A 32-bit DDS synthesizing at 1/5 Fs, yields a tuning resolution of ~ 1 ppb. So, I would imagine a slightly lower frequency is programmed into the DDS and the c-field is trimmed to yield a higher precision. If the new synthesized tone you wish to generate is an integer number of DDS codes you could start by assuming the c-field is trimmed to be on frequency, but if the new tone is a fractional number of 32-bit DDS codes you will have to manually trim if you want higher precision.
On Mon, Jan 9, 2017 at 4:48 AM, wb6bnq <wb6...@cox.net> wrote: > Hi Matt, > > Well, after rereading Mark’s paragraph in question, I think he did not > properly develop his complete thoughts. The first statement about the > Hydrogen Maser is absolute. The second statement is the one that is really > vague. The third statement is the clue taken with the fact that the first > sentence states the purpose of being used as a general purpose programmable > frequency synthesizer. > > So the answer is leaving the C-field pot untouched and taking the > difference between the “R” value and the “needed” input frequency > associated with the current “F” value to produce the original output > frequency gives a correction term to be applied to the “R” value to produce > the value you use to come up with the new “F” value used for determining > the wanted output signal. > > {After thought sentence} The above is not all that clear either, oh well. > Read on it becomes clearer. > > So lets go through the process and see if I can do this without screwing > up. The formula for the DDS chip to produce a desired output for a given > system clock frequency is the following : > > FTW (in decimal) = (Desired Output Frequency x 2^n) / SYSCLK > > However, the need is to determine what the proper input frequency is to > produce the 8388608 Hz with the given “F” value as the FTW (Frequency > Tuning Word). So the formula is the following: > > SYSCLK = (Desired Output Frequency x 2^n) / FTW (in decimal) > > In your reported numbers this produces : > first 2^32 = 4,294,967,296 times desired output of 8,388,608 Hz = > 36,028,797,018,963,968 > > SYSCLK = (Desired Output Frequency x 2^n) / FTW > (in decimal) > 50,255,055.809934059845495428970822 Hz = 36,028,797,018,963,968 / > 716918854 > > So the above 50 MHz number (SYSCLK) is the result of adjusting the C-field > so the unit is “ON” frequency for the expected 1 Hz output from the > factory. This is the SYSCLK value that should be used to find the new “F” > value for the DDS upon selecting a new output frequency such as 10 MHz (or > as close as possible without touching the C-field) if that is your wanted > output value. > > Actually, now that I have done the exercise, computing the delta between > the “R” value and the above 50 MHz makes no sense and serves no purpose. I > cannot stress enough. This is all predicated on not touching the C-field > adjustment and assuming the 1 Hz signal is precisely on frequency. > > This method does not give a lot of confidence as to preciseness. The real > value in these Rb units is they have a much lower drift rate than a > reasonably good quality Quartz oscillator. Typically less then parts in 10 > to the minus 10th or minus 11th per month. > > Bill....WB6BNQ > > > > > > Mathias Weyland wrote: > > 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/m >> ailman/listinfo/time-nuts >> and follow the instructions there. >> >> > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/m > ailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ 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.