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Old 09-21-2014, 08:04 PM   #1137 (permalink)
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I've made a bit of progress with the resolver stuff. Thank you very much, Dave, for the capacitance tips. Between tuning the circuit and adding some filter and powerbuffer capacitors, I've got a nice clean output. Oh, and another thing - I was looking into some timer IC's, and many only seem to run at much less than 5V - more like 3.6V. This actually turned out to be beneficial, because I was trying to run the system too close to the absolute limit of the op-amps. By reducing the input to 3.6V, they produced a much cleaner, less distorted output.

By the way, I was looking into the attiny IC you recommended for a 50kHz oscillator - are you referring to the ATtiny4 from Atmel? If this is it, it's basically a very basic microcontroller that could be programmed for a speed.

Another possibility, although not nearly as cheap would be the LTC6907 from Linear Technologies:
It's about $3.00 in qty 1 from digikey, but it's a dedicated timer chip, designed for this job and identical to the ATtiny in size. All it needs is a precision resistor (about $1.00) to set the speed. Perhaps for the "don't need to mess with it" factor, the extra money is worth it.

Oh, yeah - another unexpected thing - my signal generator is a source of the noise. Simply generating square waves by definition creates a lot of noise. However, my circuit filters most of it, except for the spikes at 100kHz. Tuning for a specific "clean" speed like 62.5kHz may not be necessary, because the timer chip will have its own noise issues and because filtering out 100kHz is easy and already done.

I've also got the input circuit almost working. The first test produced a nice sinewave output at 2X the rotor rpm. The main issues right now are better synchronizing with the carrier frequency and cleaning up jittery peaks. The zero crossings are very cean.
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