[time-nuts] PLL behavior

[Jim Lux]

On 9/18/12 9:48 AM, Raj wrote:
> If you break the DC control chain of the PLL with a A2D and a controller and back with a D2A .. you would program the control with any kind of behavior you want. Just a thought!
>
That is exactly what we do... the PLL is actually implemented digitally 
(DAC driving the VCO)..

But what I'm looking for is a theoretical treatment of the output 
statistics (Allan Dev, mostly) in terms of the interrupted reference input.

For context.. we do precision ranging of spacecraft in deep space by 
sending a hydrogen maser derived signal TO the spacecraft which locks a 
local VCXO to that signal, and then uses the VCXO to generate a return 
signal with a constant ratio (e.g. 880/741) to the input.

By measuring the time it takes for the round trip (essentially counting 
phase cycles on the return signal (against our hydrogen maser, again), 
we measure Range and Doppler, which is then used to determine the 
position of the spacecraft.

Typical performance is sub-meter and sub cm/sec.  (A very high 
performance would be that the transponder adds 4E-15 Allan Dev over 1000 
sec... 1E-11 or 1E-12 over 10-100 secs is more usual)

What we want to know is "what happens if the receiver and transmitter 
can't run at the same time"?  Obviously, we have less information coming 
into the system (we see the uplink half the time, so right there, we 
have a 2:1 hit) and the ground end only sees the transmit signal half 
the time (another 2:1 hit), so, from an information theory standpoint 
we've already put ourselves in a hole, but, what does the statistics 
really look like for the turnaround loop..

Full Duplex full power turnaround is expensive in power, mass, etc. (for 
instance, you have to have good filters to make sure that your receiver 
isn't corrupted by the transmitter)