Only to note that I measured at the wrong place... the reference
frequency to the DDS is 20MHz, not 10MHz as I stated :)
Regards,
Javier, EA1CRB
El 28/01/2012 15:21, Javier Herrero escribió:
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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