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

Bob Camp lists at rtty.us
Sun Jan 29 15:46:58 EST 2012


Hi

A roughly 30 second update rate makes a lot of sense. That would keep the update steps buried in the short term stability "noise".

Good news on the serial update timing and it's stability. Sounds like you could do several updates a second. That's plenty fast enough. 

Bob



On Jan 29, 2012, at 1:19 PM, Javier Herrero <jherrero at hvsistemas.es> wrote:

> El 29/01/2012 14:45, Bob Camp escribió:
>> Hi
>> 
>> Very interesting.
>> 
>> It sounds like dithering would be needed to get down to parts in 10^-14. If we do that from an external device (PC / PIC / FPGA / whatever) it would be useful to know the delay between the serial command and the DDS update. The more variable the delay, the less accurate the dither.
>> 
>> Bob
>> 
>> 
> Since I've all the things powered on the bench table, it is easy to measure: 21ms from the start of the serial message, to the end of the last DDS programming word. I've repeated several times, the time does not seem to have submitted to latencies, it is very constant.
> 
> The automatic update rate of the DDS in the unit is somewhat around 28.5 seconds (I misread an inter-word delay of 671.5us as a 671.5 seconds interval). I'm trying to get a long capture from the logic analyzer to try to plot the DDS programming variations, but then I need to post-process it since the analyzer has no SPI decoding so I'm gathering the bit streams.
> 
> My next intention is to replace the OCXO in one of my Thunderbolts with a Rb and use a small microcontroller to get the voltage correction from the Thunderbolt through the serial port and convert to a 5680A offset. I probably will use a Luminary since I've some small evaluation boards around, and I only have to add a MAX3232-class interface IC for the serial ports.
> 
> Now... my question is to decide which of the two Thunderbolt will be converted to Franken-Thunderbolt. I've bee monitoring both from some time, feeding them from same GPS signal, and both looks quite the same (I was trying to see if one of them has a worse OCXO than the other. One is from the past group purchase, and the other has been purchased not long ago - but both are very similar, with same firmware and the Trimble OCXO, and monitored with Lady Heather they are really very similar). Next... clean-up the Rb output with an OCXO (the one from the Trimble or a 10811A I've around). But this depends on other things I must do... with more priority :)
> 
> Regards,
> 
> Javier
> 
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