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

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 oscill ator, 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 l ow price in mind)

Cheers

Klaus

Reply to
klaus.kragelund
Loading thread data ...

On Wednesday, May 8, 2019 at 12:14:39 AM UTC+10, snipped-for-privacy@gmail.com wrote :

ly at square one

RC typically have 1us of jitter (found info on the web), and a crystal osci llator, standard type probably 1ns jitter. But I think that idea was crazy, a PLL clean up, would not work I guess.

d do many samples and average), I would guess I need jitter of 300ps (10%) of my 3ns reolution)

low price in mind)

This isn't a low price solution, but etched crystals were commercially avai lable to up to about 600MHz, and the jitter on their output was less than a picosecond.

Some twenty years ago I was planning on buying in a crystal oscillator that ran at 500MHz with ECL outputs, for about 100 euro.

Today's parts are more widely available and appear to go up to 2.1GHz

formatting link
mm-x/dp/2986239

formatting link

They seem to have gotten cheaper too.

Still not cheap.

--
Bill Sloman, Sydney
Reply to
bill.sloman

A standard XO is nowhere near as bad as 1 ns. That would be half a radian worth at 100 MHz.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
 Click to see the full signature
Reply to
Phil Hobbs

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.

Most XOs now have a jitter spec on their data sheet. Some spec femtosecond period jitter.

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.

--
John Larkin         Highland Technology, Inc 

lunatic fringe electronics
 Click to see the full signature
Reply to
John Larkin

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.

--
This message may be freely reproduced without limit or charge only via  
the Usenet protocol. Reproduction in whole or part through other  
 Click to see the full signature
Reply to
Cursitor Doom

Am 07.05.19 um 19:20 schrieb Cursitor Doom:

ROTFL.

A Q like a wet sand bag.

Reply to
Gerhard Hoffmann

No, but that statement is about as sensible as almost all your statements.

Reply to
Tom Gardner

Yes, I would have gone for a crystal, or at least a high-Q LC oscillator.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

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.

Reply to
bitrex

He's right about the spectral purity and the phase noise can be cleaned up by injection-locking it.

Reply to
bitrex

lly at square one

RC typically have 1us of jitter (found info on the web), and a crystal osc illator, standard type probably 1ns jitter. But I think that idea was crazy , a PLL clean up, would not work I guess.

ld do many samples and average), I would guess I need jitter of 300ps (10%) of my 3ns reolution)

h low price in mind)

That is a very good point, great catch.

I will be using it in a TDR, so short pulse, and build up waveform for refl ected pulse. Since I need up to 200m lenth, the maximum time from the emitt ed 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.

I do not know the properties of crystal jitter. Would that be sinusoidal sh aped?

I looked at Digikey. The cheapest XO (about 0.5 USD) has 3ps jitter:

formatting link

A lot better than what I need.

When looking at oscillators, the cheapest (0.4 USD) also has only 3ps:

formatting link

For crystals, I see no spec of jitter:

formatting link

But, I guess that is because that makes no sense if the inverter used for t he crystal is defined. For microcontrollers I never see a spec for the jitt er, maybe it is horrendous

I have seen jitter defined for the PLL. For example for a ST controller:

formatting link

Page 76, defines 40ps jitter. Cannot see if that is from RC or crystal cloc k. 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

Thanks for the very good info

Regards

Klaus

Reply to
klaus.kragelund

So I could use a Wien Bridge oscillator, or a cheap colpits?

Then use the cheap crystal osc in the microcontroller to compensate the measurements (measure the colpits frequency and correct the numbers)

Cheers

Klaus

Reply to
klaus.kragelund

How do you clean up the oscillator by injection? Got an example?

Cheers

Klaus

Reply to
klaus.kragelund

Hmm, Well I know little of phase noise, but if it's at all related to harmonic distortion... Then I will say I've built a little Wien bridge oscillator (audio) with light bulb AGC (ala Jim Williams, ala Bill Hewlett) And it's slick. The 2nd harmonic is hard to see without fancy kit and the 3rd harmonic is ~70 dB down. Which is better than my Rigol DDS Sig Gen. (about -70 dB 2nd and -60 on 3rd)

I'm not sure my objection has anything to do with ns phase noise.

George H.

Reply to
George Herold

Yeah me too. Curious minds want to know. In my very limited experience, the spectral purity depended on how 'strong' the AGC was.

George H.

Reply to
George Herold

This maybe.

formatting link

GH

Reply to
George Herold

OK last thing. I stuck my Wien bridge on the good SRS 720 spectrum analyzer and the 3rd was ~90 db down. (my above 70 dB number was measuring on a 'scope, not so good.)

GH FFT.)

Reply to
George Herold

While Wien bridge oscillators may have low distortion and therefore good spectral purity, they certainly aren't low noise. I mean, even the frequency-selective part is lossy, dissipative and therefore noisy.

Jitter is the uncertainty in the timing of some level crossing. This uncertainty depends on the noise level and on the rate of change of the signal around that level crossing. To get low jitter, you want the noise to be as low as possible and you want to cross the decision level as fast as possible.

So you want a low-loss resonator, a low noise feedback amplifier, high oscillation amplitude and high frequency. That pretty much rules out a Wien bridge oscillator, or any RC oscillator for that matter.

For timing a fast sampler, jitter performance doesn't need to be stellar. It shouldn't be too hard to get jitter in the few tens of picoseconds ballpark, even with an RC oscillator.

Jeroen Belleman

Reply to
Jeroen Belleman

That's the next thing. I need to find a scope that is good enough to measure the jitter. One of my work colleagues has one.

At home I have a TDS744A, which has 80ps jitter. So I can use that for rough measurements

Reply to
klaus.kragelund

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.

Oscillators are so cheap, there's no point in buying bare crystals and trying to make them oscillate.

--
John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
 Click to see the full signature
Reply to
John Larkin

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.