Wow, good work, a very fine reverse engineering attempt. On Sun, Jan 29, 2012 at 12:45 PM, Javier Herrero <jherr...@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 > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.