Home |
Search |
Today's Posts |
|
#1
|
|||
|
|||
Time and Frequency References
In article ,
Rob wrote: I think the problem is that our GPSDO has a 16-bit DAC and it is dithering the digital value to obtain the correct frequency. So when the correct DAC value would be 32000.2 it will do 32000 for 8 seconds then 32001 for 2 seconds, obtaining a long-term average that is quite good, but a wobble with 10-second period as well. Could you modify it, to "scale down" the analog control voltage coming out of the DAC to a smaller range which "trims" around a center voltage that you set manually? I did something of that sort when building a VE2ZAZ GPSDO. This has only a 10-bit DAC (or 12, I can't remember for sure) which is dithered fairly rapidly to add a couple of additional bits of precision. I low-pass-filtered the DAC output pretty heavily and then used some resistor scaling to reduce its effect to a few percent of the total 5-volt control range that my OCVCXO allows. The "coarse" setting is via an external potentiometer, driven from a good 5-volt precision reference. In addition to the oven for the crystal, I added a separate thermistor/heater board which levels out the temperature in the whole enclosure, which I then insulated pretty heavily. This approach wouldn't change the period of the wobble but would reduce its magnitude by whatever divide-down ratio you chose to implement. |
#2
|
|||
|
|||
Time and Frequency References
Dave Platt wrote:
In article , Rob wrote: I think the problem is that our GPSDO has a 16-bit DAC and it is dithering the digital value to obtain the correct frequency. So when the correct DAC value would be 32000.2 it will do 32000 for 8 seconds then 32001 for 2 seconds, obtaining a long-term average that is quite good, but a wobble with 10-second period as well. Could you modify it, to "scale down" the analog control voltage coming out of the DAC to a smaller range which "trims" around a center voltage that you set manually? This is kind of risky. It may work for some of the units and not for others. They tend to have a systematic drift, the alignment procedure is to turn a trimmer in the oscillator so the DAC outputs a value close to 32768 and then it tends to walk one way. When it approaches the end of the range (say 5000 or 65000) the center has te be re-aligned. As original, they drift about 10000 clicks in a year. Narrowing the tuning range could make it drift too quickly to be practical. As I said before, the first thing to do is to check the Thunderbolt in practice. It should be much better than the aging Datums. (they are so old that they do not even handle GPS weeknumber rollover which first occurred in august 1999) |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
SWL -newbies- High Frequency {Shortwave} Time and Frequency StandardRadio Stations | Shortwave | |||
Deutsche Welle Time and Frequency Changes | Shortwave | |||
Deutsche Welle Time and Frequency Changes | Shortwave | |||
Stair's frequency and time. | Shortwave | |||
FS: FTS 5060 Cesium Frequency and Time Standard | Boatanchors |