[time-nuts] oscillator choice question

Bob Camp lists at rtty.us
Sun May 2 19:46:29 UTC 2010


A coupe of issues with mechanical servo tuning:

1) It wears out the tuning capacitor pretty fast. They are designed for a limited number of adjustments. They loosen up with a lot of tuning and this degrades their stability. 

2) It would be much easier to tear apart the mechanical tune OCXO and put in a tuning diode than to rig a thermally isolated high resolution servo stepper 

3) Mechanical tune arrangements normally  have backlash. That's not an issue as long as the servo only goes one way. It becomes a real pain to correct for each time you reverse direction. 

4) Making a mechanical setup with a minimum step below 1 ppt is going to be more than just a simple stepper. A gear chain based system will be pretty exciting to work up. Backlash in the gears will add to what ever you have in the tune it's self. 

5) The tuning on the OCXO may not be monotonic. That's especially true if you do indeed run the trimmer at a higher resolution than a normal human could adjust it. Tuning reversals tend to drive servo loops a bit crazy. 

None of that says that it can't be done. All it says is that it will be hard to do well. 

What kind of accuracy are you trying to obtain?


On May 2, 2010, at 3:18 PM, chris at yipyap.com wrote:

> That is a really cool picture.
> Can I be like you when I grow up?
> I've figured out which of these silvered modules
> in this Schomandl sig gen is the oscillator (the one that
> got warm).  I have to figure out if it is voltage adjustable
> in some way.
> Does anyone use mechanical adjustment with a servo,
> gear train and microcontroller?
> I did hear all of those good advisers telling me to
> buy the Thunderbolt.  But I already have these pieces
> so...
> --
> Chris
> w0ep
> Niels Lueddecke wrote:
>> You see? Don't do it, don't even think about starting.
>> Go buy a trimble thunderbolt, it will save you LOTS of time!
>> http://www.dulli.org/pics/20100502%20-%20Clock.jpg
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