[time-nuts] New Subscriber, DIY GPSDO project (yes, another one)

[Attila Kinali]

On Tue, 03 Mar 2020 14:42:49 +0100
Matthias Welwarsky <time-nuts at welwarsky.de> wrote:

> In the meantime, I'm thinking of another modification to make: getting rid of 
> the HC390 ripple counter. I think I might be able to use the STM32 as a 
> frequency divider. I just have to figure out how to program the timers. What 
> might work is the following:
> 
> - Feed the 10MHz clock into the external trigger input for the timer, 
> configure the timer to count the trigger pulses instead of the internal 
> clock  source.

Better idea: feed the 10MHz input directly into the clock input
RCC_OSC_IN and let the STM32 derive its internal clock from that.
Doing that, the clock of your timer unit will be phase locked to
the 10MHz signal. Then you can use the timer for both capturing
the incomming PPS and for generating the 250kHz. The capturing
will solve the issue with aliasing. The timer output should have
a jitter in the low ps range. There is a bit more going on than
you would want in the STM32 for a low noise signal, but most
likely not so much as to degrade the output enough to matter.
If you are worried about that, you can still add a 7474, with
D connected to the STM32 timer output and the CLK to the 10MHz
to clean the jitter. But as I said, that's probably not necessary
in this case.

If you want to create a stabilized output PPS from the STM32,
you will need to do some software trickery to align the PPS,
as you cannot hope to shift the phase of the Rb in a reasonable
time to match up.

			Attila Kinali

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