### Author Topic: 40 ps Rise Time  (Read 10677 times)

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#### macboy

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##### Re: 40 ps Rise Time
« Reply #25 on: May 09, 2016, 02:12:36 pm »
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In the image you can see that the voltage increases about 200 mV (4 vertical divisions upward at 50mV per division from the 10% line to the 90% line) in about 2 horizontal divisions (each horizontal division being 20 picoseconds) for a total elapsed time of approximately 40 picoseconds.  I don't see any good way to get to a sub picosecond (less than a trillionth of a second) rise time, but it might be possible to squeeze out another 5 - 10 picoseconds with some further studying and fine tuning.
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Pardon me for saying so, but if we can't see the top of the pulse, it isn't a 40 ps rise time, it is "merely" a 5000 V/us slew rate for 40 ps. Not the same. Am I wrong?

#### Marco

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##### Re: 40 ps Rise Time
« Reply #26 on: May 09, 2016, 03:30:22 pm »
I don't see any good way to get to a sub picosecond (less than a trillionth of a second) rise time

Childs play for the optical folks, which can then be converted to fast electrical pulses ... bigger problem is getting it down a coax cable.

#### joeqsmith

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##### Re: 40 ps Rise Time
« Reply #27 on: May 09, 2016, 04:28:38 pm »

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In the image you can see that the voltage increases about 200 mV (4 vertical divisions upward at 50mV per division from the 10% line to the 90% line) in about 2 horizontal divisions (each horizontal division being 20 picoseconds) for a total elapsed time of approximately 40 picoseconds.  I don't see any good way to get to a sub picosecond (less than a trillionth of a second) rise time, but it might be possible to squeeze out another 5 - 10 picoseconds with some further studying and fine tuning.
...
Pardon me for saying so, but if we can't see the top of the pulse, it isn't a 40 ps rise time, it is "merely" a 5000 V/us slew rate for 40 ps. Not the same. Am I wrong?

#### Electro Fan

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##### Re: 40 ps Rise Time
« Reply #28 on: May 09, 2016, 10:56:35 pm »
...
In the image you can see that the voltage increases about 200 mV (4 vertical divisions upward at 50mV per division from the 10% line to the 90% line) in about 2 horizontal divisions (each horizontal division being 20 picoseconds) for a total elapsed time of approximately 40 picoseconds.  I don't see any good way to get to a sub picosecond (less than a trillionth of a second) rise time, but it might be possible to squeeze out another 5 - 10 picoseconds with some further studying and fine tuning.
...
Pardon me for saying so, but if we can't see the top of the pulse, it isn't a 40 ps rise time, it is "merely" a 5000 V/us slew rate for 40 ps. Not the same. Am I wrong?

I'm not quite sure what you are saying but I think it was a request to see the Rise Time event in the context of a longer signal duration.  Attached are a couple images along with the original.  Also attached is an illustration of the waveform being generated by the S-52 Pulse Generator as published in the S-6 Tektronix manual.  The rising edge shown in the scope images is from the first rise on the far left of waveform shown in the illustration (as indicated by the red arrow highlighted in yellow).  Let me know if I'm missing something.  Thx
« Last Edit: May 09, 2016, 11:31:00 pm by Electro Fan »

#### David Hess

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##### Re: 40 ps Rise Time
« Reply #29 on: May 21, 2016, 08:25:59 am »
Ok, thanks for that, I wasn't aware that there was a technical performance limitation, but happy to have learned something new.

This performance limitation has no effect on bandwidth; it only affects timing accuracy and perhaps not even that depending on the implementation.  For sampling oscilloscopes using random equivalent time sampling, the strobe to trigger edge has to be measured and this can be positive or negative; in a digital implementation, this requires *two* time delay counter measurements.  For sequential time sampling, the strobe always occurs after the trigger by a variable delay which is easier to determine accurately.

Bandwidth depends only on construction and the sampling gate strobe width.

Where it gets interesting is that random sampling does not require a bandwidth limiting delay line to view the leading edge while sequential sampling does unless a pretrigger pulse is available.  So in some applications, random sampling provides higher bandwidth but usually at the expense of timing accuracy.  Some sequential sampling oscilloscopes included built in delay lines but this limited their bandwidth to typically 1 or 2 GHz.

#### Earendil

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##### Re: 40 ps Rise Time
« Reply #30 on: May 21, 2016, 12:17:08 pm »
Some of things you say doesn't really make sense to me (I'm not an expert though):
- Shouldn't accuracy determine bandwidth? Because what's the point of having an analog input bandwidth say 10 Ghz when your timebase has only 1ns resolution? With that step size you won't be able to reconstruct Ghz range signals (except in some very special and limited cases).
- In sequential ETS you can set the sampling delay to a large (predetermined) value and then measure its time interval. You can use this to calibrate the timebase. Because you measure long intervals and then divide it down to the step size your measurement can be more precise. I don't think you can do similar calibration in random ETS because you also need to measure the strobe to trigger edge delay as you've mentioned.
- I agree that maximum frequency of the delay lines in those old instruments are a limiting factor. I'm not really sure why this was the case though. You can easily buy 18 Ghz semi-rigid coax delay lines nowadays. Insertion and returns loss of the coax might still be a concern.
- You should be able to measure a repetitive periodic signal's leading edge by phase locking a low phase noise oscillator to the signal and trigger from that. So delay lines are not always necessary.
« Last Edit: May 21, 2016, 03:17:33 pm by Earendil »

#### David Hess

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##### Re: 40 ps Rise Time
« Reply #31 on: May 21, 2016, 05:50:25 pm »
Some of things you say doesn't really make sense to me (I'm not an expert though):
- Shouldn't accuracy determine bandwidth? Because what's the point of having an analog input bandwidth say 10 Ghz when your timebase has only 1ns resolution? With that step size you won't be able to reconstruct Ghz range signals (except in some very special and limited cases).

Usually you would have time resolution and accuracy commensurate with your sampling bandwidth however there are exceptions.  To give an extreme example, a sampling voltmeter like a Racal-Dana 9301 or HP 3406 has no timebase or triggering at all and a bandwidth which only depends on construction and sampling gate pulse width.  These instruments can make RMS voltage measurements beyond 1 GHz.  If I connect one of my RMS voltmeters to the direct output of my sampling oscilloscope, I can do the same thing to 10+ GHz with no triggering.

Another place where timebase accuracy and resolution do not matter is X-Y displays using sampling inputs.

Quote
- In sequential ETS you can set the sampling delay to a large (predetermined) value and then measure its time interval. You can use this to calibrate the timebase. Because you measure long intervals and then divide it down to the step size your measurement can be more precise. I don't think you can do similar calibration in random ETS because you also need to measure the strobe to trigger edge delay as you've mentioned.

While this does not apply to my Tektronix 7T11A, another reason that sequential ETS can be more accurate is that assuming low jitter in the sampling strobe, multiple measurements of the delay can be taken and averaged.  Every random equivalent time sampling measurement is unque and has to be made in a single shot.  On the Tektronix 7T11A, all time measurements are single shot measurements so random ETS is just slightly less accurate than sequential ETS.  Both get down to the 10 picosecond range.

Quote
- I agree that maximum frequency of the delay lines in those old instruments are a limiting factor. I'm not really sure why this was the case though. You can easily buy 18 Ghz semi-rigid coax delay lines nowadays. Insertion and returns loss of the coax might still be a concern.

Dispersion in a transmission line limits rise time.  So for instance a Tektronix 113 delay line which uses 50 feet of 7/8" Spir-o-line (like smooth Heliax), provides a 60 nanosecond delay but limits rise time to about 100 picoseconds.  It also weighs 50 pounds and is the size of a suitcase.

For an extreme example of this, consider old 100 MHz HP analog oscilloscopes where do to age, the shield of the delay line does not make good contact between strands limiting bandwidth to below even 100 MHz.  Flexing the cable solves this.

Quote
- You should be able to measure a repetitive periodic signal's leading edge by phase locking a low phase noise oscillator to the signal and trigger from that. So delay lines are not always necessary.

That is how random equivalent time sampling works except that there is no requirement for low jitter in the source or oscillator.  If a sequential sampling timebase can support enough delay, then it can trigger on one pulse and measure the next but this depends on low jitter in the source.

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#### Earendil

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##### Re: 40 ps Rise Time
« Reply #32 on: May 22, 2016, 12:02:59 am »
Usually you would have time resolution and accuracy commensurate with your sampling bandwidth however there are exceptions.  To give an extreme example, a sampling voltmeter like a Racal-Dana 9301 or HP 3406 has no timebase or triggering at all and a bandwidth which only depends on construction and sampling gate pulse width.  These instruments can make RMS voltage measurements beyond 1 GHz.  If I connect one of my RMS voltmeters to the direct output of my sampling oscilloscope, I can do the same thing to 10+ GHz with no triggering.

Another place where timebase accuracy and resolution do not matter is X-Y displays using sampling inputs.

Very interesting. I need to wrap my head around the operating principles of those units though.
What benefits such instruments have compared to (diode detector based) RF power sensors?

Dispersion in a transmission line limits rise time.  So for instance a Tektronix 113 delay line which uses 50 feet of 7/8" Spir-o-line (like smooth Heliax), provides a 60 nanosecond delay but limits rise time to about 100 picoseconds.  It also weighs 50 pounds and is the size of a suitcase.

For an extreme example of this, consider old 100 MHz HP analog oscilloscopes where do to age, the shield of the delay line does not make good contact between strands limiting bandwidth to below even 100 MHz.  Flexing the cable solves this.
I don't know much about this but I found some references on the net that says that TEM media (like coax) are non-dispersive.
Isn't it possible that the Tektronix 113 delay line you're referring to have non-TEM modes at higher frequencies because of its connector?

Also I'm wondering if measuring the group delay of a coax delay line with a VNA could be used to compensate for this effect using post-processing. However I'm not 100% convinced that this would be really necessary. I have read quite a lot about metrology grade sampling oscilloscope calibrations recently where they compensate for various effects but I haven't seen this mentioned. But maybe this is common knowledge. I don't know.
Could you perhaps give some references?
« Last Edit: May 22, 2016, 12:12:00 am by Earendil »

#### David Hess

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##### Re: 40 ps Rise Time
« Reply #33 on: May 22, 2016, 05:06:49 am »
Usually you would have time resolution and accuracy commensurate with your sampling bandwidth however there are exceptions.  To give an extreme example, a sampling voltmeter like a Racal-Dana 9301 or HP 3406 has no timebase or triggering at all and a bandwidth which only depends on construction and sampling gate pulse width.  These instruments can make RMS voltage measurements beyond 1 GHz.  If I connect one of my RMS voltmeters to the direct output of my sampling oscilloscope, I can do the same thing to 10+ GHz with no triggering.

Another place where timebase accuracy and resolution do not matter is X-Y displays using sampling inputs.

Very interesting. I need to wrap my head around the operating principles of those units though.
What benefits such instruments have compared to (diode detector based) RF power sensors?

Besides wide bandwidth, they have a very predictable albeit non-linear sin(x)/x frequency response.  Averaging, peak detection, or RMS measurement only occurs at low frequencies.

Quote
Dispersion in a transmission line limits rise time.  So for instance a Tektronix 113 delay line which uses 50 feet of 7/8" Spir-o-line (like smooth Heliax), provides a 60 nanosecond delay but limits rise time to about 100 picoseconds.  It also weighs 50 pounds and is the size of a suitcase.

For an extreme example of this, consider old 100 MHz HP analog oscilloscopes where do to age, the shield of the delay line does not make good contact between strands limiting bandwidth to below even 100 MHz.  Flexing the cable solves this.
I don't know much about this but I found some references on the net that says that TEM media (like coax) are non-dispersive.
Isn't it possible that the Tektronix 113 delay line you're referring to have non-TEM modes at higher frequencies because of its connector?

If that was the case, then the length of the coaxial line would not matter.  As a practical manner, GR-874 connectors are good to about 50 picoseconds.

Quote
Also I'm wondering if measuring the group delay of a coax delay line with a VNA could be used to compensate for this effect using post-processing. However I'm not 100% convinced that this would be really necessary. I have read quite a lot about metrology grade sampling oscilloscope calibrations recently where they compensate for various effects but I haven't seen this mentioned. But maybe this is common knowledge. I don't know.
Could you perhaps give some references?

I do not know about the Tektronix 113 delay line, but usually a passive network is included to fix the group delay and frequency dependent attenuation.

Tektronix discussed the problem a little starting on page 183 in 062-1145-00 - Oscilloscope Vertical Amplifiers:

http://www.davmar.org/TE/TekConcepts/TekVertAmpCircuits.pdf

Here is a more technical discussions of the problem.  Who said our nuclear weapon research was not worthwhile?

http://lss.fnal.gov/archive/other/lbl-cc-2-1b.pdf

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#### Howardlong

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##### Re: 40 ps Rise Time
« Reply #34 on: May 22, 2016, 08:30:02 pm »
Regarding delay lines, the 24ns 54008b I use gives me a system rise time of 66ps compared to 33ps without using a 54121a test set. Using a professionally terminated LHR400 of about the same delay I can achieve a 49ps system rise time.

Having said that, the waveform is not very clean: while the 10-90% headline figures sound good, that last 10% takes quite some time after reaching 90% to settle to, say, 98%, of the order of 100ps or so.

#### Earendil

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##### Re: 40 ps Rise Time
« Reply #35 on: May 22, 2016, 10:12:21 pm »
It seems the dispersion is more pronounced in corrugated coax lines.
https://cds.cern.ch/record/1212915/files/th5rfp033.pdf

#### Electro Fan

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##### Re: 40 ps Rise Time
« Reply #36 on: September 13, 2016, 02:29:46 am »
I came across a Tektronix S-4.  As per the specs, it appears to be faster than the S-6.  The S-4 displays a rise time of about 35ps vs. 40ps for the S-6.  I "discovered" that the 7S12 has a button called "High Resolution" (a term that I think was less well known in popular culture in 1969 when the 7S12 manual was first printed).  It looks like in the 7904 with the 7S12 and a S-52 the S-4 is about 5ps faster than the S-6 but not quite hitting the Tektronix specs.  The S-4 is spec'd at 25ps and the S-6 is spec'd at 30ps but my setup probably has some room for error and after 45 years or so things might be prone to slowing a bit.
« Last Edit: September 13, 2016, 03:28:28 am by Electro Fan »

#### David Hess

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##### Re: 40 ps Rise Time
« Reply #37 on: September 13, 2016, 03:15:26 am »
"High Resolution" on the 7S12 does about the same thing as "Smoothing" on the 7S11; adjacent samples are effectively averaged.

The S-4 is higher bandwidth than the S-6 although I am not sure why since the traveling wave gates work the same way.  Maybe Tektronix slowed down the edge of the S-6 sampling strobe for lower noise or aberrations since the extra bandwidth was not needed for TDR applications.
« Last Edit: September 13, 2016, 03:20:28 am by David Hess »

#### Electro Fan

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##### Re: 40 ps Rise Time
« Reply #38 on: February 13, 2020, 09:17:26 pm »
I forgot to turn the Tektronix 7000 off last night and was bummed out that I needlessly left it running - hard to say after decades how long it can go.... knock on wood.

But after it had been well warmed up I noticed that the scope and pulse generator have seemingly set a new personal record for rise time.  Can't quite explain it but it looks like ~16 ps

#### gdewitte

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##### Re: 40 ps Rise Time
« Reply #39 on: February 13, 2020, 09:45:40 pm »
Lots of good reference material at https://kh6htv.com/pspl-app-notes/. AN-02a includes details about S-4 and S-6.

#### David Hess

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##### Re: 40 ps Rise Time
« Reply #40 on: February 14, 2020, 04:37:13 am »
I forgot to turn the Tektronix 7000 off last night and was bummed out that I needlessly left it running - hard to say after decades how long it can go.... knock on wood.

I have accidentally left my 7603 and 7904 on for days at a time; the 7904 has no fan and the fan in the 7603 is silent.

Quote
But after it had been well warmed up I noticed that the scope and pulse generator have seemingly set a new personal record for rise time.  Can't quite explain it but it looks like ~16 ps

It is more likely that something happened to the sampling sweep calibration.

#### Electro Fan

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##### Re: 40 ps Rise Time
« Reply #41 on: February 14, 2020, 11:12:44 pm »
Roger that David - something may have drifted- but it might be something else.

I just turned it on and got the more expected sub 40 ps rather than sub 20 ps rise time.  I should have noticed yesterday where the display showed 10 ps per div.  In the photo below it shows 20 ps as I would normally expect given that the Time div know only goes to 20 ps.

However, within a few minutes of starting to type this I noticed the display is again showing 10 ps per div - so maybe the calibration is ok but something might be happening with the Time div display value circuitry?

#### LazyJack

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##### Re: 40 ps Rise Time
« Reply #42 on: February 15, 2020, 09:08:24 pm »
Check the connector in the horozontal slot. Bad connection may cause such weirdness, as the  readout is an ingenious analog circuit. (RIP Barrie Gilbert.)

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#### David Hess

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##### Re: 40 ps Rise Time
« Reply #43 on: February 16, 2020, 11:15:37 am »
However, within a few minutes of starting to type this I noticed the display is again showing 10 ps per div - so maybe the calibration is ok but something might be happening with the Time div display value circuitry?

That is definitely possible.  Bad solder joints, leakage, drift, and miscalibration can cause the decoded readout to shift in value.  My 7T11A sampling sweep is a little flaky in this regard in one of my mainframes.

#### Howardlong

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##### Re: 40 ps Rise Time
« Reply #44 on: February 16, 2020, 02:35:10 pm »
Check the connector in the horozontal slot. Bad connection may cause such weirdness, as the  readout is an ingenious analog circuit. (RIP Barrie Gilbert.)

Thanks for the Barrie Gilbert reference, I only know him from the Gilbert Cell, which has been such a key invention within modern RF design, particularly ZIF and Low IF. I had not realised how many other things he’d been involved with. RIP.

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#### David Hess

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##### Re: 40 ps Rise Time
« Reply #45 on: February 16, 2020, 04:16:01 pm »
Here is an excellent article about Barrie Gilbert:

http://hephaestusaudio.com/media/2009/06/the-gears-of-genius.pdf

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