[time-nuts] Linear Interpolator

Stephan Sandenbergh stephan at rrsg.ee.uct.ac.za
Thu Jun 29 09:19:33 EDT 2006


>> So you suggest, I take the 10MHz output of my OCXO lock that to a stable
>> VCXO (say 64MHz as not to be a harmonic multiple of 10MHz) and then clock
>> my FPGA with the resultant output.

>I would rephrase that to say that you should lock your low jitter VCXO (of
>say 64 MHz) to your stable OCXO (of 10 MHz).

Thanks for correcting me. This is actually what I meant.

>Oh, what 64 MHz oscillator do you have?

Currently I don't have a 64MHz VCO yet. I'm just weighing my options. My
original idea was too run the FPGA off a 64MHz free-running XO. This would
have given me 15.6ns resolution with a simple start-stop type phase detector
(and not too much EMI problems). However, in retrospect, I doubt if that
will give me the 1ns dithered accuracy I want. It will be a shame to add
substantial noise to the low 2ns jitter of the M12+T. I guess the final
dithered accuracy depends on one's pre-averaging time.

Well, I still think it might be better to run off a free-running XO to have
the dithering advantage. My application requires good stability (ideally sub
ns) between distant locations (maybe in the order of 100s of meters to 10s
of kilometres) on time scales of 100s of seconds to a few minutes. 

I guess one can view the PLL as a very sharp roll-off low-pass filter. If
one could have less jitter (via sawtooth correction and a good resolution
phase detector) at the 1PPS input in the first place, one could set your PLL
bandwidth higher to allow for a longer stability hold-over. 

I suppose one might be able to do the math in order to remove all the guess
work, however I have never seen the measured phase noise of the M12+T's 1PPS
or anything similar. So, I am just designing so that the M12+T, the DACs and
my OCXO will be the limiting factors, and not the measured phase resolution.
The reason - because it seems easy enough to do so. Maybe, after all, I
should just go for a faster clock. This would allow for dithering down to
about 2ns-3ns accuracy (same as M12+T). I'll just have to endure the EMI -
keep it away from the sensitive circuitry etc. 

What are your thoughts? Am I just being paranoid?  

Best regards,


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