Chris wrote on 8/3/2017 3:05 PM:
On 08/03/17 17:27, rickman wrote:


Not sure if you are referring to the Shortt clock or the PLL. But the
statement applies equally to both. There is no magical stability in the
PLL. It is a control loop and as such the thing being controlled will
*never* remain in phase or at the same frequency as the reference.


I think the difference is that while a pll always has a phase offset
the reference and vco are in phase lockstep once the loop has aquired
lock. It's a closed loop system whereas the Shortt clock is an open
loop system, only getting a kick back into sync from time to time.

Like a hit and miss governor ?...

I don't know what you guys are seeing. The two pendulums of the Shortt
clock are in lock step. The fact that they are only compared every 30
seconds does not change the nature of the design.

The phase comparison signal from a PLL is typically "grainy" in the same way
and has to be filtered to become a control signal. The only reason you say
they are in "lock step" is because the grain is very fine. The Shortt clock
grain is very fine as well typically adjusting only every other 30 second
period.

I guess the difference is the Shortt clock is adjusting the instantaneous
phase and the average frequency while a typical PLL adjusts the
instantaneous frequency to try to keep the phase aligned. Both will see
variations in phase over time.

--

Rick C