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[danlavry]

[quote title=bobkatz wrote on Wed, 23 February 2005 13:23][quote title=danlavry wrote on Tue, 22 February 2005 11:29][quote title=bobkatz wrote on Sun, 20 February 2005 21:45]

Timeline wrote on Sat, 08 January 2005 23:34

Curious, What meathod would one use to 'introduce' controlled jitter for a measurment? What might the jitter source be? The late Julian Dunn invented the J-Test signal which has been very successful. And a simple 1/4 sample rate sine wave test signal does reveal differences between various converters quite well. In addition, TC Electronic has written an excellent white paper on a simple 12 kHz sine wave test: Please see: http://www.tcelectronic.com/default.asp?id=1573  and click on "Clock and Synchronization inthe System 6000" Quote: It is not only the concept that I question. The measurement itself may yield some approximate results while inducing high level of jitter, but as you approach tiny jitter, the measurement loses accuracy. If they have the money to purchase a clock jitter analyser, as you say, not a poor man's tool) but ultimately it is the effect on the analog signal that counts. And the job of the testing engineer to see the effect on the analog signal. How little jitter do you want to have in a clock signal? The answer: As little jitter as is necessary so that it has no audible (or measurable) affect on the analog signal.

I am well familiar with Dunns paper and did read the TC paper as well. I am even more fimiliar with the math of what it takes to make those 1KHz FFT spikes. It takes a 1KHz sine wave to modulate the phase of a 10KHz (or 12KHZ for the TC paper), and the amplitude has to be pretty high. It sorts of work to indicate the rejection at 1KHz as long as the FFT is long, the FFT window is good and the 1KHz jitter is much higher then the real system jitter. I could not resolve to any degree of accuracy accurately a simulation of 100psec 1KHz sine wave jitter on an 8K FFT with BH4 window. That was a simulation, NO real noise. NO random noise!

So that "tool" is great for instructive purposes, but not for measuring real jitter. Unfortunately, some folks are "marketing" their gear as low jitter based on such measurements.

Again, that measurement becomes very inaccurate at low jitter levels, and does not tell you information about anything outside of the loop rejection at 1KHz to relatively high jitter, which by itself is of little value, because it ignores the rest of the picture.

Also, it is not difficult to build a jitter generator for inducing a 10KHz digital tone with 3.5nsec jitter, because the presence of say 100psec random wide band jitter it relatively negligible.

But try to build a 10KHz generator with say 50psec of 1KHz tone. This time, you can not afford 100psec random jitter, and even 25psec random jitter is undesirable...

So the jitter generator is a problem. Injecting low level jitter is a problem too. The FFT does not read low level jitter accurately. The information is restricted to 1KHz only... Other then that it is a great jitter test!

The way to measure low level jitter is to have an extremely wide band measurement gear! I read today that Agilent makes a probe with 220fF (0.22pF) capacitance!  
 
Of course I agree that the end result is what counts, and that less jitter is better. I am sad to see people in audio make such a big deal out of that almost worthless test. I think it is being used for just more marketing hype. You called the test "successful", I disagree. We seem to agree that real jitter measurement tools are very expansive! Some of it way upward of $50000! I just do not see the expanse as justification or a "permit" to come up with an inadequate and inconclusive "test".  

Regards
Dan Lavry
www.lavryengineering.com

[bobkatz]

Dear Dan:

I cannot speak mathematically to the test's resolution , but I can say that the J-test has proved extremely revealing for me as far as the jitter is concerned in a D/A converter. There are DACs which measure terribly, and there are a handful of DACs which show no artifacts at all, when measuring with a 64k point FFT, averaging over about 4 seconds, with 64 Bit float accuracy, on a 44.1 kHz sampled signal.

Have you read any of John Atkinson's measurements using the J-Test signal and a special analyser that really just identifies the spectral products by their distance and how they surround the 11.05 kHz fundamental?

I can send you various measurements I have made of D/A and A/D converters. The cheap ones measure worse, for sure! I can find a small quantity (less than a handful) of DACs whose jitter artifacts are equal to or below their noise floor! What does that tell you?

I personally found the signal to be very informative and correlate with the sonics of various converters that are/are not susceptible to jitter. I have measured one DAC which uses the TI ASRC chip but in the first design was driven from a defective crystal. This same DAC shows EXTREME improvement (no artifacts in the FFT) after the crystal was fixed.

I wish I could speak to magnitudes of jitter, but as I said, Atkinson's measurements show quantities that are inferred from this indirect method of measurement.

Since a ladder converter and a delta-sigma converter perform quite differently with respect to a jittery clock (see Bob Adam's papers) how do you go about evaluating the sonic effects of the jitter without taking into account the effects on the analog side? In other words, a true ladder DAC can be fed much higher levels of jitter and exhibit far fewer artifacts on the analog side than a delta-sigma DAC.

Quote:

But try to build a 10KHz generator with say 50psec of 1KHz tone. This time, you can not afford 100psec random jitter, and even 25psec random jitter is undesirable...

That's the hard part, of course, building an analog generator with little jitter itself for the A/D converter under test! The Audio Precision that I borrow occasionally (old model, analog only, Portable one) does not have low enough THD! But the generator in the Audio Toolbox seems to have very low artifacts...