Motor PWM causes errors on quadrature encoder

I've run into another problem caused by my robot's head. Right around the time I added this, one of my wheel encoders started acting up, reporting
less movement than the wheel actually did, so that wheel run full speed. After much hair pulling, I determined simply unplugging the head motor makes the quadrature work fine.
I spent last night tracing the encoder paths with my oscilloscope, and didn't see anything unusual. I can't see a difference in the signal when the head motor is connected or not, nor between the encoder that works or doesn't.
The encoder is being read in a half-quadrature fashion. Each encoder uses one interrupt on an ATmega16, with the other read on a normal input pin.
While I'm sure I can just add capacitors until it works fairly quickly, I would like to actually find what is messing things up, as a learning experience if nothing else. Can anyone suggest what a next testing step might be (IE, using a freq/dev that is x% of the PWM frequency), or a site with such a methodology --- or "Give up and install caps" :-)
-Chris
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Chris Candreva -- snipped-for-privacy@westnet.com -- (914) 967-7816
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what type of motor is in head? do you have voltage drop in this motor when it is plugged in (but not "activated")?
Rich
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snipped-for-privacy@gmail.com wrote: : what type of motor is in head?
DC Gear motor
: do you have voltage drop in this motor : when it is plugged in (but not "activated")?
Ooops -- edited this fact out. The motor is being controlled with locked antiphase PWM, so there is a pulse generated even when the motor isn't active.
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The problem was twofold. First, I wasn't useing the scope correctly. As soon as I remembered that AC means "The AC part of DC" and not house current, I saw the +-.5v spikes being introduced on the +5 of the board.
Second -- for some reason I had used .01uf bypass caps instead of .1uf, and they were useless. .1 work, .47 work even better.
I've spent too much time lately looking at the big changes, I forgot how to think small. (Mostly I use my scope as for working on vector-based arcade games, either in XY mode as a vector monitor, or to trace those waveforms, which are about +-20v. I was still thinking big changes, not small ones.)
Thanks to Larry Barello for getting me pointed in the right direction.
-Chris
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