[time-nuts] FE-5650A option 58 tuning word for 10 MHz output
Scott Stobbe
scott.j.stobbe at gmail.com
Mon Jan 9 10:40:25 EST 2017
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 <wb6bnq at 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
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