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Old 16th January 2015, 10:45 AM   #1
udok is offline udok  Austria
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Default Oscillator Phase Noise?

Hi all,

Has anybody build an oscillator phase noise measurement system?

There are some threads here dealing with ultra low jitter oscillator designs and it would be great to see some real measurements.

I have read some basics and basically you need a mixer and a low noise amplifier. The amp output could go into a soundcard input or to a spectrum analyzer.
A precision reference oscillator is not needed if you use two identical test oscillators....

Is anybody interested in building such a thing?

Greetings,
Udo
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Old 16th January 2015, 10:59 AM   #2
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I thought you guys could hear those 2ps of jitter, otherwise what is all the fuss about!

You can use the DAC itself as the jitter measuring device, since it translates time errors into amplitude errors. Play a pure tone through the DAC, and the tone will get sidebands that are copies of the master clock's phase noise spectrum. These can be seen on an audio distortion analyser, or a soundcard (if the soundcard's ADC master clock has lower jitter than the one you are trying to measure, and your FFT-fu is strong)

I think the mixer and low noise amplifier approach places equal demands on the spectrum analyser's phase noise performance to just analysing the output of a DAC, but I could be wrong.
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Old 16th January 2015, 11:14 AM   #3
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Quote:
Originally Posted by scopeboy View Post
Play a pure tone through the DAC.
Low-frequency oscillators are likely to have far higher phase noise than a ~10 MHz crystal oscillator. A direct measurement is thus far more sensitive.

Basically you need to construct a PLL with very low cutoff frequency. Using two fixed-frequency oscillators doesn't work well, as they need to have a steady long-term phase relation in order to make the phase detector work properly. Conceptually easy, but for good results excellent design practice is needed.

In the < 50 MHz range, a XOR gate is pretty good as phase detector. Unfortunately using this may cause the PLL to never lock. A way around this is to add a phase-frequency detector for the PLL, and use the XOR just for the phase noise measurement.

Samuel
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Old 16th January 2015, 11:19 AM   #4
udok is offline udok  Austria
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@scopeboy:
Hey, this is a playing field, i don't have pressure to finish things in time here :-)

And if i build a discrete oscillator, I care about the 2ps, even if it may not be audible (2ps are not very ambitious btw).
I like to build things "perfect", or at least understand how they work. I want to know the limitations and be able to measure the performance. It is not my first goal to fit some time tables or have a finished product.

Otherwise i could buy a finished crystal oscillator circuit with specification... or the finished DAC and integrated amplifier... but this is DIY.
Anybody defines for himself how far he want to go.

The trick with the mixer is that the phase noise is translated to DC (zero frequency), so that the spectrum analyzer phase noise is not important.
The spectrum analyer noise is not important too as the low noise amp (after the mixer) does enough amplification to lever the signal above the spectrum analyzers noise.
More details are here:
Low-Cost Phase Noise Measurement | Wenzel Associates, Inc.
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Old 16th January 2015, 11:23 AM   #5
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Quote:
Originally Posted by Samuel Groner View Post
Low-frequency oscillators are likely to have far higher phase noise than a ~10 MHz crystal oscillator. A direct measurement is thus far more sensitive.
Where is the low frequency oscillator? Any tone you can play through a DAC, you can generate digitally. The only oscillator involved in that process is the DAC's master clock, the very thing we are trying to test.

I guess the issue might be the low frequency reference oscillator to compare the tone against. If you used a notch-type THD analyser, again there is no oscillator involved. But maybe the notch filter in that has some sort of noise analogous to phase noise in an oscillator?

Quote:
Basically you need to construct a PLL with very low cutoff frequency.
The problem with this is finding a VCO for your PLL with audiophile grade jitter specs. The lowest jitter VCO I can think of would be something like a Tent Labs VCXO. You could lock this to incoming MCLK from a SPDIF receiver, and get a very good readout of the link jitter from an XOR gate. But could you measure anything useful by locking it to another crystal oscillator?
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Old 16th January 2015, 11:31 AM   #6
udok is offline udok  Austria
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Can you please elaborate the idea with the phase-frequency detector in more detail?
A picture would be of great help.

As the two oscillators are crystal stabilized, the PLL does not need a big pulling range (+-100 ppm should suffer).
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Old 16th January 2015, 11:39 AM   #7
udok is offline udok  Austria
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What is the Tent VCXO phase noise specification?

A good reference is the Morion MV89. It is affordable and has very good specifications which is verified independently.
(-145dBc@100Hz http://www.morion.com.ru/catalog_pdf/MV89-OCXO.pdf)

But the trick with the PLL method is that you can use two test oscillators for the measurement (one must be pullable).
You do not need an expensive reference for the measurement. If the two oscillators are identical, your result is 3 dB too high.
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Old 16th January 2015, 11:50 AM   #8
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Tent specify "less than 3ps RMS (3 sigma)". VCXO

Other VCXOs are available, but Tent are the only company I know who stock them in the usual digital audio master clock frequencies.

Quote:
But the trick with the PLL method is that you can use two test oscillators for the measurement (one must be pullable).
You do not need an expensive reference for the measurement. If the two oscillators are identical, your result is 3 dB too high.
A pullable oscillator will always have worse jitter and phase noise than an ordinary XO. The wider the pulling range, the worse the phase noise. If I am reading the Morion spec right, you are paying for the extraordinary phase noise with a pulling range of only 0.25ppm. Not to mention a sine wave output that will need squared before it could drive a DAC. At the levels we are discussing, the noise floor of the squaring circuit would probably dominate the jitter performance. (the squaring operation transforms amplitude noise into phase noise)
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Last edited by scopeboy; 16th January 2015 at 11:55 AM.
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Old 16th January 2015, 11:57 AM   #9
dmills is offline dmills  United Kingdom
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Look at the amazing work Martein Baker PA3AKE did on measuring phase noise for his high performance receiver design.

The quadrature method works, and can see close in, where notch filtering is limited to about a 2KHz offset from the carrier.

It is overkill for audio, but low noise sources are a fascinating area.

Regards, Dan.
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Old 16th January 2015, 12:03 PM   #10
HpW is offline HpW  Switzerland
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Quote:
Originally Posted by udok View Post
Has anybody build an oscillator phase noise measurement system?

There are some threads here dealing with ultra low jitter oscillator designs and it would be great to see some real measurements.

I have read some basics and basically you need a mixer and a low noise amplifier. The amp output could go into a soundcard input or to a spectrum analyzer.

Is anybody interested in building such a thing?Udo
Well, I implemented already the SW part supporting a (cross spectrum) dual channel conjugate complex averaging (as Demian requested) on the power spectrum density (with 1/Hz normalization) to get the last possible noise floor.

I know other(s) who deal with power noise & amplifier noise issues while this part is also critical.

You are invited to deal with the evaluation version

Hp
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