[time-nuts] Performance verification for time counters
hmurray at megapathdsl.net
Thu Nov 30 02:04:02 EST 2017
kb8tq at n1k.org said:
> The gotcha with a DDS in this case are the sawtooth spurs. To get them down
> to the 5 ps level, you would need a DDS with a clock that is well into the
> 100’s of GHz.
I think that assumes you want to be able to generate an arbitrary frequency.
Suppose I start with 10 MHz. It's easy to get a clean 1 MHz or 5 MHz, but
not so easy to get 3 MHz. But if I start with 9 MHz, I can get a clean 3 MHz.
How do I figure out how many crystals I need?
How good are PLLs relative to 5 ps? Can I use them to make pseduo-crystals?
What's the term for PLLs where I divide the output by M and the reference by
N? As M and N get bigger I get more possible output frequencies but the
output of the phase comparator gets lower in frequency and hence harder to
filter. How low can I go before 5 ps gets interesting? (Is that a
If I have an array of crystals and PLLs, I can get some more flexibility if
the crystals are VXOs.
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