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

[Bob Camp]

Hi

That is indeed an odd update rate. The short term stability of the FE-5680 is in the parts in 10^-13 range once you get past 100 seconds. I would think updates of a DDS lsb or more would mess up the stability.

Of course if it is temperature correction, it may still be in the noise.

Bob



On Jan 29, 2012, at 8:22 AM, Javier Herrero <jherrero at hvsistemas.es> wrote:

> El 29/01/2012 13:57, Magnus Danielson escribió:
>> Hi Javier,
>> 
>> On 01/29/2012 12:45 PM, Javier Herrero 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.
>> 
>> Good work Javier!
>> 
>> It also makes perfect sense from the hardware architecture of these newer 5680A. It's nothing wrong with it, it's just different.
>> 
>> Somebody put this up on the wiki.
>> 
>> I will receive new 5680A when I pickup the packet on Monday, I only have an older variant with serial port and DDS output.
>> 
>> Cheers,
>> Magnus
>> 
> I've just gathered the following message from the unit using the serial tool:
> 
> Cmd 0x22 0x0D byte reply: [22] [0D] [00] [2F] [44] [02] [62] [EF] [43] [FD] [CC] [87] [3E]
> Cmd 0x22 ASCII (.): D.b.C...
> Cmd 0x22 ASCII ( ): D b C
> 
> It seems that these are the current DDS values. Since now they are a slightly different of what I've logged before, I suppose that they are updated and are not some stored start-up values. So another reverse-engineered commad :)
> 
> I'm currently logging with the logic analyzer the SPI activity to the DDS, it seems to be updated at a somewhat outrangeous interval of 671.5 seconds or something like that. It will take several hours to fill the analyzer memory :)
> 
> Regards,
> 
> Javier, EA1CRB
> 
> 
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