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30th November 2010, 02:31 AM  #1 
diyAudio Member
Join Date: Nov 2010

Rubidium clock
Dear members,
Has anyone tried to compare an Rb clock, used as a master clock in a CD transport, to a combination of an external Rb clock used as a wordclock and connected to a CD transport, like does Esoteric with the G0/P0 models ? In the first case, the clock's output frequency is 11M2896 or 16M9344 etc... depending on the CD transport model and in the second case, the clock's output frequency is 44K1, for the CD standard sampling frequency. Which is the best way to go ? Thanks. 
30th November 2010, 04:09 AM  #2 
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Join Date: Dec 2001
Location: Schaffhausen Switzerland

An Rb clock may have amazing long term stability, but for CD playback you are looking for shorter term jitter, and in this they are VERY poor.
Avery well done xtal osc will sound far better. Regards, Allen 
30th November 2010, 05:47 AM  #3 
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the better spec is low, close to carrier (audible frequency offset) phase noise

30th November 2010, 12:10 PM  #4 
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8th December 2010, 06:52 AM  #5 
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Join Date: May 2010
Location: Vejen

Well... Maxime is producing this little baby here. MAX3624
Excellent S/R when it comes to Supply voltage and extreme low jitter. Combined with a Rubidium clock do I think you could get the best from both worlds. Long term and short term stability, but I might be wrong. 
9th December 2010, 12:46 AM  #6 
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Join Date: Feb 2003

MAX3624 looks good, but its output frequencies are fixed for network applications. MAX3639 seems a better choice for audio?
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9th December 2010, 02:25 AM  #7 
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these chips only address higher frequency offset phase noise
theoretically for audio you want low "close in" phase noise  look at the 1KHz noise Crystech makes xtals in std DIP cans with better than 120 dB@100Hz, 140 dB@1Khz if you want to throw money at the nonproblem then Vectron EMXO series evacuated SC cut with low vibration sensitivity may make more sense  they claim to be able to "significantly" better the 130 dB@100 Hz close in noise on request  but you'll probably have to pony up for a 100 piece min order when human jitter perception in audio is looked for in DBT studies the sensitivity is absurdly low  100s of nS  not ps 
9th December 2010, 04:45 AM  #8 
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Join Date: May 2010
Location: Vejen

jcx
A recent thread linked to a graph showing jitterversusdynamic range curves. I can't seem to find that one right now, but for the worstcase scenario the math isn't too hard. Here goes. What jitter does is shifting the sampling instant. The result is that the larger a signal's slew rate, the larger the effect of jitter. As a result, higher frequencies will suffer more from jitter. Take a sine wave. Its maximum slew rate occurs at zerocrossing, for: dV/dt = 2*pi*f*A*cos(0) = 2*pi*f*A, where A is the amplitude. For a 0dBFS (rms) sine, a maximum occurs at the top of the frequency band, say 20kHz: dV/dt = 2*pi*20000*sqrt(2) = 178k/sec = 105dBFS for a 1second RMS jitter. This also shows that a 10x increase in jitter will increase its impact (raises the jitterinduced noise floor) by 20dB. So a firstorder approximation is: 10ns jitter > (10520*log(10^8)) = 55dBFS 1ns jitter > 75dBFS 100ps jitter > 95dBFS 10ps jitter > 115dBFS 3ps jitter > 125dBFS Keep in mind that these are worstcase jitter effects for 20kHz 0dBFS sine waves, decreasing by 6dB per octave that the frequency is lowered. This HF impact is named as the reason why jitter is often felt to 'decrease the airiness' and 'muddle up the high end'. So yes, for a CDplayer 3ps jitter could be considered overkill, but not by very much 
9th December 2010, 06:25 AM  #9 
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theory of when jitter starts to degrade S/N and demonstrated perceptual jitter detection are 2 different things
signal correlated jitter or sinusoidal jitter may be easier to detect, but a quality external clock's phase noise/jitter should be random and uncorrelated  for which the DBT thresholds are 10s to 100s of nS google: Ashihara jitter 
9th December 2010, 10:16 AM  #10 
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Join Date: Nov 2010

In a manner, can the phase noise be compared to the oscillator's harmonics attenuation around its fundamental ? In this case, if the oscillator feeds a passband filter, it must have a high selectivity feature (Q factor) to maintain the oscillator's phase noise low ?

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