[time-nuts] femtosecond jitter anyone?

Chris Mack / N1SKY sometimesyoufeellikeanut at twentylogten.com
Wed Apr 8 23:32:20 UTC 2009

>> This is a good idea for testing..
> Applying jitter frequencies for jitter tolerance testing is standard
> stuff and needs to be done. Jitter tolerance curves match up with MTIE
> tolerance curves very neatly.

Of course, here is the weird part... It's not SONET; but it is a chip  
that can be used for SONET...  This is for a very specific form of  
audio clocking (not audiophile, nor consumer) for a mastering  
engineering application.  Common input clock frequencies: 44.1kHz to  
96kHz or also a 10MHz rubidium.

The DSP PLL is this chip (I am still learning the intricacies of this  


The system clock (to drive the DSP and the DSP's DCO) is essentially  
a jitter reference, pins XA and XB (differential, single ended  
capable); Jitter is transferred nearly 1:1 from XA,XB to CLK_OUT.   
This is the 38.88 MHz reference from Vectron with some skirting  
issues to be filtered before connected to the XA and XB pins.

The input (on CLK_IN pins) is the source clock to be cleaned (e.g.,  
44.1kHz to 96kHz or 10MHz Rb).

The output (on CLK_OUT pins)  is 11 MHz to 25MHz for 256x  
oversampling master clock for ADCs and DACs

24-bit accuracy for 40kHz (88.2kHz to 96kHz sample rate encompassing  
a 45/55 anti-alias filter) shows the need for sub picosecond timing  
aperture uncertainty.

Of course 24-bit in the real world is hard to achieve (even the new  
"32-bit" converters have a problem with it) with issues internal to  
the sampling mechanisms in a DAC / ADC, but with some out-of-band  
dither and thermal management, coupled with low jitter sampling  
clock, there may be an additional bit or so to be obtained.  This is  
all part of the experiment....

>> I have Howard Johnson's book for

>> I think a normal LC tank would be more suitable for that task.
> It's a good introductional level book for digital signals, but isn't
> very applicable to waveshaping or clock characterisation and testing

Yes, HJ's books leaves me wanting a little more... seems like an  
analogue / RF book for digital folks.

I am looking for sharp Q to get rid of any skirt around the 38,88MHz  
of the Vectron OCXO.

Temperature can be obtained from cooling componentry already in situ,  
such that a known temperature is established.


More information about the time-nuts mailing list