[time-nuts] FE-.5680A trimming resolution

Azelio Boriani azelio.boriani at screen.it
Sun Jan 29 07:24:25 EST 2012


Wow, good work, a very fine reverse engineering attempt.

On Sun, Jan 29, 2012 at 12:45 PM, Javier Herrero <jherrero at hvsistemas.es>wrote:

> Hello,
>
> 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.
>
> Best regards,
>
> Javier, EA1CRB
>
>
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