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Old 17th June 2013, 12:26 AM   #11
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Thats just silly talk. I test many CDCE and CDCM jitter cleaner EVMs as they are made.
The PLL can lock onto a frequency many times the crystal. When divided back down to
a frequency similar to the original, the phase noise is in hundreds of femtoseconds...

Long term phase noise is still no better than the crystal, but the short term can be
many times better. I see it. I measure it. Not that type of EVM every day, but often
enough...

Temperature of the crystal would be the thing to tighten up on. I see cheap crystals
drift all over, compared to either of my rubidium refs...

Last edited by kenpeter; 17th June 2013 at 12:33 AM.
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Old 17th June 2013, 10:22 AM   #12
DF96 is offline DF96  England
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A PLL is necessarily no better than the device it locks to for low frequency jitter. At mid frequencies jitter is set by the loop filter, so it could be better or worse. At high frequencies jitter is set by the PLL's own oscillator which in most cases will be worse than a crystal. Even if the PLL uses a crystal itself it needs to vary the frequency and this will worsen the jitter as it is an extra source of noise.

Long term drift is irrelevant for audio, so there is no need for rubidium references. Short-term jumps in frequency might be a problem, but decent crystal quality should avoid this.

Perhaps some of the confusion arises because to me 'PLL' is a circuit architecture, but to others 'PLL' might be a frequency source or type of oscillator.
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Old 18th June 2013, 04:55 AM   #13
oshifis is offline oshifis  Hungary
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Quote:
Originally Posted by kenpeter View Post
Long term phase noise is still no better than the crystal, but the short term can be many times better. I see it. I measure it.
What is the definition of long term/short term phase noise, and how do you measure it? I guess you need a super accurate and ultra low noise in the frequency and/or amplitude domain test rig.
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Old 20th June 2013, 01:10 AM   #14
wa2ise is offline wa2ise  United States
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Quote:
Originally Posted by DavesNotHere View Post
crystal clocks are not accurate.
Crystals are a lot better than ceramic resonators I've seen in cheaper CD players.
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Old 20th June 2013, 03:50 AM   #15
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by oshifis View Post
What is the definition of long term/short term phase noise, and how do you measure it? I guess you need a super accurate and ultra low noise in the frequency and/or amplitude domain test rig.
This is a good question because there is an exact answer that does not depend on opinion.

Error in the phase of a sine wave can be expressed ad some added frequency component. Phase noise is defined as the power of those added components per Hz of signal.

It is probably better th=o think of "jitter" which is 100% that same thing as phase noise but expressed in the time domain. Jitter is simply the error in the period of the clock. For example a 1000Hz clock should have a period of 0.001 second. But might have a 1 sigma error of .00001 seconds.

We all know that periods and be converted to and from frequency. Phase noise is just looking a "jitter" from the point of view of frequency. If you one you know both.

So there you are clocking samples out of a DAC. The question is how bad can the error in the clock get before you can hear it. Tiny errors in the clock produce frequncy errors that no tweeter on Earth can reproduce, For example nanosecond level errors produce frequncy errors in the radio spectrum

On the other hand a millisecond level error is dead in the middle of the human voice range. Clearly we can hear the voice and clearly can can't hear microwaves. So the threshold of tolerable error is some place between.

No the thing is that even a cheap crystal has nanosecond level or better error.

Next topic LONG term error. This is easy to understand. The crystal is very tiny "jitter" but what if the freqnucy of "off". This causes a tiny shift on pitch. Can you hear it? that depends on how much "off" the crystal is. But again even a cheap crystal is at worst 100 parts per million. Far better then any's ability to hear.
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