Hi Which still gets us back to - why the really odd sweep on the FE's? and should you center the VCXO as a matter of routine maintenance?
Bob -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Magnus Danielson Sent: Tuesday, February 14, 2012 6:09 PM To: [email protected] Subject: Re: [time-nuts] FE-5680A Question 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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
