[time-nuts] (Datum) FTS-4040A cesium, and STEL-1173 NCO chip

Magnus Danielson magnus at rubidium.dyndns.org
Tue Apr 30 13:59:57 UTC 2013


Hi Stu,

Got your private email, but it slipped through the cracks of time.

On 04/29/2013 07:51 AM, Stewart Cobb wrote:
> Guys,
>
> I'm working on a sick FTS-4040A cesium frequency reference, which is
> basically a box and power supply wrapped around the FTS-5045 cesium beam
> module.  (Datum bought FTS, then Symmetricom bought Datum, but this 1995
> device is too old to appear on the Symmetricom website.
>
> This was one of the first "digital" cesiums.  It uses a microcontroller to
> drive a frequency synthesizer to sample many different points in the cesium
> tube's frequency response.  Part of the synthesizer is a 12.6+ MHz DDS,
> built from a Stanford Telecom STEL-1173 digital numerically controlled
> oscillator (NCO) chip and an Analog Devices fast D/A chip.
> In my unit, the STEL chip seems to be broken.  All the inputs seem to be
> correct, but the output bits to the D/A are all continuously low, so there
> is no 12.6 MHz signal and thus no lock.  The unit also throws a fault code
> "11", which translates as "12.6 MHz signal power low", confirming this
> diagnosis.

The output drivers of the STEL-1173 seems to fade, so they are obviously 
not designed with sufficient margin.

> Web searching gives the impression that these STEL-1173 chips are known to
> be fragile.  There was a reference on time-nuts in 2006 to repairing a
> cesium by replacing the STEL chip, In 2009, a Steve Swift who read the 2006
> thread offered to sell some STEL chips to the thread originator, but the
> deal (if there was one) was conducted off-list and I can't find valid
> contact information for Steve Swift.

I have some STEL-1173 chips, but not of the same packaging as the one I 
got from another source which actually solved the issue. I do not know 
which is the correct one for the FTS-4040A caesium, and I have not 
cross-checked.

> So it's time to consult the hive mind:
>
> 1) Are these STEL-1173 NCO chips known to fail early?  Are there ways to
> "baby" them so they don't fail early?

It seems to be a common problem, their output degrades over time until 
bit after bit get stuck with interesting results to the loop until it 
completely looses lock.

> 2) Does anyone know a source for replacement STEL-1173 chips?

They are not easy to find.

> 3) Is there any detailed documentation (schematics, procedures, test
> points, etc) available for the FTS-5045 cesium module?  (I have the 4040A
> operators manual, but it doesn't show much detail.

I have not seen any.

> 4) Is there a way to tell whether the cesium beam tube itself is good,
> without the frequency synth working?  All other telemetry points (ion pump,
> electron multiplier, etc) seem to settle to nominal values after warmup, so
> I have some hope that the tube is still good.

I had about the same issue. Everything else looked good, so it was not 
until someone experienced helped me to check the STEL-1173 outputs.

> 5) Any other suggestions or hints for repairing this unit?

There are folks that knows them better than me. I has an experienced 
hand to assist me.

Cheers,
Magnus



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