 Audio modulation of PLL with active loop filter
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GeordieBoy
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Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
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> ...tuned it to operate the coil at the high-frequency end of the resonance curve, mainly because a slightly capacitive load should fit the inverter better.
The load presented to the inverter by the TC when driven above it's natural frequency is net inductive. But, you are right that this region of operation is best for a the inverter. MOSFETs are happiest switching into loads that are at least slightly inductive.
If you configure the VCO to operate over a wider frequency range you are really increasing the gain of the VCO component. This has the effect of increasing the loop gain of the whole PLL. So it doesn't sound unreasonable that distortion might drop and performance would get better as you increase the loop gain. More loop gain means less steady-state error... *provided that the system doesn't go unstable*
Another thing to consider is that the VCO's control law is probably most linear when operated over a small slice of it's capable frequency range, than when modulated over it's entire range. Even most non-linear systems can be approximated to linear behaviour if the signal excursions are small enough.
For example: It's easy to make a singing Tesla Coil that is quiet and remarkably clear, but making one that is really loud and still excellent fidelity is significantly more challenging!
-Richie,
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