Hi
> On Jan 1, 2017, at 7:31 PM, wb6bnq <[email protected]> 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/[email protected]/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 >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> > > <FE5650A_Operation-Maint.pdf>_______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
