[time-nuts] FE-.5680A trimming resolution
jherrero at hvsistemas.es
Sun Jan 29 11:45:52 UTC 2012
As it has been discussed in the past days, the architecture of the newer
FE-5680A that has been recently purchased by a lot of us does not led to
a trimming resolution through the serial port of 1.7854e-7Hz, but rather
leds to think that the trimming resolution is in fact 6.80789e-6Hz
(relative frequency resolution 6.807e-13).
I've hang a logic analyzer to the DDS SPI bus, and an SPI message
appears inmediately after updating the offset through the serial port.
I've found that the DDS is programmed in two frequencies, separated
1400Hz near exactly, for each serial port offset setting, and that one
bit increment in the serial port offset setting is translated to a
one-bit increment for both DDS frequencies. The DDS frequencies are
alternated at 416.6666666Hz rate through FSELECT pin, at an invariable
50% duty cycle, presumably to perform synchronous detection in the same
way as explained in the FRS-C manual.
For example, the following data has been gathered:
Serial offset 00 00 00 00
DDS A word: 44 02 62 F6 = 1141007094 = 5 313 228.32219 Hz
DDS B word: 43 FD CC 8E = 1140706446 = 5 311 828.32085 Hz
Serial offset 00 00 00 01
DDS A word: 44 02 62 F7 = 1141007095 = 5 313 228.32685 Hz
DDS B word: 43 FD CC 8F = 1140706447 = 5 311 828.32550 Hz
Serial offset 00 00 00 02
DDS A word: 44 02 62 F8 = 1141007096 = 5 313 228.33151 Hz
DDS B word: 43 FD CC A0 = 1140706448 = 5 311 828.33016 Hz
I've seen that these values seems to vary slightly from time to time in
the less significative digits, I've been then change in the order of 2-3
units from one data take to a different one minutes later. I've checked
that the unit updates each several seconds the DDS control words, and
I've seen changes in the lower significant bits at minutes intervals,
although most of the times, same previous words are sent. I suspect this
is some form of unit self-compensation, perhaps to temperature changes.
Last, I've sent an offset of 1468879 units, that shoudl correspond to a
10Hz frequency change assuming a trimming resolution of 6.90789e-6Hz,
and after a temporary unit unlock, it has locked exactly at 10 000
010.000 Hz. So I can conclude that these units are not fully compliant
with the manual we are handling, and that the trimming resolution is
6.80789e-6Hz and not the stated 1.7854e-7Hz.
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