[time-nuts] FE-5680A Question

[Magnus Danielson]

On 02/14/2012 12:11 PM, Rex wrote:
> The Efrotoms (FRS-C. Lpro) find the lock by modulating the microwave
> frequency with an audio signal (127 Hz if I remember right) which causes
> the light sense modulated signal to double in frequency when centered on
> the hyperfine frequency. See the manuals for nice description. The 5680A
> seems to accomplish the same thing by stepping the frequency +/- 700 Hz
> rather than mixing in modulation. Never saw any documentation on that,
> but seems to be implied by the great hacking Javier Herrero has done on
> the loop frequencies.
>
> Seems to me that finding lock, that is finding the dip, may be a bit
> harder with the stepping than with the modulation. Maybe the observed
> drop in frequency during start up is part of the algorithm to walk the
> stepped frequency to center on the hyperfine light transmission dip.

The modulation (may it be sine or square-wave) is about tracking the 
absorption dip. However, the initial frequency error of the OCXO can be 
so large that you don't even hit the dip at all. So, to achieve lock the 
non-locked state is detected by lack of response, and a sweeping action 
of the OCXO is done. If sufficient signal is detected, then the sweeping 
action is stopped and the loop is steered by the detected response which 
acts like a frequency locked loop. A little to much onto either side and 
a positive or negative response is given. When in the middle a maximum 
is achieved on the second harmonic.

So, the initial large end-to-end sweeps is about to try to lock the OCXO 
onto the rubidium reference. That will fail until the OCXO has heated up 
enough and also the rubidium is heated enough.

For some rubidiums you may need to hand-trim the oscillator in order to 
achieve lock, since their oscillators (crystals and tuning-cap) has 
wandered to far astray from locking-range.

Rubidiums is a bit intricate, but the pieces fall together eventually.

Cheers,
Magnus