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Registered Member #1519
Joined: Thu Jun 05 2008, 12:30AM
Location:
Posts: 1
Previously Steve Conner stated that when using his PLL circuit to build a Singing Tesla Coil, locking to the input from the function generator is a different problem from tunning the feedback from the Tesla Coil. To let the PLL lock the function generator, we need an extra Resistor in series with the Capacitor in the loop filter.
I don't really understand why is it different?
The output from the Tesla Coil is a relatively nice sine wave as far as I have known. Isn't it so for the function generator too (which generates nice sine wave too)? The comparator compares the input with VCO's waveform and generate an output as square waveforms no matter what kind of input you put in, isn't it? So why having an input from the function generator would be a different case to the one from Tesla Coil feedback?
And, Which loop filter are we talking about (is it the one between the the comparator output and the input of VCO?)
Sorry for asking these stupid questions… I'll be really appreciated if anyone could answer my questions
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Unfortunately you'll need some knowledge of control theory and calculus to have any clue what I'm talking about, but I'll try anyway.
When tuning a Tesla coil, the output is always the same frequency as the input, because there is no other oscillator in the system than the VCO. A frequency change will take some time to propagate through the system because of the Q of the resonator, but it will always get there. It is impossible for the PLL to be out of lock in the classical sense that you might use when tuning up a radio receiver or whatever.
Hence, if we look at this as a "plant" in the terms of control theory, with its input being the VCO control voltage, and the output being the phase detector output voltage, the plant transfer function is a low-pass filter, whose break frequency depends on the resonator Q.
I can't remember if it's first-order or second-order, but it hardly matters. You can control this plant with an integrator, so my simple loop filter from the Mk1 works, and so does the op-amp integrator in my Mk2 driver.
Now, if we consider locking to a function generator, there are two oscillators that needn't be at the same frequency. A constant error in VCO control voltage will cause a ramp in the phase detector output, as the oscillators drift apart.
Therefore, the plant transfer function is now an integrator, and trying to use another integrator as the controller will violate the Nyquist criterion and make it unstable. The appropriate controller is now a lead-lag network (I think) or simply a proportional gain. So the Mk1 may or may not lock to a function generator, and the Mk2 certainly won't.
Sorry if you don't understand this, but I can't think of a simpler way to explain it. Maybe Richie can help :|
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Problem with Steve's PLL circuit locking
