[time-nuts] Fwd: QL-E0pfx7I3.pdf

Peter Bell bell.peter at gmail.com
Mon Oct 31 23:22:23 EDT 2011


Hi, Magnus

I assume the cell temperature is correct - if I swap over the
absorption cell (just the glass bits in a piece of heatshrink) from
the working unit, it locks - the temperature of the outside shield is
about 62c - the RF looks to be on the right frequency based both of
the fact you can swap over the snap diode / filter assembly without
problems and on the 6.834GHz leaking from the lens.

With the bad cell, there is no detectable modulation from the
photocell - the VCXO is sweeping up and down and going through the
correct point, but nothing is detected even if you set the scope
trigger just above the noise level and leave it for 30 minutes.  With
the good cell, it behaves pretty much the way I would expect - once
the VCXO gets in range the fundamental signal appears (416.6... Hz on
these units) and then the 833Hz comes up, finally stabilizing as a
clean sine wave at the 2nd harmonic.

Since this is a comparison test between the two sets of cells and
everything else (lamp / heater / photocell / housing) is the same, it
strongly suggests that the fault is in the cell itself.  I'm just
finding this strange, since the only bad absorption cells I've ever
seen before were obviously broken just by visual inspection - and this
one looks fine.  It is rather amusing to compare the size of this cell
to the ones that were in the old Efratom oscillators, though.

Regards,

Pete



On Tue, Nov 1, 2011 at 9:19 AM, Magnus Danielson
<magnus at rubidium.dyndns.org> wrote:
> On 10/31/2011 04:57 PM, Peter Bell wrote:
>>
>> Thank you Gerald - it does look very similar - the only obvious
>> difference is that the FE-5680A has an LM2941 subregulator on the lamp
>> supply that does not appear to be in the 5650 - hopefully I won't have
>> to remove the lamp, since it sounds (and looks) like a rather fiddly
>> job.  On both these units, the photocell signal is good if you put the
>> working Rb absorption cell into them, so I think the lamps are
>> probably OK.
>>
>> I tired reheating the absorption cell to get the Rb back into the
>> nipple, but it doesn't seem to have made any difference - still no
>> lock...
>
> What's your
>
> Lamp intensity
> Harmonic strength
> 2nd Harmonic strength
>
> If you have lamp intensity but no harmonic, then you can have misalignment
> between the physics package and your oscillator. Tuning the oscillator may
> help, but it could just as well be that the temperature of the cells are
> off.
>
> Cheers,
> Magnus
>
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