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
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