[time-nuts] Cheap DMTD for Rb / OCXO locking (was: True Position GPSDP + Rb X72)

Attila Kinali attila at kinali.ch
Fri Sep 22 15:01:10 EDT 2017

On Fri, 22 Sep 2017 11:50:29 -0400
Bob kb8tq <kb8tq at n1k.org> wrote:

> Given the noise levels of typical Rb’s - use XOR gates as the mixers. 

Right. That makes it a lot more robust than using a diode mixer.
> If you do the filters as R/C’s they are cheap. If you just re-clock, you
> maybe add a flip flop or two to each channel. Total BOM for the DMTD
> part of it would be < $1 not counting the OCXO. 

Recklocking shouldn't be necessary. A simple inverter should raise
the signal back to an approriate signal level. After that a division
chain is still needed as otherwise the rate of transitions will be
too high for the uC to handle.

Probably the cheapest way to do this would be to use a 38.4MHz or 38.88MHz
oscillator (available for 1-2$) and using a 74LVC74 to divide it by 4.
That gives 400kHz and 280kHz offset frequency, respectively. Dividing
both beat signals by 256 using two 74LVC161/163 each should get it
down enough to be workable with a modern uC.

> All of the previous comments about “why would you want to do this?” 
> still apply. Most of the small Rb’s we can get our hands on have really 
> awful phase noise and spurs. That’s not true of all small Rb’s, but buying
> them new in small volume at $2K each is not something most of us 
> are interested in doing ….


If anything, I'd do it the other way. Discipline a Rb to GPS using
a long time constant. Then slave an OCXO onto that using the method above
and use a loop time constant in the order of 1-100s

			Attila Kinali
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson

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