[time-nuts] GPS receiver local oscillator (was: 1PPS for the beginner)

[Attila Kinali]

On Tue, 14 Aug 2018 11:05:17 -0400
Bob kb8tq <kb8tq at n1k.org> wrote:

> > Is there a need to make the math easier?
> > This is what we have microprocessors for.
> 
> There is *always* a need to make the math easier if I’m quickly typing up an example. For proof I
> reference the hundreds of posts in the archives where I didn’t quite get it right :)

I am pretty sure that the math is less the problem than being able
to control the oscillator precisely enough while staying at low cost.

The average TCVCXO has a tuning range of...let's say 10ppm = 1e-5
over a range of 0-3V. Now, we want to be able to shift the phase
such, that we are within 1ns at the next pulse (actually we want it
to be better, but lets keep the numbers simple). This means we need
to control the frequency with a precision of 1e-9. Hene we would need
a DAC resolution of 1e-4 or 14bit. That's already a DAC that has a price
tag, multiple times that of the TCXO. Hence it's cheaper just to
shift the frequency/phase digitally and not touch the TCXO.


			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, Neal Stephenson