[time-nuts] FE-5680A FAQ update: question about frequency synthesizer architecture

[Javier Herrero]

El 27/01/2012 19:27, beale escribió:
> I added a bit to the "electronics" section of the FE-5680A FAQ as below.
> http://www.ko4bb.com/dokuwiki/doku.php?id=precision_timing:fe5680a_faq#electronic
>
> (Note- until today, I had the 8 and 6 digits transposed, calling it the fe5860a. But no one noticed :-)
>
> The updated section is below. I measured the 20 MHz input and 5.3 MHz output of the DDS, but I'm puzzled by how the tuning resolution (4.6 mHz) of the DDS output is divided by such a large factor to achieve 0.18 uHz resolution at the final 10 MHz output. Can any frequency synthesizer gurus explain how this is done?
The frequencies inside the unit are quite similar to those found in the 
FRS-C, 60 and 5.3125MHz. The FRS-C excites the cavity at 60MHz x 114 - 
5.3125MHz = 6.8346875GHz (a bit over the 6.834682608GHz Rb natural 
resonance - so I suppose that the resonance is driven to 6.8346875GHz 
using the C-Field), so I understand that the FE-5680A operates in the 
same way. Since in the multiplication process the 60MHz frequency is 
multiplied by 114 and the 5.3125MHz only by one, 1Hz offset in the 
5.3125MHz frequency will need 1/114Hz offset in the 60MHz signal to 
obtain the same resonant frequency.

I've checked that the DDS is driven by 10MHz, not 20MHz (I've just 
checked it), so the 5.3125MHz is probably an image and not a fundamental 
DDS output. Hence, the minimum DDS step is 2.23mHz. A change of 2.23mHz 
in the 5.3125MHz frequency is compensated by an approximately 20.45uHz 
change at the 60MHz frequency, and so, a 3.41uHz at the 10MHz output, 
i.e. one part in 3.41^-13. This lets to a factor of 19 between the 
adjustment attainable directly by modifiying a 1LSB and the claimed 
1.7854^-14 adjustment. Probably this is done by modifying the duty cycle 
of the FSELECT signal. I suspect that the 416.6666667 signal at FSELECT 
is used to produce the modulation on the cavity excitation to perform a 
synchronous detection (the same way it is done in the FRS-C at 127Hz), 
to obtain a null, so the null can be slightly "moved" by variying the 
duty cycle at FSELECT.

I will try to play a bit more this evening :)

Best regards,

Javier



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Javier Herrero
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