[time-nuts] PLL behavior

Bob Camp lists at rtty.us
Wed Sep 19 11:38:14 UTC 2012


Hi

Commonly this sort of thing is done with a sample and hold in the loop. No reference in / put the loop voltage in hold. You still have a phase drift and need to cope with the phase offset when the reference comes back.

Bob

On Sep 19, 2012, at 4:08 AM, Azelio Boriani <azelio.boriani at screen.it> wrote:

> In my opinion you fall in the case of disciplining with holdover... this is
> more like a disciplined oscillator (like a GPSDO) problem than a PLL.
> 
> On Wed, Sep 19, 2012 at 1:45 AM, Jim Lux <jimlux at earthlink.net> wrote:
> 
>> 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)
>> 
>> 
>> 
>> 
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