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Building a phase noise measurement system for digital audio
Building a phase noise measurement system for digital audio
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Old 9th November 2017, 02:18 PM   #41
udok is offline udok  Austria
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And can you hear a difference in the files i posted?
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Old 9th November 2017, 03:08 PM   #42
EUVL is offline EUVL  Europe
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> The Well Tempered Master Clock - Building a low phase noise/jitter crystal oscillator

Copied the circuit from someone else (no longer in this forum), gave it a new name,
without naming the originator for his credit, or asking for his consent ?
The Well Tempered Master Clock - Building a low phase noise/jitter crystal oscillator


Patrick
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Old 9th November 2017, 03:20 PM   #43
TNT is offline TNT  Sweden
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Quote:
Originally Posted by udok View Post
A simple jitter experiment:

In the zip file are two 440 Hz 16 bits/48kHz sine signales with the spectrum
as in the picture.

The second has random jitter of 10 nanoseconds (not picoseconds) added to
the sampling clock.

I will bet the first one a beer, that can hear a difference using the foobar ABX test plugin!
You have an odd taste in music.

//
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Old 9th November 2017, 03:35 PM   #44
udok is offline udok  Austria
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The sine signal is a good test case because it help you to concentrate on the difference between the two files. A real music file is much more difficult, because real music
has a lot of harmonics which mask little imperfections like THD or phase noise. And the sine signal is easy to generate in Matlab too.

My intention is to safe you from investing 10000 Euros and 2+ years in building
a top phase noise measuring system, getting a lot of grey hairs in the way, and in the end hearing no difference :-)

If you think that you can not hear a difference with 10ns of sample jitter, i add a file with 100ns of jitter here.
This is about 100000 more than a crystal oscillator.
The picture shows the histogram of the sampling jitter.
With this example you can hear that the sampling time jitter sounds like random noise, no spectacular modulation effects are happening.
Attached Images
File Type: jpg ab100ns-histogram.jpg (30.4 KB, 237 views)
Attached Files
File Type: zip ab100ns.zip (177.2 KB, 8 views)

Last edited by udok; 9th November 2017 at 03:54 PM.
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Old 9th November 2017, 04:41 PM   #45
EUVL is offline EUVL  Europe
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> My intention is to safe you from investing 10000 Euros and 2+ years in building
> a top phase noise measuring system, getting a lot of grey hairs in the way, and in the end hearing no difference :-)

Maybe they just want to do it for the fun.


Cheers,
Patrick
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Old 9th November 2017, 05:46 PM   #46
udok is offline udok  Austria
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If it for the fun of it, i can help

The picture shows the theoretical jitter curves.
The phase noise measurement should be a few dB better
Attached Images
File Type: jpg snr_jitter.jpg (67.9 KB, 232 views)
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Old 16th November 2017, 11:52 PM   #47
AverPaul is offline AverPaul  United States
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Building a phase noise measurement system for digital audio
I'm coming very late to this galactic-sized thread, but here is a scheme that I've used very successfully. It's not dirt-cheap, but it's not insanely expensive either. Best of all, it can measure down to RMS number in the below 10 ps range. For slightly less money it will measure down to, say, less than 50 ps.

John Miles KE5FX, mentioned elsewhere in this thread, has a toolkit that includes very nice software to measure phase noise. It's meant to be connected to an RF spectrum analyzer via IEEE-488 (GPIB). Here's what you need to have/borrow/buy:

A spectrum analyzer. I have an HP8566B, which I got off of eBay for $1500 several years ago. There are others, but this one is particularly sweet for this task, as the close in phase noise is very good, and rivals anything new in the $50K+ price range. Get one in good condition and be sure to see pictures of the screen display. Look closely at the picture and see where the display intensity is set. If it's dialed up all the way, keep looking. It means the tube is very weak. That said, there are very nice LCD retrofit kits for these, but they're a little pricey (~$750, as I recall).

A Prologix USB to IEEE-488 adapter or Prologix Ethernet to IEEE-488 adapter. They ~$200 or so, work perfectly, and get the job done. I prefer the Ethernet version, but either one is fine. Any other computer to GPIB interface that John's software supports will work as well.

A DC block. Get one from Minicircuits Labs, or build one with a couple of connectors and a 100pf or 1000pf NPO cap. This is non-critical, and NOT OPTIONAL, since applying DC, or the DC offset that might be present on a digital signal will destroy your SA. Put this on the input connector and leave it there. Use a 10X probe to connect to your circuit to be safe and only go to a 1X probe when you are sure you know what you're doing.

Use the low-frequency (BNC) input on the HP8566B, since it can measure closer to DC than the other (default) input.

Optionally, for extra low phase noise/jitter measurements, build a GPS Disciplined Oscillator (GPSDO). There are Trimble GPS receivers available on eBay for around ~$200, and antennas (make sure it's one with the proper preamp build in to work with the Trimble) for another $50-100. John Miles toolkit also has software that will talk to these receivers to configure, test, and evaluate them. These receivers were meant for cell tower use, and have ended up so cheap on the surplus market when one giant phone company bought another giant phone company and dumped all their new infrastructure. Using one of these as the 10MHz timebase for the SA will drive the residual jitter of the SA down into the 5-10ps range.

Don't measure the baseband (Word Clock) frequency, if you can at all avoid it. 48kHz is not that far from 0 and your measurement will be limited by having the carrier so close to zero Hz on the SA. Most good clock generators run at a much higher frequency (typically 25 or 50 MHz, but maybe in the 12 MHz range). Measure that signal to get good measurement.

Using John's Phase Noise app let's you directly measure jitter, since RMS jitter is the integral of phase noise over some bandwidth. His program lets you set a low and high frequency range for the calculation and reads out in RMS pS. Typically, you want to calculate jitter over a 100Hz to at least 40 kHz range, depending on your circuit and application. Going below 100Hz is difficult because you're starting to see the skirt of the SA's local oscillator, but you can also tell what you're looking at by the shape of the phase noise plot.

This scheme works really well, and if you're seriously trying to measure phase noise, this is how you do it. There are many other ways to do this, and some of them can be done for little cost. But what you'll find with those is the residual jitter of those methods will be one or two orders of magnitude worse than any worthy clock source, so you won't get much joy--other than building something. Which is cool, of course!

By the way, the math for what jitter translates to in terms of noise floor vs. frequency is:

SNR = 20 log(1/(2*PI*freq*jitter)

where freq is in Hz and jitter is in Seconds. So to get an SNR of 110dB at 15kHz, you need a clock source with jitter less than about 25 ps. THIS is why jitter matters...

Paul
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Old 19th November 2017, 08:09 AM   #48
gentlevoice is offline gentlevoice  Denmark
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Building a phase noise measurement system for digital audio
Hi Paul,

... Interesting input and perspectives you add ... A couple of comments from me:

* First, it seems to me from the discussion in the TWTMC thread (Andrea Mori et al's build-thread for a low phase noise/jitter clock oscillator) that what may be the most important frequencies to measure in relation to audio are the very close-in frequencies, i.e. the frequencies from, well, "DC" to maybe 1 kHz.

* Second, the phase noise/jitter range that has been looked at in this thread is below 1 ps.

Thus, if I read your post correctly, none of these criteria really are met with the suggestions you make (right?)?

Also, personally - and being DIY- I am looking to find out what may be called the "components" of jitter: I.e. testing some circuitries/solutions to find out which of them are low phase noise solutions - and then using these solutions/principles at other frequencies as well.

To this end I reckon that Herbert's DC Receiver (link in my previous post) may be a good choice, although for a start it means building a couple more oscillators. But it should allow phase noise measurements at close to carrier frequencies - which is where I would mainly like to look.

I notice also that you are a relative newcomer to the diyaudio forums - so welcome

Cheers,

Jesper
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Old 19th November 2017, 06:45 PM   #49
AverPaul is offline AverPaul  United States
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Building a phase noise measurement system for digital audio
Hi Jesper,

1ps RMS jitter would result in jitter-induced residual noise of below -130dB at 44kHz. Are you aware of any digital audio systems with a noise floor that low? At 1kHz this is -164dB, which is 20dB better than the limit of 24-bit data.

While I agree that better is always better, from an engineering standpoint the goal should be to properly size the solution to the problem. On the other hand, if the goal it to explore the boundaries of the problem domain, then I'm right there with you.

It's true that there are different way to view the effects of jitter, and close-in noise is certainly a valid one. Again, however, I think you have to view all parameters like this from a systems perspective. All things being equal, if the RMS jitter over the audio bandwidth is well below the system noise floor, the close-in jitter will be as well, assuming that the PDF of the jitter is not oddly shaped and there are no spectral frequencies ("birdies") in the jitter. Looking at the phase noise over the audio bandwidth will provide information on this.

I applaud the search for lower and lower jitter. It used to be, and still is, common to see audio interfaces with clock generators with 1nS jitter, and 200pS used to be considered "good", so there is certainly room for improvement.

I look forward to seeing how your search goes.

Best,

Paul
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Old 20th November 2017, 06:46 AM   #50
1audio is offline 1audio  United States
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Building a phase noise measurement system for digital audio
Just to put a bit of a baseline to this the AK5394A is 123 dB SNR. Its possible to use 4 and get an additional 6 dB. https://www.akm.com/akm/en/file/datasheet/AK5394AVS.pdf
The AK4490 is 123 dN SNR in mono mode: https://www.akm.com/akm/en/file/datasheet/AK4490EQ.pdf

Jitter impact on SNR is a bit more complex since its tied to how the chip uses the clocks and what type of chip. The numbers are correct for a classic ladder dac with sample and hold. Delta Sigma gets better performance from the same clocks, and the sample clock doesn't figure the same way.

The close in phase noise is an issue but its unclear how much of an issue it is. Possible the best way would be a Allen Variance plot. The longer intervals probably don't mean much but the short intervals may. That requires a different method to measure entirely.

For audio there are really two clock familys you need to test- 44.1 and 48. Typically you would be looking at 22 and 24 MHz clocks. because of this if really low residual is needed you would use the double balanced mixer method. Ideally two tunable oscillators and two PLL loops and correlation software to remove the reference oscillator. Its all in the lit and a lot of hassle.

My quickie method using a specific FM tuner will get pretty close (The Yamaha 950 has possibly the lowest noise of any commercial FM tuner) and will show those spurs which are far more significant. Tuning to the 4th harmonic amplifies the phase noise by 12 dB further enhancing the measurement. Doing the math to convert to standard phase noise or jitter however is beyond me. Its not simple.

Except for troubleshooting I think measuring the jitter/noise at the output of a DAC is what is important. If its good go on. If not you have issues. All you need is a reasonable ADC and FFT software.

However in my experience jitter is a solved problem. A good SPDIF receiver can get 20 pS or less from a really bad source. USB is trivial to get very low jitter AS LONG AS the basics are done well. Sharing a gate oscillator chip with any other function and you probably have a real problem.
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