[time-nuts] Lying to Lady Heather

Bob Camp lists at rtty.us
Tue May 11 00:22:08 UTC 2010


I've spent some time lying to Lady Heather, with some interesting results:

1) Classical control loop theory would suggest that damping should be fairly close to 1 for reasonable operation. Greater than 10 should be highly damped. Less than 0.1 should ring quite a bit. The TBolt doesn't seem to work this way. You can go to << 0.1 and still have a stable response to a step. You can go out to > 100 and not get a "lazy" response to a step. You can get to a point that it will ring, but it's down < 0.001. Obviously the TBolt and I read different books.

2) In a PID setup, you would have control on each coefficient. With the TBolt setup the "gain" seems to be the only way to impact the D part of the PID. You can watch the DAC output as you increase the gain. The swing of the DAC responding to the GPS pps jumping will decrease as you increase the gain number. It sounds backwards, but it makes sense. With "correct" gain, each time there is a step in the GPS PPS, the DAC immediately changes, no matter what the damping or time constant. Again, seems strange, but that's the way it works. 

3) Time Constant does seem to slow down the "integrator" in the PID. 

Why lie to Lady Heather?

On a very stable unit - watch the DAC voltage. It's climbing up and down like crazy on a second to second basis. It's reasonable to believe that the OCXO is more stable than GPS at one second. The DAC should be fairly quiet second to second. DAC LSB's are around 1 ppt. That's around (like a factor or 3 or 5) the stability of the OCXO at 1 second. One or two LSB per second might make sense. Anything 5 or 10X  than that is mostly noise that you simply don't need. 

Tell the unit enough lies (like gain = -60) and sure enough the DAC slows down and hops 1 LSB every so often. When GPS is stable it will stay in one state for 10's of seconds. Even with 10 ns hops in the GPS, it still stays down in the 1 to 2 LSB range. That's *got* to be more stable. 

Why is this good - nice as a frequency standard. 

Why this is bad - TBolt pps does not track GPS PPS very closely. Not good for E911 service. 

Bottom line - there's lots of ways to optimize a TBolt.


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