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Building a phase noise measurement system for digital audio
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Old 26th August 2016, 11:18 AM   #1
andrea_mori is offline andrea_mori  Italy
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Building a phase noise measurement system for digital audio
Default Building a phase noise measurement system for digital audio

Considering the interest in building a phase noise measurement tool to test the oscillators we design for digital audio, I start this thread to investigate the opportunity to develop such this system.

Commercial tools are available but they are too expensive and not affordable for the hobbyist.

Gerhard, Demian, Ambrosia168 and all the other members, please use this thread for the discussion about the project.
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Old 27th August 2016, 10:19 AM   #2
gerhard is offline gerhard  Germany
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Andrea Mori wrote in the oscillator thread:

I don't own an FFT, can you suggest a suitable soundcard and the right software (cheap) to do the test? Or other inexpensive gear to do this?



I heard people say that Juli@ cards would be quite ok. I did some experiments
with an external USB soundcard Terratec DMX6fire USB but the driver situation
then was hopeless. Probably that has been solved over the years.

I also have a DG6SAQ vector network analyzer that works as a sound device and that
has own audio codecs. In the beginning there was some nuisance now and then
when Windows knew better about the proper use of sound devices.
The whole board of the VNWA is maybe 10 * 6 cm, so there cannot be too much
involved with the codecs. There is even an onboard hub so it needs only 1 USB cable.

Someone has teamed an external ADC chip to a BeagleBoneBlack and he
controls it from a smartphone. Quite impressive.
< https://www.element14.com/community/...d-web-based-ui >

For me, the problem is solved with the Agilent 89441A. It is available sometimes
quite cheaply because it's underrated. Most people think one can use it only to
develop the pre-previous generation of cell phones. The phone manufacturers need
better stuff for LTE, so the old ones are on the market. When the RF unit is
missing, that is no problem for you since YOUR signals need not to be received
from the air. But make sure you get the dual channel option and the source module
so you can make Bode plots.

regards, Gerhard
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Old 27th August 2016, 11:47 AM   #3
gerhard is offline gerhard  Germany
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Demian Martin wrote in the oscillator thread:

remaining questions on this-

1) reference noise source- couldn't a voltage modulated oscillator provide a suitable reference if all the details are worked out? A sinusoidal modulation with peak voltage matching the frequency offset from a known voltage could be translated into a known phase noise level? Getting some reference is important. I think there is something in the Wenzel lit on that issue as well. They also have a procedure for manual calibration that is not real easy to follow on paper.


You cannot wideband modulate a crystal oscillator. Maybe a few Hz. And it would
be really Fm modulation. That could be looked upon as phase modulation also,
the two are linked by integrating/differentiating but the PN then would be different
for each offset frequency.
Modulating the osc is like constantly accelerating / decelerating a flywheel,
and the crystal is quite heavy a flywheel.

It would be easier to get phase modulation from a RC lowpass, where the
C is a varactor diode. That is really done.

I got the idea for the PN standard from
< http://tf.nist.gov/general/pdf/1000.pdf >
And they say that taxpayer's money is wasted...

Really, there is not much to it. In the picture, there are 2 power splitters/combiners
MCL PSC2-1. The left one is used to split an incoming test carrier into two outputs.
One leaves the box on the leftmost SMA connector immediately as the ref signal.
The other output goes to the right PSC2-1, this time used as a combiner. The
second input comes from the noise source. The output of the combiner is the
contaminated version of the reference.

With the switch & the two relays I can select high/low noise or an external dirt
input that can double as "clean" with a 50 Ohm termination instead.

The noise source is a 50 Ohm resistor whose voltage noise is amplified with
2 LMH6702. Probably the first amplifier adds more noise than the resistor, but
the only thing that counts is that it is constant and reproducible.
Note that there are still some wires missing in the box.

With the addition of a lambda/4 coax cable delay, a ring mixer and a sensitive
FFT analyzer you can already measure the added phase noise of amplifiers.
That is actually much easier than for oscillators since there is only 1 frequency
involved, no need for PLLs, VCXOs and other stuff.


2) Could we start with 24.568/22.5792 MHz and a multiplier/divider for all the variations? or do we need discrete oscillators for each frequency?

Each halving of the frequency makes it 6 dB easier for use as a reference
and 6 dB harder for use as a test object.

3) Do we need an input signal conditioning stage? A limiting amplifier to get the necessary level and remove amplitude noise?[/QUOTE]

You get rid of the amplitude noise already by the use of the lambda/4 delay line.
Of course you can also measure the amplitude noise by enforcing 0 phase
difference at the mixer inputs.
Having some amplifiers capable of 17 dBm or more is handy.
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Last edited by gerhard; 27th August 2016 at 11:51 AM.
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Old 27th August 2016, 03:06 PM   #4
jborden is offline jborden  United States
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Building a phase noise measurement system for digital audio
Default Building a phase noise measurement system for digital audio

Is this an option: https://by-rutgers.nl/PDFiles/DC-receiver.pdf ?
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Old 28th August 2016, 08:11 AM   #5
1audio is offline 1audio  United States
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Building a phase noise measurement system for digital audio
Good doc. I'll need to study it more but it seems to be on the right track. The stuff about vibration sensitivity may be far more important than the other stuff we are looking at.


Gerhard:
The Juli@ is a very good soundcard. Its limited by the performance of the ADC and DAC but external ADC and DAC through the spdif interface work well. my trick of injection locking the crystals works well and keeps the card working when the external oscillator is missing.

I looked at the Agilent and the prices I saw were still quite high. Its a nice box. It highlights what happens when technology moves on. Yesterday's $100K box is worth little more than scrap.
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Old 28th August 2016, 08:12 AM   #6
1audio is offline 1audio  United States
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Building a phase noise measurement system for digital audio
Further general comment:
What I would really like is to be able to measure with some confidence and accuracy the jitter/noise impact on the audio output. Perfect oscillator in a poorly implemented DAC may not get the desired results. The DAC may have internal jitter issues that swamp all these efforts. I can get really good results from USB DAC's already. How do I confirm the improvements from better oscillators, if there are any?
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Old 28th August 2016, 04:16 PM   #7
jborden is offline jborden  United States
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Building a phase noise measurement system for digital audio
Quote:
Originally Posted by 1audio View Post
Good doc. I'll need to study it more but it seems to be on the right track. The stuff about vibration sensitivity may be far more important than the other stuff we are looking at.
Good vibration isolation particularly for very low frequency vibration is not trivial (and would have its effect on the very close in phase noise we are most concerned with)

The problem is that regular springs couple. Roller balls are great for horizontal isolation but vertical is more of a challenge. The graviton measuring folks have literature on this. For example: http://www.gravity.uwa.edu.au/docs/H...ran_Thesis.pdf

The "Euler spring" is an interesting design that can be constructed in a number of ways including carbon fiber tubes arranged in tetrahedra or spiral.

Jonathan
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Old 28th August 2016, 05:57 PM   #8
1audio is offline 1audio  United States
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Building a phase noise measurement system for digital audio
Biggest challenge is the lack of mass of crystals or crystal oscillators. Maybe imbedding in a cast iron block on even rubber would be enough. Wenzel has stuff on active vibration stabilization. The crystals are more sensitive in one axis so its less of a challenge than it first seems.

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Old 28th August 2016, 06:08 PM   #9
gentlevoice is offline gentlevoice  Denmark
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Building a phase noise measurement system for digital audio
Quote:
Maybe imbedding in a cast iron block on even rubber would be enough.
... or how about Panzerholz ..? Very quick damping and reasonably frequency linear as well:

https://www.lessloss.com/page.html?id=80 .... e.g. 2/3 down the webpage.

Cheers,

Jesper
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Old 28th August 2016, 06:23 PM   #10
jborden is offline jborden  United States
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Building a phase noise measurement system for digital audio
Default Building a phase noise measurement system for digital audio

Quote:
Originally Posted by 1audio View Post
Biggest challenge is the lack of mass of crystals or crystal oscillators. Maybe imbedding in a cast iron block on even rubber would be enough. Wenzel has stuff on active vibration stabilization. The crystals are more sensitive in one axis so its less of a challenge than it first seems.

The vibrations of concern come from the earth itself. The simplest setup is an 18" 3/4" birch plywood on top of an inner tube. Then roller blocks. (2" disc bowls in 7071 alum and SiC ball bearings) Then 3/4 inch slab of marble into which equipment is placed.

The inner tube does couple low frequency vertical waves/vibrations. The marble block supplies mass -- yes the heavier the better within reason.

This setup is easy to do and ideal for a workbench. Euler springs for better vertical isolation would be better than inner tube. Other spring like isolators including magnets are just springs and couple the vibrations the same way -- more expensive than inner tubes but not fundamentally better.

Active -- interesting probably a whole 'nother order of complexity/development.
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