How do servos work?

I'm wondering if anybody here has a link to a website that explains *in
detail* how servos (like Hitec and Futaba) work? I found a few websites
but none of them really dig into the logic of the servo controller.

Here's why I'm asking: when I was first exposed to servos (in an EE
class), I thought that the control was based on the length of the duty
cycle. It wasn't until lurking around here for a while (yay ivy league
education) that I finally realized that it was actually based only on
the length of the positive pulse, hence the name.

So if you send servo pulses at 25khz instead of 50khz, you substantially
improve the torque? This leads to the question, when manufacturers quote
torque in the tech specs, what refresh rate are they generally using?
And also, what is the relationship between refresh rate and torque. If I
double the refresh rate, does that double torque? Or is it log(2)?

Finally, if this is what PWM truly refers to, then when we use a similar
technique on a DC motor (varying the duty cycle by keeping a constant
refresh rate and changing the pulse widths), is that still considered
PWM? Because its really more like duty-cycle modulation, with true PWM
seeming to refer to the scheme that servos use.

So if my understanding is right, how do servos actually work? I
understand that they compare the desired signal to the actual signal,
and I think the response is proportional (?) to the error signal. But
how often does this compare-respond cycle operate...just when positive
pulses are received? If so, then 50khz seems very low to provide the
torque that I know servos have. Thanks for any light you can shed--

Mark

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mehaase(at)sas(dot)upenn(dot)edu