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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Jan 2 17:02:54 EST 2017


Maybe a waterjet cutter would  imapct less on the shielding properties of 
tthe mumetal?

Bruce

On Monday, January 02, 2017 12:58:55 PM Bob Camp wrote:
> Hi
> 
> > 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 …..
> 
> Bob
> 
> > 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


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