Forums

1ns max jitter oscillator, cheap - for fast 4 diode sampler

Started by Unknown May 7, 2019
On Tue, 7 May 2019 12:42:22 -0700 (PDT), klaus.kragelund@gmail.com
wrote:

>On Tuesday, 7 May 2019 17:18:48 UTC+2, John Larkin wrote: >> On Tue, 7 May 2019 07:14:33 -0700 (PDT), klaus.kragelund@gmail.com >> wrote: >> >> >Hi >> > >> >I'm working on my ~3ns 4 diode sampler (preferable 1ns if possible) >> > >> >So I need a pretty good oscillator, with low jitter >> > >> >I have never needed a good oscillator before, so on this topic I am totally at square one >> > >> >First I was thinking about an RC oscillator, and cleaning up the jitter. RC typically have 1us of jitter (found info on the web), and a crystal oscillator, standard type probably 1ns jitter. But I think that idea was crazy, a PLL clean up, would not work I guess. >> > >> >In order to not mess up my measurement and keep the averaging low (I could do many samples and average), I would guess I need jitter of 300ps (10%) of my 3ns reolution) >> > >> >But jitter is not listed as a search parameter. So where to start? (with low price in mind) >> > >> >Cheers >> > >> >Klaus >> >> Do you want a continuous running oscillator, namely a crystal >> oscillator? That works if the measured event and the sampler timebase >> can run off the same clock. Even cheap XOs have picosecond or >> sub-picosecond jitter measured over short time spans. Longer spans are >> trashed by low frequency phase noise, numbers in the nanoseconds per >> second for cheap XOs, picoseconds per second for good OCXOs. >> >That is a very good point, great catch. > >I will be using it in a TDR, so short pulse, and build up waveform for reflected pulse. Since I need up to 200m lenth, the maximum time from the emitted pulse to reflected is 3us. So if the jitter is slowly changing over time, it may be a lot less in only that time span. >
The simplest timebase is a linear RC ramp and a comparator and a DAC, no clock at all. RMS jitter of 1 part in 20,000 isn't difficult, 1:50000 is challenging. So 3 us/20000 would be 150 ps RMS jitter, which is probably OK. The echo from 200m of coax will be very soft, and you can average to reduce displayed jitter. Cheat a little. You can switch the ramp capacitor or charging current to have a couple of different delay ranges, and get less jitter on the short range. A TDR can use the same clock for the launch pulse as for counting coarse timebase delay, so an XO for coarse counts and a vernier ramp for fine delays could hugely reduce sampling jitter. Like say, a 50 MHz clock followed by a 20 ns analog ramp.
>I do not know the properties of crystal jitter. Would that be sinusoidal shaped? > >> Most XOs now have a jitter spec on their data sheet. Some spec >> femtosecond period jitter. >> > >I looked at Digikey. The cheapest XO (about 0.5 USD) has 3ps jitter: > >https://www.sitime.com/datasheet/SiT8008 > >A lot better than what I need. > >When looking at oscillators, the cheapest (0.4 USD) also has only 3ps: > >https://www.sitime.com/datasheet/SiT2001 > >For crystals, I see no spec of jitter: > >https://abracon.com/Resonators/abls.pdf
Oscillators are so cheap, there's no point in buying bare crystals and trying to make them oscillate.
> >But, I guess that is because that makes no sense if the inverter used for the crystal is defined. For microcontrollers I never see a spec for the jitter, maybe it is horrendous > >I have seen jitter defined for the PLL. For example for a ST controller: > >https://www.st.com/resource/en/datasheet/stm32g071cb.pdf > >Page 76, defines 40ps jitter. Cannot see if that is from RC or crystal clock. But is most likely crystal clock. So it seems, I can use a cheap crystal for the microontroller and get a sufficient low jitter figure > >> Sampling oscilloscopes typically need async triggered timebase >> oscillators, which are more difficult. Jitters like 1 part in 50,000 >> (jitter 20 PPM RMS times timed delay) are more common for a triggered >> LC, like on an 11801. 1 part per million is possible; I'm doing that >> now. >> >> A triggered oscillator can be phase locked to a good XO while >> preserving the trigger alignment. >> >> >Thanks for the very good info > >Regards > >Klaus
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On Tue, 7 May 2019 15:39:04 -0400, bitrex <user@example.net> wrote:

>On 5/7/19 3:08 PM, Tom Gardner wrote: >> On 07/05/19 18:20, Cursitor Doom wrote: >>> On Tue, 07 May 2019 07:14:33 -0700, klaus.kragelund wrote: >>> >>>> Hi >>>> >>>> I'm working on my ~3ns 4 diode sampler (preferable 1ns if possible) >>> >>> I know I'll appear a dinosaur by saying this, but you really can't beat a >>> good old fashioned Wien Bridge oscillator when it comes to spectral >>> purity and low phase noise. They certainly beat the crap out of any >>> digital synthesis technique IMV. >> >> No, but that statement is about as sensible as almost >> all your statements. > >He's right about the spectral purity and the phase noise can be cleaned >up by injection-locking it.
A sampler time base needs to stay phase coherent to a trigger. Injection locking whacks randomly the phase. We care about time, not frequency. It is possible to build an instant-start LC oscillator, and phase-lock it to a low phase noise XO, and preserve the original trigger timing with picosecond precision, but I can't tell how. But Klaus can do a totally synchronous system, for TDR, so doesn't need a triggered oscillator. Could do a simple all analog ramp for the timebase. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On Tue, 7 May 2019 15:37:52 -0400, bitrex <user@example.net> wrote:

>On 5/7/19 1:20 PM, Cursitor Doom wrote: >> On Tue, 07 May 2019 07:14:33 -0700, klaus.kragelund wrote: >> >>> Hi >>> >>> I'm working on my ~3ns 4 diode sampler (preferable 1ns if possible) >> >> I know I'll appear a dinosaur by saying this, but you really can't beat a >> good old fashioned Wien Bridge oscillator when it comes to spectral >> purity and low phase noise. They certainly beat the crap out of any >> digital synthesis technique IMV. >> >> >> >> > >In a rare moment of partial agreement with my arch-nemesis "Cursitor >Doom" an injecton-locked Wien bridge oscillator can provide a >near-perfect combination of very low phase noise and very low wideband >noise floor and distortion. And certainly meets the low-price requirement.
If Klaus needs an oscillator at all, he needs many MHz. A $3 quartz crystal oscillator would have picosecond jitter. A Wein bridge wouldn't be practical at that frequency and would have ghastly phase noise. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On Tue, 7 May 2019 12:45:47 -0700 (PDT), klaus.kragelund@gmail.com
wrote:

>On Tuesday, 7 May 2019 19:20:49 UTC+2, Cursitor Doom wrote: >> On Tue, 07 May 2019 07:14:33 -0700, klaus.kragelund wrote: >> >> > Hi >> > >> > I'm working on my ~3ns 4 diode sampler (preferable 1ns if possible) >> >> I know I'll appear a dinosaur by saying this, but you really can't beat a >> good old fashioned Wien Bridge oscillator when it comes to spectral >> purity and low phase noise. They certainly beat the crap out of any >> digital synthesis technique IMV. >> >> > >So I could use a Wien Bridge oscillator, or a cheap colpits?
No, NO! Buy a cheap cmos crystal oscillator with a suitable jitter spec. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On Tuesday, 7 May 2019 22:50:59 UTC+2, John Larkin  wrote:
> On Tue, 7 May 2019 12:42:22 -0700 (PDT), klaus.kragelund@gmail.com > wrote: > > >On Tuesday, 7 May 2019 17:18:48 UTC+2, John Larkin wrote: > >> On Tue, 7 May 2019 07:14:33 -0700 (PDT), klaus.kragelund@gmail.com > >> wrote: > >> > >> >Hi > >> > > >> >I'm working on my ~3ns 4 diode sampler (preferable 1ns if possible) > >> > > >> >So I need a pretty good oscillator, with low jitter > >> > > >> >I have never needed a good oscillator before, so on this topic I am totally at square one > >> > > >> >First I was thinking about an RC oscillator, and cleaning up the jitter. RC typically have 1us of jitter (found info on the web), and a crystal oscillator, standard type probably 1ns jitter. But I think that idea was crazy, a PLL clean up, would not work I guess. > >> > > >> >In order to not mess up my measurement and keep the averaging low (I could do many samples and average), I would guess I need jitter of 300ps (10%) of my 3ns reolution) > >> > > >> >But jitter is not listed as a search parameter. So where to start? (with low price in mind) > >> > > >> >Cheers > >> > > >> >Klaus > >> > >> Do you want a continuous running oscillator, namely a crystal > >> oscillator? That works if the measured event and the sampler timebase > >> can run off the same clock. Even cheap XOs have picosecond or > >> sub-picosecond jitter measured over short time spans. Longer spans are > >> trashed by low frequency phase noise, numbers in the nanoseconds per > >> second for cheap XOs, picoseconds per second for good OCXOs. > >> > >That is a very good point, great catch. > > > >I will be using it in a TDR, so short pulse, and build up waveform for reflected pulse. Since I need up to 200m lenth, the maximum time from the emitted pulse to reflected is 3us. So if the jitter is slowly changing over time, it may be a lot less in only that time span. > > > > The simplest timebase is a linear RC ramp and a comparator and a DAC, > no clock at all. RMS jitter of 1 part in 20,000 isn't difficult, > 1:50000 is challenging. So 3 us/20000 would be 150 ps RMS jitter, > which is probably OK. The echo from 200m of coax will be very soft, > and you can average to reduce displayed jitter. Cheat a little. > > You can switch the ramp capacitor or charging current to have a couple > of different delay ranges, and get less jitter on the short range. > > A TDR can use the same clock for the launch pulse as for counting > coarse timebase delay, so an XO for coarse counts and a vernier ramp > for fine delays could hugely reduce sampling jitter. Like say, a 50 > MHz clock followed by a 20 ns analog ramp. > >
I was actually going the digital way Clean clock to drive the microcontroller that generates the TDR pulse with a HR timer The microcontroller has picosecond timing https://www.st.com/resource/en/datasheet/stm32f334k6.pdf Page 80, 217ps I will let that HR timer trigger the 4 diode sampler, then use slow aquisition to sample and store for later analysis But, your way may be cheaper Cheers Klaus
On Tuesday, 7 May 2019 22:50:59 UTC+2, John Larkin  wrote:
> On Tue, 7 May 2019 12:42:22 -0700 (PDT), klaus.kragelund@gmail.com > wrote: > > >On Tuesday, 7 May 2019 17:18:48 UTC+2, John Larkin wrote: > >> On Tue, 7 May 2019 07:14:33 -0700 (PDT), klaus.kragelund@gmail.com > >> wrote: > >> > >> >Hi > >> > > >> >I'm working on my ~3ns 4 diode sampler (preferable 1ns if possible) > >> > > >> >So I need a pretty good oscillator, with low jitter > >> > > >> >I have never needed a good oscillator before, so on this topic I am totally at square one > >> > > >> >First I was thinking about an RC oscillator, and cleaning up the jitter. RC typically have 1us of jitter (found info on the web), and a crystal oscillator, standard type probably 1ns jitter. But I think that idea was crazy, a PLL clean up, would not work I guess. > >> > > >> >In order to not mess up my measurement and keep the averaging low (I could do many samples and average), I would guess I need jitter of 300ps (10%) of my 3ns reolution) > >> > > >> >But jitter is not listed as a search parameter. So where to start? (with low price in mind) > >> > > >> >Cheers > >> > > >> >Klaus > >> > >> Do you want a continuous running oscillator, namely a crystal > >> oscillator? That works if the measured event and the sampler timebase > >> can run off the same clock. Even cheap XOs have picosecond or > >> sub-picosecond jitter measured over short time spans. Longer spans are > >> trashed by low frequency phase noise, numbers in the nanoseconds per > >> second for cheap XOs, picoseconds per second for good OCXOs. > >> > >That is a very good point, great catch. > > > >I will be using it in a TDR, so short pulse, and build up waveform for reflected pulse. Since I need up to 200m lenth, the maximum time from the emitted pulse to reflected is 3us. So if the jitter is slowly changing over time, it may be a lot less in only that time span. > > > > The simplest timebase is a linear RC ramp and a comparator and a DAC, > no clock at all. RMS jitter of 1 part in 20,000 isn't difficult, > 1:50000 is challenging. So 3 us/20000 would be 150 ps RMS jitter, > which is probably OK. The echo from 200m of coax will be very soft, > and you can average to reduce displayed jitter. Cheat a little.
That would then be a little sensitive to the DAC noise, which would cause jitter directly. But that DAC signal can be heavily filtered I would need a linear ramp then. But in other threads you have shown more or less how that can be done
> > You can switch the ramp capacitor or charging current to have a couple > of different delay ranges, and get less jitter on the short range. > > A TDR can use the same clock for the launch pulse as for counting > coarse timebase delay, so an XO for coarse counts and a vernier ramp > for fine delays could hugely reduce sampling jitter. Like say, a 50 > MHz clock followed by a 20 ns analog ramp. > >
That is sort of how they are doing the ps timebase. They use a standard timer (16 bit or whatever), max clock of 144MHz (7ns resolution). Then they add a delay line, to generate the 217ps smaller intervals Cheers Klaus
On Tuesday, May 7, 2019 at 10:20:49 AM UTC-7, Cursitor Doom wrote:
> On Tue, 07 May 2019 07:14:33 -0700, klaus.kragelund wrote:
> > I'm working on my ~3ns 4 diode sampler (preferable 1ns if possible)
[and want a low-jitter oscillator]
> I know I'll appear a dinosaur by saying this, but you really can't beat a > good old fashioned Wien Bridge oscillator when it comes to spectral > purity and low phase noise. They certainly beat the crap out of any > digital synthesis technique IMV.
The best timing performance requires significant stored energy, if only for Heisenberg uncertainty principles. That means LC beats RC circuitry (the resistors don't store energy, they just waste it). A rock has the full momentum of the standing wave acoustics, so a crystal is better than LC. Short of maser/resonant cavity references, the possibilities are good for plain old wires as delay lines (distributed L, C) also. World-class timing uses superconducting cavities, if that matters.
John Larkin <jjlarkin@highland_snip_technology.com> wrote:

> It is possible to build an instant-start LC oscillator, and phase-lock > it to a low phase noise XO, and preserve the original trigger timing > with picosecond precision, but I can't tell how.
"preserve the original trigger timing" <- compared to what?
On 5/7/19 5:01 PM, John Larkin wrote:
> On Tue, 7 May 2019 15:37:52 -0400, bitrex <user@example.net> wrote: > >> On 5/7/19 1:20 PM, Cursitor Doom wrote: >>> On Tue, 07 May 2019 07:14:33 -0700, klaus.kragelund wrote: >>> >>>> Hi >>>> >>>> I'm working on my ~3ns 4 diode sampler (preferable 1ns if possible) >>> >>> I know I'll appear a dinosaur by saying this, but you really can't beat a >>> good old fashioned Wien Bridge oscillator when it comes to spectral >>> purity and low phase noise. They certainly beat the crap out of any >>> digital synthesis technique IMV. >>> >>> >>> >>> >> >> In a rare moment of partial agreement with my arch-nemesis "Cursitor >> Doom" an injecton-locked Wien bridge oscillator can provide a >> near-perfect combination of very low phase noise and very low wideband >> noise floor and distortion. And certainly meets the low-price requirement. > > If Klaus needs an oscillator at all, he needs many MHz. A $3 quartz > crystal oscillator would have picosecond jitter. A Wein bridge > wouldn't be practical at that frequency and would have ghastly phase > noise. > >
Sometimes folks assume we're all intimately familiar with the requirements of the projects they're working on ("I'm working on my...") sadly I'm not, sure would make my life easier if that crystal ball were up and running. Not even sure if requirement is sine or square. Analog Devices likes the injection-locked Wien bridge for low phase noise, low distortion sines but at say, 10kHz. Down there the performance of that lash-up does look amazing.
On Wednesday, 8 May 2019 00:27:09 UTC+2, bitrex  wrote:
> On 5/7/19 5:01 PM, John Larkin wrote: > > On Tue, 7 May 2019 15:37:52 -0400, bitrex <user@example.net> wrote: > > > >> On 5/7/19 1:20 PM, Cursitor Doom wrote: > >>> On Tue, 07 May 2019 07:14:33 -0700, klaus.kragelund wrote: > >>> > >>>> Hi > >>>> > >>>> I'm working on my ~3ns 4 diode sampler (preferable 1ns if possible) > >>> > >>> I know I'll appear a dinosaur by saying this, but you really can't beat a > >>> good old fashioned Wien Bridge oscillator when it comes to spectral > >>> purity and low phase noise. They certainly beat the crap out of any > >>> digital synthesis technique IMV. > >>> > >>> > >>> > >>> > >> > >> In a rare moment of partial agreement with my arch-nemesis "Cursitor > >> Doom" an injecton-locked Wien bridge oscillator can provide a > >> near-perfect combination of very low phase noise and very low wideband > >> noise floor and distortion. And certainly meets the low-price requirement. > > > > If Klaus needs an oscillator at all, he needs many MHz. A $3 quartz > > crystal oscillator would have picosecond jitter. A Wein bridge > > wouldn't be practical at that frequency and would have ghastly phase > > noise. > > > > > > Sometimes folks assume we're all intimately familiar with the > requirements of the projects they're working on ("I'm working on my...") > sadly I'm not, sure would make my life easier if that crystal ball were > up and running. Not even sure if requirement is sine or square. > > Analog Devices likes the injection-locked Wien bridge for low phase > noise, low distortion sines but at say, 10kHz. Down there the > performance of that lash-up does look amazing.
There's another thread about a 1ns sampling stage which details that I am working on a ns sample hold, that will be used for a cheap TDR function https://groups.google.com/forum/#!topic/sci.electronics.design/Rzeziuv-4q8 Cheers Klaus