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
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