[time-nuts] FE-5650A option 58 tuning word for 10 MHz output

Bob Camp kb8tq at n1k.org
Mon Jan 2 12:58:55 EST 2017


> On Jan 1, 2017, at 7:31 PM, wb6bnq <wb6bnq at cox.net> wrote:
> Hello Mathias,
> I think you did not quite understand how the calibration process works with that unit.  The factory procedure is to set the pot to minimum and then bring the DDS up to the step that produces the closest frequency just below the wanted one and then use the C-field pot as the final tweak.
> The "R" value is composed of a number of factors that can only be determined after the Rb cell is made.  A number of variables, in the Rb cell itself, will determine the actual "ON" resonance frequency and that is what is programmed into the "R" number at the end of final assembly.
> That "R" number is actually the Rb frequency divided by 136 plus the upward fine tuning of the C-field pot.  See PDF page 16 & 17 of the attached PDF operational manual for the FEI-5650.  The only difference between all the variants comes after the Physics Package as shown on PDF page 16 block diagram.
> The "F" number is truly a 32 bit number, not just the first 8 digits but all 16 of them.
> Your problem is being able to have a frequency reference that is, obviously, way better then the Rb.  As Cesium and Hydrogen references are quite expensive that leaves just the GPS satellites as the only other reference available that will do the job.  Of course that entails more than just the GPS "timing" receiver as you will need some other equipment to help in the comparison process.  Also it takes time to do many small adjustments to achieve that final comparison.  It won't happen in just one day.
> It would probably have been better to purchase one of the Rb's that already output 10 MHz instead of hacking the option 58.  BUT that is just my opinion.

This heads you down another rabbit hole. Since the 10 MHz output units sell better than the PPS only versions, there is a cottage industry going to convert
one to the other. Parts are missing on the board to make a “proper” 10 MHz unit. The conversion often is a bit ugly. No matter how well it is done
electrically, bashing a hole in the magnetic shield case for the SMA is a really bad idea unless you have the proper gear to heat create it after you are done. 

So yes, the 10 MHz version is a better way to go, but only if it came from the factory as a 10 MHz unit …..


> Good luck,
> Bill Houlne....WB6BNQ
> Mathias Weyland wrote:
>> Hello guys
>> I'm new to this list. I got myself a FE-5650A Rubidium Standard off of ebay.
>> It's the "option 58" 1 pps output variant, hence I have to modify the tuning
>> word used in the DDS phase accumulator to get 10 MHz out. I found a vast amount
>> of awesome descriptions on how to do that on the web and in particular on this
>> list. One write-up that stood out was this one by Mark Sims:
>> http://www.mail-archive.com/time-nuts@febo.com/msg13486.html
>> I think I can pull this off since everything is documented so nicely. However,
>> I'm having trouble calculating the right tuning word and this is why: Mark notes
>> that the reference frequency reported by the unit is the one with the C-field
>> pot at the lowest frequency position. He gives a number of suggestions on how to
>> deal with that. Since I didn't get that hydrogen maser for Christmas, the best
>> approach seems to be "to calculate the true reference frequency from the saved
>> (minimum C-field) R=reference frequency and F=divisor word and use that value to
>> calculate divisor words." I don't understand how the saved minimum C-field
>> reference ties into this calculation.
>> My approach would have been to calculate the true reference frequency from the
>> saved divisor alone, ignoring the minimum C-field calculation. I don't see how
>> the minimum C-field reference frequency would help me since the C-field pot is
>> not in the min position anymore due to factory tweaking. To be specific, this is
>> what I would do:
>> The unit returns the following string upon 'S':
>> OK50255055.760840Hz F=2ABB5046B34A2E00
>> Now based on this, the tuning word should be coded in the first 8 characters, of
>> F, i.e. '2ABB5046'. I'm a bit confused about the remaining characters being
>> non-zero. Any documentation I came across has a number that ends in 8 zeroes...
>> In any case, 0x2ABB5046 is 716918854 in decimal and the resolution would
>> therefore be
>> 2^23 / 716918854 = approx. 0.0117 Hz which makes sense.
>> The physics package would then output a frequency of
>> f_ref = (2^23 / 716918854) * 2^32 = approx. 50255055.809934 Hz
>> This is higher than the reference given in the 'S' output, which is in line with
>> what Mark wrote. However, scaling this with the average correction factor he
>> gave yields
>> f_ref * 1.000000002150 = approx. 50255055.917982 Hz
>> Which is higher than what I would expect. Then again I'm not entirely sure what
>> I would expect because various errors add up in the above calculation. I'd be
>> interested in what people with more experience think about those results.
>> I would then use
>> M = 10000000/(2^23/716918854) = approx. 854633872.509003
>> to find the 10 MHz tuning word, which I would then round up (unfortunately it's
>> smack in the middle between two integers...) and convert to hex, yielding
>> 0x32F0AD91. This does in fact result in a 10.000 MHz output waveform but I have
>> no means to check its accuracy (yet?). I'd appreciate any hints about where
>> things could have potentially gone wrong, especially with respect to the minimum
>> C-field reference frequency that I ended up not using.
>> On a slightly related note, I have cooked up a small PCB with a local 5 V
>> regulator and status LEDs that mates with the amphenol connector used on this
>> standard. I have to complete the write-up on it and will probably put up a
>> video about the mod on my youtube channel; once this is done I'll be sitting on
>> 9 spare boards since I got 10 boards done. If there is interest, I could send
>> off the spares without profit, i.e. for about 5 bucks or so. I imagine this
>> could be of use to those who have the same standard. The board doesn't do
>> anything funky, it is just neat. In any case I'd like to ask if it would be OK
>> to formally place this offer on the list once I got everything ready.
>> Thanks a lot and best regards!
>> Matt
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