Jitter measurement
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Posted by budgie on January 19, 2006, 11:09 pm
Please log in for more thread options Well, it's probably jitter and "wobble" as well .... I have a PLL with a 10MHz xtal oscillator, comparison frequency 10kHz, reference source being a GPS receiver which outputs 10k. This all works very well, but I was hoping to somehow quantify the residual phase variation, and without requiring particularly elaborate gear. I've done visual comparisons on the CRO with the OCXO timebase oscillator in my frequency counter, which showed no discernible wobble in the phase relationship. I've also fed the buffered 10MHz into the counter which resolves to 1Hz (1 sec gate) and the variation in this "sampling" is the 1Hz you'd expect in a sampled stream (10,000,000 or 10,000,001). I've also listened to the 10MHz on a narrowband FM receiver 9comms test set) and can't discern any change in the residual instrument noise when the oscillator is on or off, but that only indicates that the comms test set has a noise floor. I can work out the freq/voltage slope at the varicap and watch the control voltage on a CRO, but that is going to incorporate additional effects into the calculation. Any suggestions welcome. | |||||||||||||||||||||||||||||||
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Posted by John_H on January 20, 2006, 1:09 am
Please log in for more thread options budgie wrote: > source being a GPS receiver which outputs 10k. This all works very well, but I
relationship.
> was hoping to somehow quantify the residual phase variation, and without > requiring particularly elaborate gear. > > I've done visual comparisons on the CRO with the OCXO timebase oscillator in my > frequency counter, which showed no discernible wobble in the phase >
and
> I've also fed the buffered 10MHz into the counter which resolves to 1Hz (1 sec > gate) and the variation in this "sampling" is the 1Hz you'd expect in a sampled > stream (10,000,000 or 10,000,001). > > I've also listened to the 10MHz on a narrowband FM receiver 9comms test set) > can't discern any change in the residual instrument noise when the oscillator
is
> on or off, but that only indicates that the comms test set has a noise floor.
> > I can work out the freq/voltage slope at the varicap and watch the control > voltage on a CRO, but that is going to incorporate additional effects into the > calculation. > > Any suggestions welcome. My favorite technique is to use a digital oscilloscope in X-Y mode. A stable reference that's locked to the signal to be measured gets split and filtered into a sine and cosine with reasonably inexpensive RF components such as those from minicircuits.com (or passives for the lower frequencies). The reference sine and cosine feed 2 channels of the scope in XY mode resulting in a circle. The signal with the jitter goes to the trigger. With one point per trigger, the scope shows a spot, a smear, or a "smile" showing the amount of jitter within one reference cycle (degrees within the 360 degree circle). The scope I used a dozen years ago had a timebase jitter of about 75 ps, limiting the measurement. The requirement to lock on to the incoming signal means a little extra work. A VCXO with a low frequency cutoff tracks the incoming signal below the cutoff but only delivers the VCXO noise above the cutoff. With this reference, you can watch the jitter. Or use a SERDES and an FPGA to take the incoming signal and resolve it down to less than 1ns of time interval resolution per clock. | |||||||||||||||||||||||||||||||
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Posted by Rene Tschaggelar on January 20, 2006, 6:31 am
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budgie wrote: > Well, it's probably jitter and "wobble" as well ....
reference
> > I have a PLL with a 10MHz xtal oscillator, comparison frequency 10kHz, > source being a GPS receiver which outputs 10k. This all works very well, but I
relationship.
> was hoping to somehow quantify the residual phase variation, and without > requiring particularly elaborate gear. > > I've done visual comparisons on the CRO with the OCXO timebase oscillator in my > frequency counter, which showed no discernible wobble in the phase >
and
> I've also fed the buffered 10MHz into the counter which resolves to 1Hz (1 sec > gate) and the variation in this "sampling" is the 1Hz you'd expect in a sampled > stream (10,000,000 or 10,000,001). > > I've also listened to the 10MHz on a narrowband FM receiver 9comms test set) > can't discern any change in the residual instrument noise when the oscillator
is
> on or off, but that only indicates that the comms test set has a noise floor.
> > I can work out the freq/voltage slope at the varicap and watch the control > voltage on a CRO, but that is going to incorporate additional effects into the > calculation. > > Any suggestions welcome. Having a look at the noise of the VCO control voltage may be one way, but if not done carefully may introduce additional jitter. The usual method is to measure the slopes of the carrier. EG have it running on 10MHz and measure from 10Hz to 1MHz beside the carrier. Then you can integrate the slope to get ps_rms-jitter. Rene -- Ing.Buero R.Tschaggelar - http://www.ibrtses.com & commercial newsgroups - http://www.talkto.net | |||||||||||||||||||||||||||||||
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Posted by Ken Smith on January 20, 2006, 10:06 am
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>Well, it's probably jitter and "wobble" as well ....
> >I have a PLL with a 10MHz xtal oscillator, comparison frequency 10kHz, reference >source being a GPS receiver which outputs 10k. This all works very well, but I >was hoping to somehow quantify the residual phase variation, and without >requiring particularly elaborate gear. Can you make a second system? In theory, the 10k from two different GPSes should be the same frequency. The scope display of both outputs should have no slip action so you can blow up the timescale to see the jitter. -- -- kensmith@rahul.net forging knowledge | |||||||||||||||||||||||||||||||
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Posted by budgie on January 21, 2006, 6:24 am
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On Fri, 20 Jan 2006 15:06:53 +0000 (UTC), kensmith@green.rahul.net (Ken Smith) wrote: >>Well, it's probably jitter and "wobble" as well ....
reference
>> >>I have a PLL with a 10MHz xtal oscillator, comparison frequency 10kHz, >>source being a GPS receiver which outputs 10k. This all works very well, but I
>>was hoping to somehow quantify the residual phase variation, and without >>requiring particularly elaborate gear. >
>Can you make a second system? > >In theory, the 10k from two different GPSes should be the same frequency. >The scope display of both outputs should have no slip action so you can >blow up the timescale to see the jitter. What is unknown is how the 10kHz signal is derived. It is possible that the pulse count is 10,000 per sec but the duty cycle varies, or a number of other possibilities.. The data sheet isn't revealing, and the manufacturer doesn't want to know legacy receivers, so there is no detail on the derivation. (Having seen how the 50/60Hz was derived from colour-burst crystals in the old Fairchild 5369 devices, I am leery of making assumptions In that family, M cycles were counted in the high state, and N in the low state, with M<>N. IIRC they also
varied M across one output cycle to achieve "proper" division - the 5369EYRN
produced 50Hz from a 3.579545 crystal by this form of "fudged" non-integer division.) For this reason, I see it as conceivable that two devices may show jitter but still deliver zero slip. That is why I used the OCXO from the frequency counter timebase in a CRO comparison - the jitter or variation in the counter timebase would certainly be independent of the GPS-derived oscillator's variation. That disclosed no discernible perturbation to a slow but steady slip, indicating that neither was particularly bad. What I am trying to achieve is some improvements in the area of the loop filter and the oscillator itself. With a methodology for quantifying the variation, this becomes a more scientific process. | |||||||||||||||||||||||||||||||
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>
> I have a PLL with a 10MHz xtal oscillator, comparison frequency 10kHz,