Re: [time-nuts] FE-5650A option 58 tuning word for 10 MHz output
On 2017-01-04 10:16, wb6bnq wrote: Hello Bill Thanks for re-iterating over this. Yes, I do think the outer can covering is a MU-metal shield. The bottom plate where the connector is located is not. That is reassuring thank you! I know the calculator that comes with Windows XP will produce the correct mathematical results. I think the Windows version 7 does the same. I do not have Windows 10 and therefore cannot address that one, if there is one. Even EXCEL spreadsheet does not do the job properly. So use caution with your calculations. OK noted. The original calculations were done with a calculator that was designed for high precision (in the floating point sense). I did re-run the calculations in windows calculator for kicks, and the result is different, although the difference is too small to have an effect on the integer phase accumulator increment (fingers crossed!) However, with all that said, it means nothing if you cannot properly measure the final value against an external standard of greater accuracy. Acquiring the equipment to do the external measurements is where the real cost comes in. Yes, I think that I am aware of that and I have the opportunity to do that with somebody else's gear. I also understand that I'm supposed to do that on a regular basis. Hopefully the above helps to clear up your query ? Yes most of it is clear, thank you. Unfortunately though my original question, i.e. how to incorporate the reported R value into the calculation, is still kind of open. I'm still convinced that what I did, i.e. not taking the R number into account, is no worse than using it. But this might be incorrect, and if it is I'd like to know why. Regards and thanks again Matt ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] FE-5650A option 58 tuning word for 10 MHz output
On 2017-01-04 11:08, Attila Kinali wrote: Hoi Attila So it's the usual curiosity and the need to have a reference for your 23cm radios? Yes exactly. Advantage of an GPSDO over an Rb is that you know it's on frequency, while with an Rb it can be off without you knowing it, if the electronics or the physics package is defect. This is in line with what I've read. Maybe the decision to go for the Rb was not the best one but I don't think that it's that bad either. I was offered the opportunity to calibrate my modified Rb standard against a fellow ham's two 10 MHz standards and his GPSDO. BTW: there is a whole new way of looking at statistics of time series in the field of time and frequency control, which you might find interesting. The NIST Technote 1337[1] and NIST special publication 1065[2] give a nice overview of the methods applied. This is very interesting thank you for sharing those! Unfortunately, it's not that easy :-) I don't know the exact construction of the FE-5650, but usually the physics package is just a normal aluminium microwave cavity, without shielding. The mu-metal shield is usually the housing of the whole device itself. Hm I thought I had done my homework on this one but I never have issues with being proven wrong. For what it's worth, here are two pictures of the unit in question: http://2.bp.blogspot.com/_8uoOi3ZInLQ/SdZfVfpcBaI/BB8/YvZ2-l0GhMI/s400/DSCN4592.JPG http://www.dty.sakura.ne.jp/keisoku/rubidium/FE5650-2.jpg The shiny face plate is where I would be drilling. For what it's worth, it is very light, nonmagnetic, does not look like the mu-metal I've seen in disassembled hard disks and very much looks like machined aluminum... Would a compromised mu-metal shield show during calibration if the standard was physically rotated? Best regards and thank you Matt ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] FE-5650A option 58 tuning word for 10 MHz output
On 2017-01-02 12:18, Attila Kinali wrote: Attila! Nice to see you around here ond au der es guez nois! May I ask what you want to achieve? Resp. what you need a 10MHz reference for? I've always been wondering about those devices, I guess out of pure curiosity. Once I learned that those were available for cheap I knew I had to get one. But there is a secondary goal, which is to have a standard around to check my other gear against every now and then. Lately, I've been building and repairing 1.3 GHz radios and lost some time because I did not realize how much off both of my service monitors where. I could get away with a properly calibrated OCXO but the Rb was so much more sexy. So for the most part I need the 10 MHz for my own entertainment. :-) In general I would agree with Bill Houlne's comment that it's probably easier to use a different reference that already has a 10MHz output. Also keep in mind, that an output that has been designed as an PPS output might not work as well as an output for 10MHz. Yes I agree with that, but I'd also say that based on the reading I have done, it would be foolish to assume to get something that is guaranteed to work anyway. With all the things that could go wrong in general (dead unit, worn-out discharge lamp, drifted sweeping boundaries of the oscillator etc.), the 1 pps to 10 MHz conversion for this particular model(!) seems fairly straight forward. In fact the only issue I've been facing so far is the one that I'm trying to resolve in this thread. And even if this cannot be resolved, the converted unit is still suitable for my application. On the output issue you are rising: This particular model has a DDS that takes a 50ish MHz reference and synthesizes another frequency which is a power of 2. A coax cable is taking that to a divider board where it's brought down to 1 Hz. This is all happening outside of the physics package. The plan is to reconfigure the DDS to synthesize 10 MHz instead and bring out that coax. I would have to open a hole on the face plate to do that, but I'm fairly confident that this outer enclosure does not contribute to the actual magnetic shielding of the chamber... All modifications are strictly outside of the physics package (i.e. no water jet cutting...) I am not master of desaster on this list, but it's usually ok to offer this kind of stuff, as long as it is time-nut related. OK thank you and hope to meet you soon! Matt ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] FE-5650A option 58 tuning word for 10 MHz output
On 2017-01-02 01:31, wb6bnq wrote: Hello Bill Thanks for your response and the pdf manual. There's lots of good information in there. You are certainly right in that there's a few things that I have not understood. Unfortunately this has not changed after having read your response: 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. In the comprehensive post[*] that I quote in my original request, Mark states that "THE R= REFERENCE FREQUENCY IS NOT THE FREQUENCY THAT THE PHYSICS PACKAGE IS SHIPPED TUNED TO!!! It is the frequency produced by the physics package at the minimum setting of the C-field potentiometer." This seems to be contradictory to what you wrote. Either the R value is determined at the C-field pot minimum frequency position as Mark stated, in which case it does not contain the upward C-field tweaking. Or it is determined after the final tweaking as you say. Either way I don't understand how to incorporate the R reference frequency into my calculations, which is what Mark seems to be suggesting. The "F" number is truly a 32 bit number, not just the first 8 digits but all 16 of them. The F number has 16 hex digits. Since each of those digits represents a nibble (i.e. half a byte, 4 bits), I end up with 16*4 = 64 bits. Not just 32. The first 8 digits of the F number however represent a 32 bit number, which is also the size of the the phase accumulator in the AD9830 which makes sense in my opinion. I think I absolutely failed to grasp what you're trying to tell me here :-( Re-reading the paragraph in [*] that starts with "You can set the divisor" did clear up something for me though -- looks like there's nothing weird with the F-number stored in my unit; the last byte is indeed 0x00. I still don't quite understand why this parameter is potentially stored at a higher precision than what the DDS can deliver. 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. You are probably right on that, yet getting any of those surplus standards off of ebay seems to be a bit of a hit-or-miss thing anyway. At least I get the learning experience, that's gotta count for something :-). Thanks again for your response and best regards Matt ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
[time-nuts] FE-5650A option 58 tuning word for 10 MHz output
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 * 1.2150 = 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 = 1000/(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 -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
