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
--
------------------------------------------------------------------------
Javier Herrero
Chief Technology Officer EMAIL: [email protected]
HV Sistemas S.L. PHONE: +34 949 336 806
Los Charcones, 17 FAX: +34 949 336 792
19170 El Casar - Guadalajara - Spain WEB: http://www.hvsistemas.com
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