[time-nuts] FE-5680A Question

[Bob Camp]

Hi

If you look at an LPRO or FRS or any of the other brands of Rb's their sweep
is very different. They don't spend very much time at the ends of the sweep
at all. It's pretty much a triangle wave sweep up and back sort of thing.
They don't seem to care weather they catch the lock going high to low or
going low to high. The rate of the sweep is *much* slower than what FE is
doing on these units. Since there isn't a "brain" in most of them, there's
no pattern watching stuff there. They just look for a lock signal (amplitude
out of the detector) and stop sweeping when they see it. 

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Magnus Danielson
Sent: Wednesday, February 15, 2012 3:34 PM
To: time-nuts at febo.com
Subject: Re: [time-nuts] FE-5680A Question

Hi Bob,

On 02/15/2012 06:12 PM, Bob Camp wrote:
> Hi
>
> Which still gets us back to - why the really odd sweep on the FE's?

They didn't look all that odd to me. As long as you do the sweeps every 
now and then you will lock up. I think you need to tell me what's so odd 
about them.

> and should you center the VCXO as a matter of routine maintenance?

Yes, you should. It's fairly trivial as you just adjust it so the CV is 
mid-scale. If you do it regularly enough, every other year or so, 
getting back on track will never be a problem.

Cheers,
Magnus

> Bob
>
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of Magnus Danielson
> Sent: Tuesday, February 14, 2012 6:09 PM
> To: time-nuts at febo.com
> 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
>
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