[time-nuts] Using CPLD/FPGA or similar for frequency

Bob Camp kb8tq at n1k.org
Mon Jun 15 19:31:50 EDT 2015


Coming up with a reference clock can be harder than you might think. 

Most of the jitter numbers you see published on frequency sources are based on a
“jitter mask” that runs from (maybe) 10 KHz up to 20 MHz. That’s fine for a specific
telecom need. It may not in any way apply to capturing a 1 pps pulse. *IF* you believe
the phase noise to jitter integration stuff (and there are reasons not to) the jitter goes way
up as you integrate closer to carrier. A 0.1 ps source can quickly turn into a 10 or 100 ps 
source with a change of the lower bound on the jitter mask.

Now, once you have the jitter number, you are only part way to knowing it’s impact on
your measurement. There are *always* more things to consider.


> On Jun 15, 2015, at 6:14 PM, Attila Kinali <attila at kinali.ch> wrote:
> On Thu, 11 Jun 2015 14:22:58 +0000
> Alan Ambrose <alan.ambrose at anagram.net> wrote:
>> A clever interpolator for frequency or TIC would kill it -
>> for TIC essentially a PICTIC on steroids. The PICTIC does 19pS with
>> a 10 bit ADC and a 66MHz clock, an SR620 does 4pS with a 12 bit ADC
>> and an 80 MHz clock - so ... cough ... Spartan 3E at 256MHz with 16
>> bit ADC - and 1pS should be easy
> Which architecture for the FPGA do you have in mind? The delay
> line method (which is the most common one for FPGAs) has an intrinsic
> limit around 10-20ps. But the SR620 and the PICTIC use both a time to amplitude
> conversion by charging a capacitor (both include a Nutt interpolator).
> Using this technique, it might be possible to get into the 1ps ballpark,
> if the design is done carefully (according to Richard McCorkle, the
> limiting factor for the PICTIC II was the ADC of the PIC, followed by
> the stability of the reference clock).
> 			Attila Kinali
> -- 
> I must not become metastable. 
> Metastability is the mind-killer.
> Metastability is the little-death that brings total obliteration.
> I will face my metastability. 
> I will permit it to pass over me and through me. 
> And when it has gone past I will turn the inner eye to see its path. 
> Where the metastability has gone there will be nothing. Only I will remain.
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