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Low-noise oscillator
Low-noise oscillator
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Old 25th November 2002, 07:34 PM   #21
jwb is offline jwb  United States
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My[1] scope can't measure jitter directly either (despite having the "advanced math module"[2]), but there's no shame in the cursor method.[3]

-jwb

1: "My" being the scope I have at the office, not the amazing Tek 465 at home.

2: Advanced math apparently means addition, subtraction, and multiplication.

3: Set the scope to accumulate or high intensity and use the cursors to find the excursion on the time axis.
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Old 25th November 2002, 10:13 PM   #22
Pjotr is offline Pjotr
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Huh, err... And that way you want to measure down to 1 ps ??? Hopefully your scope’s time base has that low jitter too. A better approach would be to build two identical oscillators. Feed them to a mixer and measure the sideband noise with your soundcard and a FFT program. I am just thinking of this. But maybe it is easier to contact some old colleagues from the university to dig up a nice new Tek TDS scope.

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Old 26th November 2002, 07:10 AM   #23
Jocko Homo is offline Jocko Homo  United States
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That will give you a phase noise measurement, which will not be the same as a jitter measurement. Jitter measurements take into account voltage variations from ground bounce, Vcc shift, and stuff like that. Those fancy 'scopes have fancy A/Ds that are supposed to be good for that. But then they can cost $40K. For that price, they ought to be good.

Jocko
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Old 26th November 2002, 10:09 AM   #24
Pjotr is offline Pjotr
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True Jocko,

But about what are we talking here? Jitter of the oscillator or conversion-clock jitter at the DAC?

RMS jitter and sideband noise are closely related if it concerns an oscillator. If you measure the sideband noise you can calculate the RMS value of the jitter. It’s true you can’t measure the pp value of the jitter that way.

Note that if an oscillator is specified for say 5 ps jitter it is most time given in RMS. The pp value can easily be as high as 20 ps to 50 ns.

But why does anyone want to measure the numerical value of the jitter if you can listen to the musical result? Measuring is only handy to see if you make progress on a testbed. Relative measurements will suffice then. If you look at the down converted sideband spectrum, the sideband lobe need to be as narrow as possible and the white noise floor as low as possible. If you want to investigate psu-related noise you need to power each oscillator from different psu’s to avoid correlation of the psu noise.
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Old 26th November 2002, 09:56 PM   #25
Jocko Homo is offline Jocko Homo  United States
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The thermal phase noise of the oscillator, and the jitter introduced by the circuitry that follows, that can degraded its performance.

I prefer the noise spectrum method, but it would be nice to have a fancy 'scope that could give a quick check of the amount of deviation in pS. The noise spectrum will give you more insight, from a design standpoint. Which I would find more interesting.

Jocko
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Old 26th November 2002, 10:05 PM   #26
ALW is offline ALW  United Kingdom
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Default Exotic 'scopes

Unless anyone has a 'scope that is capable of measuring jitter, don't bother.

Triggering the 'scope on the jittered signal gives misleading results, related to the 'scope trigger interval and the transition interval being measured.

The frequency spectrum also has an effect, at some frequencies jitter will not be shown at all, and at others it could appear doubled in amplitude, what one is really displaying is interval variations.

Andy.
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Old 26th November 2002, 10:40 PM   #27
jwb is offline jwb  United States
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Seems like the main trick here is going to be clean power supplies. The oscillator obviously needs one (9V) and the inverter or comparator needs a different one (5V). I guess I'll derive the latter from the former.

Also I suspect that the comparator will have better PSRR than the inverter for turning sine into square. With the inverter, any supply noise is going to translate directly into timing errors on the output. But with the comparator the reference voltage can be set with something quiet (LED, buried zener, etc.).
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Old 2nd December 2002, 06:45 AM   #28
pjkunz is offline pjkunz
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Default ? on BJT's

Hi Jocko,
I've etched a small board for the oscillator and destroyed yet another shirt with ferric chloride stains...(the schematic with the power supply) and am in the processs of stuffing it. But I have a question on BJT's...

I'm using Zetex 694b's and 795a's in place of the 2sc2240's and 2sa970's, these seem to match up pretty well, but what is the reason for using these very high hfe types, just curious.

Also is there anything specific to keep in mind if substituting for the 2sd438's? I have a ton of MPS8099's from a Leach amp project....(yes I've read the new law) I'd be more than happy to use something better, but I'm not sure what I'm looking for in this application?

Thanks again,

Pete Kunz
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Old 2nd December 2002, 08:22 AM   #29
sonnya is offline sonnya  Denmark
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Those Zetex have pretty high cbe... about 10 - 20 times higher than 2sc2240...
I would guess that they would mess up the oscillator circuit Jocko have designed pretty much!!!!

You would have to recalculate c1 and c2

Sonny
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Old 2nd December 2002, 08:38 AM   #30
peranders is offline peranders  Sweden
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Low-noise oscillator
Default Re: ? on BJT's

Quote:
Originally posted by pjkunz
Hi Jocko,
I've etched a small board for the oscillator and destroyed yet another shirt with ferric chloride stains...
Check here for alternative etching method.

http://www.diyaudio.com/forums/showt...highlight=h2o2
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