm »
It's a SPICE simulation of the rise time.  The capacitances are a guesstimate.  I've attached the figure Pickering sent me

I found a 1993 dissertation at UC Santa Barbara by Michael Case on non-linear transmission lines which looks very interesting.

https://www.ece.ucsb.edu/Faculty/rodwell/publications_and_presentations/theses/theses.html

Bottom of page.

It's more applicable to ICs than discretes,  but a stripline on Rogers laminate and some diodes is cheap relative to a 2.4 mm connector.
 

Offline JohnG

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Re: Generating a <3 pS rise time step
« Reply #10 on: March 16, 2019, 03:38:21 am »
I highly suggest adding some inductance to the simulation and see what happens. I would suggest sticking adding some where yo think there might be interfaces, let's say 50 pH and 100 pH to start. This might the inductance of a solder bump for a die attach or similar.

John
 

Offline T3sl4co1l

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Re: Generating a <3 ps rise time step
« Reply #11 on: March 16, 2019, 04:31:39 am »
Note that 3ps requires a lumped equivalent of say 75pH and 60fF (for 50 ohms), and stub lengths much less than 1mm.

This rules out all but the tiniest diodes (single dice in a hybrid assembly?), and even rules out a lot of transmission lines, since the width and height of the TL are similarly limited, lest TE/TM modes take over and introduce weird dispersion.  Needless to say, as the cross section of the TL goes down, the losses go way up, and so the length must be that much smaller to maintain sharpness.

This requires monolithic construction.  There is actually nothing you can do with conventional materials (PCBs and SMT components) or a normally-equipped machine shop that will succeed here.

The next best thing seems to be bulk physics -- optics and lasers -- as has been mentioned.  Not really easier, but stands a chance of doing it with off the shelf components, materials and a machine shop.

Tim
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Online DaJMasta

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Re: Generating a <3 pS rise time step
« Reply #12 on: March 16, 2019, 04:36:14 am »
Yes, I would agree that some key parasitics are missing.  There is a reason this isn't an every day thing even in test equipment: that it's REALLY difficult to get this fast based on the parasitics of even very high frequency connectors.  As has been mentioned a few times now, <3ps rise time or fall time is not going to happen with hand assembly of discrete parts, the parasitics of bond wires can even be too far out of tolerance at this sort of level.


Going for fast is great and a worthy target, <3ps is not realistic.
 

Offline rhb

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Re: Generating a <3 pS rise time step
« Reply #13 on: March 16, 2019, 03:37:10 pm »
I verified last night that a mercury wetted relay is not viable for a sampling scope because of the length of the required delay line (~50 ft).

This is a best effort project.  If I can compress the 100 pS pulse from Leo's unit to even 10-20 pS it will be better than what I have now.  <3 pS is a goal, not an expectation.

I'd be much more interested in suggestions for diodes.  For example, might a microwave transistor have lower junction capacitance than readily available diodes?

I can get the required 50 ohm impedance by simply scaling the diameter of the wire to the height of  whatever device I use.  Spacing is more problematic, but it's also periodic.  I still don't know the mathematical details.

Unfortunately, my printer appears to be having trouble with some pages of the dissertation.  At least that's the most common problem, but it is fairly old.  So it may be that it is dying.
 

Online Marco

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Re: Generating a <3 pS rise time step
« Reply #14 on: March 16, 2019, 04:01:06 pm »
As I've said before in threads like this, the fastest edge I've seen on this forum is here. It's the falling edge though, not the rising one. I suspect the differentiator causes the relatively slow rising edge, so a step without the differentiator might retain the speed of the falling edge.

More information about the principle.
« Last Edit: March 16, 2019, 04:33:15 pm by Marco »
 

Offline rhb

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Re: Generating a <3 pS rise time step
« Reply #15 on: March 16, 2019, 04:29:06 pm »
I have two of Leo's units.  The square wave version has a measured rise time of 36 pS.  The impulse version has measured rise time of 48 pS and a duration of 100 pS.

That's significantly better than the other units.  Leo's units may well be the physical limit.  They are extremely good.  I'm likely to get a 3.5 or 2.4 mm unit when they become available again.

The logical next step is some basic calculations followed by a simulation using openCEM if the hand calculations don't kill the idea.

Ex cathedra pronouncements have no value at all.  Mathematical physics calculations are the only authority I  recognize.  And then only after I've reviewed them.
 

Online Marco

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Re: Generating a <3 pS rise time step
« Reply #16 on: March 16, 2019, 04:34:50 pm »
Oops, linked the wrong thread. This is the one.

Measurements trump math, he copy pasted a result from his  LeCroy WAVEMASTER 8620A ... it is objectively the fastest edge on this forum AFAICS. Substantially faster than 36 ps.
 

Offline rhb

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Re: Generating a <3 pS rise time step
« Reply #17 on: March 16, 2019, 05:31:45 pm »
I already read that thread.  I see no justification for saying that unit is faster.  The fall time might be, but the rise time is not and the pulse is wider.

I'm not aware of a thread that discusses an attempt to produce NLTL pulse compression using discrete parts.  I had never heard of NLTL when I started the thread, but I suspected someone on the forum would toss a stone in the right direction.  So this thread has morphed into attempting to build an effective  NLTL compressor for use with Leo's pulsers.  Once I have my 11801 working I'll be able to proceed.

Measurements trump math only if they are correctly made.  I've got measurements made by Leo for my units done with a CSA803  which, based on Leo's comments, were done using an SD-30 (40 GHz) head.

Leo is using a Maxim LED driver with a 21 pS rise time.  The main limitation on my units is the BNC connector.

I bought an 11801 because I thought Leo's measurements were so cool.  Now having acquired a system which outstrips my current verification capability I'm looking to improve that.  It's a vicious cycle.  There's a very long forum thread on that topic.
« Last Edit: March 16, 2019, 05:39:22 pm by rhb »
 

Online Marco

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Re: Generating a <3 pS rise time step
« Reply #18 on: March 16, 2019, 05:54:58 pm »
I already read that thread.  I see no justification for saying that unit is faster.  The fall time might be, but the rise time is not and the pulse is wider.
The topic asked for a step, not a pulse.
Quote
I'm not aware of a thread that discusses an attempt to produce NLTL pulse compression using discrete parts.
I haven't seen anyone DIY one for the hell of it, but here's an academic description of one (they call it hybrid, but they just use reflow soldered SMD components, so I don't see what's hybrid about it). They don't measure the rise time though.

PS. if you have a spare ~3K$ there's the Macom MLPNC-7103 :) This is a module for a modular microwave development system, but I doubt the Macom unit on its own is much cheaper.
« Last Edit: March 16, 2019, 06:06:46 pm by Marco »
 

Offline rhb

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Re: Generating a <3 pS rise time step
« Reply #19 on: March 16, 2019, 07:28:54 pm »
I'd have posted the step response, but I don't know where it is.  The thread you linked showed a spike and Case's dissertation is focuses on spike compression.  I used a step in the subject line because I'd never heard of NLTL pulse shaping.  I like spikes better as the math is easier, but I'll take whatever I can get.

Interesting paper.  Thanks for the link.  Looking at their figures it appears they were getting around 50-75 pS rise times.

Keysight sells a 3 pS rise time unit.  I haven't bothered to look at the price.  I can justify 100 diodes and a 2.4 mm connector.  The latter will take some searching to get for under $100.

Edit:  I've added the step response for my square wave unit.
« Last Edit: March 16, 2019, 08:52:36 pm by rhb »
 

Offline rs20

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Re: Generating a <3 pS rise time step
« Reply #20 on: March 16, 2019, 09:30:35 pm »
I know that in this thread it's obvious from context, but that's no excuse to use poor terminology. It's a bad habit. Capital S is Siemens (conductance), small s is seconds (time). You want a <3 ps rise time step, not <3 pS.
 
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Offline David Hess

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Re: Generating a <3 ps rise time step
« Reply #21 on: March 16, 2019, 10:53:45 pm »
This rules out all but the tiniest diodes (single dice in a hybrid assembly?), and even rules out a lot of transmission lines, since the width and height of the TL are similarly limited, lest TE/TM modes take over and introduce weird dispersion.  Needless to say, as the cross section of the TL goes down, the losses go way up, and so the length must be that much smaller to maintain sharpness.

This is what limited the performance of Tektronix's early sampling heads.  The size of the GR-874 connectors allowed other propagation modes but nobody had an instrument fast enough to measure or confirm it initially.

Quote
This requires monolithic construction.  There is actually nothing you can do with conventional materials (PCBs and SMT components) or a normally-equipped machine shop that will succeed here.

On a larger scale it requires transmission line construction for the entire switching assembly which suggests placing a mercury wetted reed relay inside a coaxial transmission line or coplanar waveguide such that the impedance is maintained through the entire structure.  But the reed relay's glass envelope alters the dielectric constant so that much be taken into account and as you point out, the physical size will allow other propagation modes limiting performance above a certain frequency.  I assume that is what limits performance of the Tektronix 284 pulse generator to 70 picoseconds even with the tunnel diode embedded in the coaxial transmission line.

The S-2 sampling head and Tektronix 284 with GR-874 sized transmission lines are both limited to 75 or 70 picoseconds respectively.  I suspect changing the connector on the S-2 would improve its performance to at least 8GHz.

Like you say, that rules out such a fast pulse generator using commonly available construction techniques unless you can build it into the rigid cable used with 3.5mm or smaller RF connectors.  The NLTL assemblies I have seen for sharpening an existing pulse used MMICs and hybrid construction.

This is a best effort project.  If I can compress the 100 pS pulse from Leo's unit to even 10-20 pS it will be better than what I have now.  <3 pS is a goal, not an expectation.

I'd be much more interested in suggestions for diodes.  For example, might a microwave transistor have lower junction capacitance than readily available diodes?

Look for a suitable varactor or step-recovery diode which are essentially the same thing intended and specified for different applications.  I have been told than some bipolar RF transistor junctions are also very good for this but I think you are going to have to qualify any part you want to use.

 

Offline David Hess

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Re: Generating a <3 pS rise time step
« Reply #22 on: March 16, 2019, 10:56:14 pm »
I know that in this thread it's obvious from context, but that's no excuse to use poor terminology. It's a bad habit. Capital S is Siemens (conductance), small s is seconds (time). You want a <3 ps rise time step, not <3 pS.

If only there had been a previous unit of conductance with the same dimensions which could not be confused with seconds or samples.
 

Offline rhb

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Re: Generating a <3 pS rise time step
« Reply #23 on: March 17, 2019, 12:07:45 am »
I think the point about pS vs ps perfectly valid even if there was no ambiguity.

The paper that Marco linked used a microwave varactor.  Hopefully one that is not too expensive.

For an initial test I'm going to use Leo's pulsers as the source and focus on constructing the NLTL dead bug style over a ground plane.  The varactors will be tombstoned and a wire of the proper dimension to produce 50 ohm impedance for the height of the varactors running between a pair of SMA connectors soldered to the tops of the varactors.  That is tractable with manual calculations and perhaps some manual wire drawing.

So assuming at least 1 channel on my two eBay SD-22s works, I should be able to see an effect.  If it does work, I'll switch to using the same LED driver that Leo is using to eliminate the ~15 ps reduction in the rise time introduced by the BNC and buy 3.5 and 2.4 mm connectors to replace the SMAs.

As David pointed out earlier, a mercury relay is not usable at these speeds.  I had not been aware of the length of the trigger delay required.

I finally manage to get Michael Case's dissertation printed, so I'll have a better grasp of the mathematics in a few days.  My chief concern at this point is the horizontal spacing of the diodes.

All of this is Leo's fault.  He sells these absolutely wonderful fast edge pulsers at prices one can justify for a hobby.  He even supplies plots for the particular unit you receive made on his sampling scope. 

I got sucked into wanting a sampling scope and now that I have one, I want a signal source capable of verifying its performance.  With the rise times of the SD-30 (9 ps) and the SD-32 (7 ps) I need something faster to quantify their performance.  The SD-22 and SD-24 are slower, so if I can't beat the SD-24 step I'll stop and not buy an SD-30 or SD-32.

I'd much rather have access to all the data at a supermajor oil company, but there's no way I can get that.  So I'm playing with electronics.
 
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Offline David Hess

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Re: Generating a <3 pS rise time step
« Reply #24 on: March 17, 2019, 12:19:30 am »
I think the point about pS vs ps perfectly valid even if there was no ambiguity.

Oh, I agree.  I just hate the SI unit for conductance because it replaced a better unit.

Quote
I got sucked into wanting a sampling scope and now that I have one, I want a signal source capable of verifying its performance.  With the rise times of the SD-30 (9 ps) and the SD-32 (7 ps) I need something faster to quantify their performance.  The SD-22 and SD-24 are slower, so if I can't beat the SD-24 step I'll stop and not buy an SD-30 or SD-32.

Tektronix made a calibration generator for these oscilloscopes which was built into some of the mainframes.  You might be able to find a junked one which has the calibration generator.
 
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