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Old 09-16-2014, 09:33 PM   #1116 (permalink)
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Ok, I've got the new version working. It's actually getting simpler!
I've been able to get rid of one level of filtering by optimising the first filter. So now, I'm down to 4 IC's to drive the resolver, instead of 5. The final circuit will use two.

There isn't as much gain as the previous version, it now puts out 8.5V p-p. This is due to my limited selection of resistors. I suppose by spending a bit more time I could improve that by combining resistors.

I tested it, and it works fine with the resolver; the power amp seemed to run a little bit cooler, I have no idea why.

One problem I've noticed is the resolver puts out a bunch of noise during the 0 crossovers. This can be reduced by changing the carrier frequency.

One neat feature of my 'scope is that is has a spectrum analyzer. Especially when you are dealing with sine waves or square waves - ESPECIALLY square waves - there will be certain frequencies that have resonant characteristics. This is the EMI noise that can wreak all sorts of havok. This circuit, when combined with the resolver, showed an especially good characteristic at 62.5kHz and 83kHz. There was the primary (desired) frequency peak and almost no others. It was one of the cleanest plots I've seen!

Is there any easy way we can put in 62.5kHz, rather than 50kHz? I'm going to try to figure out why this happened, so maybe I can tune this feature for 50kHz... Otherwise, I may have to add another level of filtering to eliminate the higher than 75kHz noise. Adding another set of op-amps is really NBD. However, you always compromise phase or have to add more gain later.

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MPaulHolmes (09-17-2014)