[time-nuts] femtosecond jitter anyone?

Chris Mack sometimesyoufeellikeanut at twentylogten.com
Thu Apr 16 14:02:18 UTC 2009

Awesome... Thanks for being a sounding wall everyone....

Indeed I am using the SRCs from TI for both the 2 DACs and the single  

The 127dB discrete ADCs out there are approximately half bit more in  
dyn range than the 124dB core in the ADC... Indeed, there probably is  
some significant jitter in the clock section for the IC ADC (and it is  
single ended, which the Si5326 also supports form CMOS output)...   
However, I am hoping to try some thermal experiments and some dither  
in the analog domain to see what additional envelope pushing can be  

Unfortunately the data sheets for the ADC do not have the crucial  
information that normal high speed ADCs would have including  
temperature data and SFDR, SINAD, ENOB etc..  The specs are kinda  
brain-dead in this regard...

But with distribution and PLLs, the jitter budget should be a little  
bit (pun intended?) better that what the clock portion of the  
converters can do (which is an unknown for this experiment), and who  
knows if the internal clock sections of said converters get better  
with temperature change....  Worse case, I get some Johnson noise out  
of the system perhaps...  Or the temperature systems are removed and  
not included in the prototype, but the placeholder is there if it is  

On the low jitter PLL side... In addition to the Si5326, I found that  
MultiGig has chips that will take 38.88MHz and create 2X or 4X with  
"0.34ps" jitter....caveat bandwidth of measurement...  For the Si5326,  
the higher the reference (jitter reference / DSP clock) frequency the  
better the jitter from CLK_IN to CLK_OUT and jitter cleaning...

Nice chips from Analog too...

I am amazed at how low jitter can be....


On Apr 9, 2009, at 7:46 AM, J.D. Bakker wrote:

> At 22:03 -0400 08-04-2009, Chris Mack / N1SKY wrote:
>> On Apr 8, 2009, at 8:50 PM, Bruce Griffiths wrote:
>>> Chris
>>> If you divide the output down to ~38MHz using a noiseless divider  
>>> then
>>> the performance is 20dB or more worse than can be achieved with a  
>>> good
>>> ~38MHz crystal oscillator.
>> Ah, this would work, but there is a synchronization aspect since
>> framing on AES/EBU is in the mix (pun intended?) and there are more
>> pieces of external equipment that all need to be synched (within
>> AES11 framing sync margins)...
> From a jitter POV the AES11 profiles are insanely wide, and they make
> little sense if you're not using digital tape or other media which
> take their time to slew to their final speed. Look at the recent TI
> Asynchronous Sample Rate Converters (ASRCs); their rate estimators
> have a bandwidth much tighter than needed to track a worst-case AES11
> signal.
>> This 38.88MHz is a DSP clock, essentially a microprocessor clock
>> (albeit a very nice microprocessor clock) where the DSP simulates a
>> PLL operating on an incoming clock source, and makes an output clock
>> of a different frequency, [...]
>> The output of the DSP PLL in this box / design of interest is 11MHz
>> to 24MHz to feed the oversample clocks on the ADCs and DACs,
>> synchronized to the external 44.1kHz to 10MHz master house clock a la
>> the PLL and the rest of the equipment on the other side of the  
>> room...
> By 'DSP PLL', do you imply that the DSP controls a DDS? If so, is the
> DDS a separate chip or do you use a DAC hooked to your DSP?
> For best jitter performance in an audio system you may want to
> consider getting a free-running low noise XO with a frequency that is
> NOT a multiple of your sampling rate(s), have it drive your ADC/DAC
> converter directly and use an ASRC (either integrated or, preferably,
> FPGA/DSP) to go to your target output rate.
> As for fs jitter: I've yet to see a converter chip with differential
> clock inputs, and for a single ended clock input I expect that the
> total input-referred noise due to ground bounce &co is in the order
> of a ps, if not worse. (The story changes a bit for discrete
> converter designs, as those can have diff clock inputs with specified
> noise performance).
> JDB.
> [all things being equal, voltage pullability == lower Q == more phase
> noise. And that's even before you consider control port noise
> injection...]
> -- 
> LART. 250 MIPS under one Watt. Free hardware design files.
> http://www.lartmaker.nl/
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