Re: Do you think NI can fix my PLL? -- Details

Joerg wrote:
Oddly, I haven't touched a 555 since I was a kid. I use a few HC221 and HC123 when I need a reliable one-shot and can tolerate a few % variation in timing mostly due to cap tempco. Never had a problem with these.


I haven't either. But I must confess that I also used the 121 in the old TTL days. Not anymore, and I hope that can be chalked up to teenage sins :-)

Well then I guess I'm still a sinner. But I do avoid the "pulses running all over the place" design method rightly castigated in AoE.

I was thinking of using LM2907 with one phase of the 500pp/rev encoder signal directly. This should be easy enough.


But mind it's linearity error. +/-1% isn't exactly something to write home about.

Oh yeah, that's not so hot.

Oh I see, by setting a constant pulse width for each encoder tick, then high speed->high duty cycle / low speed->low duty. So low-pass filtering that gives a smooth result.

I don't get why an integrator would be desired. That would accumulate V(t)*dt until saturation.

A low-pass filter != an integrator.


Sorry, I meant integrator plus S&H. A lowpass will still let substantial ripple pass. Ideally you'd want nice steps where each level corresponds to the velocity, like the output of a DAC. That makes filtering a lot easier. So an integrator would start integrating, then hold and store when the one-shot time is up. Now it is reset and waits for the next pulse. You will be up to one pulse behind in phase when doing that, something that may matter in a clutch application.

Pardon my density, but all I see happening here is that I integrate the pulses, which are always the same width.

Oh wait, I think I get it. I integrate the not-pulse time! So I get a measure of the period of each cycle, with a small error equal to the width of the one-shot pulses. Then hold the integrator value in the S&H over the duration of the pulses, while the integrator also resets.

Right?

Now I have to go see how the LM2907 works again. I didn't try to understand it actually the last time I used one. Just needed a quick hack.

Man, you guys must have a very comfy R&D budget over there.


Heh, heh. Yeah. There was actually a dearth of equipment when I started. I've built things up quite a bit over 7 years. But haven't asked for anything over about $13k. Still no decent spectrum analyzer, but at least a trio of good scopes, a logic analyzer which I haven't used yet :-( and 3 function gens including one of those new Tek models with the graphical screens. A nice new Kronhite 8-pole Bessel filter. Still a bit low on power supplies. They get borrowed almost as fast as I buy them.



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Christopher R. Carlen
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