[time-nuts] Re: problem with Trimble Mini-T

[Magnus Danielson]

Erik,

That looks to me as the beating from being unable to lock. One possible 
reason is that the oscillator drifted too far so it no longer can really 
reach 10 MHz. You keep getting cycle-slips.

It might be better if you measured it in free-running state, so you 
start the Mini-T up with no antenna connected, but measure the 10 MHz 
output. Then you can observe the oscillators behaviour without the PLL 
pulling it here and there.

The beat-frequency indicates the unmodulated oscillators frequency 
offset. Usually should this provide a component for the PI loop to pull 
it in, but if the steering range is not working, then that will stop it.

You should be able to diagnoze more with Lady Heather or other analysis 
of PLL state.

Cheers,
Magnus

On 2023-11-04 12:49, Erik Kaashoek via time-nuts wrote:
> While evaluating a Trimble Mini-T GPSDO it appears the 10 MHz output 
> frequency,  with loop control enabled, is jumping up/down every second 
> or so in the order of 5e-10Â  (green trace in attached image) resulting 
> in a 0.1 s ADEV of 8e-11.
> In holdover mode (no GPS input and no PPS output) the output frequency 
> has a much better short term stability  (purple trace in attached 
> image) resulting in a 0.1 s ADEV of 5e-12
> The time constant was set to 100s and the damping factor to 1. The 
> Mini-T was powered by a linear bench supply.
> The DOCXO used has reference has a 0.1s - 10 s ADEV of at most 5e-12.
> Is there anything in the tuning that can reduce the jumps or is this 
> to be expected from a Mini-T?
> Erik.
>
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