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

Bob Camp kb8tq at n1k.org
Tue Jun 16 07:06:45 EDT 2015


> On Jun 15, 2015, at 11:26 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
> kb8tq at n1k.org said:
>> Since the internal PLL’s have jitter in the 20 to 30 ps RMS range, that
>> limits a lot of  the data you get. 
> I haven't looked recently, but I doubt if much has changed.  Xilinx uses DLLs 
> rather than PLLs.

The jitter on both clock sources looks pretty gaussian. 

> They have a long chain of buffers and a giant multiplexor to select the right 
> tap.
> Does anybody have data on what the "jitter" actually looks like?  I'd expect 
> several blurry peaks, with the spacing between peaks being the step size of 
> the delay/mux chain and the blur being wider if there is more random logic.

The calibration output is a mess …


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