[time-nuts] FEC-5680A Won't Lock

[Bob Camp]

Hi

If you adjust the free running VCXO frequency, you need to monitor just where you set it to. Admittedly this is a bit like saying you need an accurate watch to set your accurate watch. You at least need a counter / standard you can trust to 0.1 ppm. That's usually do-able. If not there are easy ways to get there, so ask.

The VCXO ramps up and down while it tries to lock. It should swing a couple hundred hertz low of 10 MHz and at least 10 Hz high. The swing hangs at each end for a while, so it's not to hard to check. I suspect that anything more than 100 Hz high or less than 100 Hz low could be an issue. The main reason is that I've not seen one of these that's outside that range. 

If the range is OK, then you have a problem elsewhere. If so there are things that can be checked. First thing is to validate the VCXO.

Bob


On Apr 23, 2012, at 4:13 PM, CORNACCHIA wrote:

> I got a EFC-5680A it will not lock. Someone suggested I adjust C217 which worked, but it drifts slightly back and forth some times when started from a cold start it will go out of lock, and may or may not relock.
> Any suggestions will be appreciated.Thank You
> 
> ----- Original Message -----
> From: Magnus Danielson <magnus at rubidium.dyndns.org>
> To: time-nuts at febo.com
> Cc: 
> Sent: Monday, April 23, 2012 4:08:08 PM
> Subject: Re: [time-nuts] Antique Rubidium Standard Questions
> 
> Ed,
> 
> On 04/23/2012 07:54 AM, Ed Palmer wrote:
>> I'm playing with a Tracor 304-B Rubidium Standard from 1969. I'm using
>> it as a learning exercise to find out more about the guts of a Rubidium
>> standard and how it works.
>> 
>> This thing is a beast! Rack-mount, 3U high, 39 pounds (~18 kilos), 9
>> plug in circuit boards. The OCXO is an oddball frequency that is
>> multiplied directly to 6.8 GHz. There's no synthesizer in that chain. A
>> synthesizer is used to convert the oddball frequency to a 5 MHz output.
> 
> OK. A bit different to some of the rubidiums, but looking around it is not as uncommon as one might think. This technique is used in some of the FEI 5680 and also the later GPS rubidiums.
> 
>> It's sort of working. The error signal isn't up to spec, but it's strong
>> enough to give a stable lock although there's no trace of a second
>> harmonic signal. Allan Deviation is in the Xe-12 range from 1K to 10K
>> seconds. The OCXO has a not-yet-resolved issue that is probably
>> degrading the results.
> 
> What is the OCXO issue?
> 
>> The lock frequency suggests that the Rubidium cell has drifted down by
>> ~30ppt over the 40+ years since it was built. Is that reasonable? That's
>> much more drift than the specification states, but I doubt if the spec
>> was intended to be valid for 40 years!
>> 
>> Could the drift be at least partially responsible for the lack of second
>> harmonic? A message on the list (
>> http://www.febo.com/pipermail/time-nuts/2006-April/020562.html ) said
>> that you could peak the second harmonic by adjusting the cavity tuning.
>> If the cell and the cavity are out of sync would that kill the second
>> harmonic? How close to they have to be? If this thing has a cavity
>> tuning adjustment I haven't found it.
> 
> You should see second harmonics regardless, it's an effect of the dip itself. I would check if you can observe the second harmonics on the signal from the detector by some other means. If you seems to lock up but does not see second harmonics, it just could be something in the second harmonics detection which needs some LTC.
> 
> There are many sources for shifting the frequency, including the power supply (check voltage and clean-ness), the trimming of the C-field, the leakage of helium, resonator tuning, temperature trimming of lamp and resonator may shift amplitude and hence frequency through light pulling.
> 
> It would be good if you could hook up the modulation sine on X and return signal on Y on a scope, that would give you a clear display of the resonance dip.
> 
> Oh, and check if you have leakage problems around the integrator cap, that would also shift the frequency.
> 
> Cheers,
> Magnus
> 
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