[time-nuts] Good clean-up oscillators

[Poul-Henning Kamp]

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In message <ae327d38-2987-3546-5e84-cb1b960ed977 at rubidium.se>, Magnus Danielson writes:

>Consider for instance that we have a LPRO rubidium and we want to use a 
>clean-up oscillator to provide better phase-noise, what would be a good 
>selection?

There are special devices for this, like Vectrons TRU-50, but they
are hard to get hold of.

I have an item far down my todo-list simply labeled "Inverse PPSDIV":

For not relevant reasons I once put an AMD Opteron CPU into a two
instruction-loop and monitored an address line with a 'scope and
that was a _very_ clean signal.

In high speed digital chips, like high-end CPUs, close in phase-noise
is a, if not the most, serious limitation on clock rate.  The DEC
Alpha silicon pioneered the use of a SAW resonator at the pointy
end of the PLL, but I'm not quite sure what they do today.

The TODO point is to take a RaspBerry Pi, run a tight loop with all
which wiggles a GPIO pin with all interrupts disabled, and measure
the phase noise.

It's going to wander all over the place, because it comes up from
a 1 cent Xtal, but the phase noise will be from the on-chip resonator,
whatever that is.

If it's any good, buy a Pi-Zero, rip out the X-tal and feed it
from your LPRO instead.  If you only need one GPIO pin, I doubt
the exact clock frequency matters much.

(The BeagleBone is interesting too, since the PRU units run
autonomously at 200 MHz from their own memory, so the main
CPU could do other jobs for you.)

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