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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Hi Plasma
In a PLL driven Tesla coil, the output from the secondary coil is always the same frequency as the drive signal. (How could it not be?)
Therefore a PLL is always "locked" in the usual sense of the word, and so the usual lock detector circuits, which look for a difference in frequency, do nothing.
In my Mk2 PLL driver, I added LEDs for "Frequency too low" and "Frequency too high". When these are both off, the circuit is what you might call "locked". LED3 and LED4 in this schematic:
But lock detection in a PLL SSTC is generally quite a hard problem. The above circuit only works because of the integrator, turning it into a Type 2 loop, which means that the integrator will hit one of its end stops (the LEDs) if it can't find a stable working point with zero phase error. Which I guess is what you and I mean when we say "locked".
Registered Member #1376
Joined: Wed Mar 05 2008, 08:31AM
Location:
Posts: 49
Hello,
Many thanks for your prompt reply Steve, the information is highly appreciated!
I have another question about feedback indication. There is a circuit attached below that should work in conjunction with the 4046 pll chip.
Apparently when the feedback coil is out of resonance the output of the Op Amp is high and is sent to pin 14 of the 4046 PLL chip. This lets the PLL circuit know that resonance has not yet been found.
When resonance is detected, the output of the Op Amp will have a 50% square wave at the same frequency indicating that the resonant frequency has been detected.
The second picture attached is a simulated oscilloscope image of the circuit. The person running these tests used MultiSim 11 Pro Edition.
The left image is the resonant signal from the feedback coil. The top waveform is the output signal from the Op Amp and the bottom waveform is the resonant feedback coil signal.
The image on the right is the non resonant signal. The top is the non-resonant output of the Op Amp and the bottom is just a sqaure wave from the feedback coil.
So basicaly the circuit is in resonance when the input signal to the Op Amp is Bipolar out of resonance when the signal is Unipolar.
Registered Member #1376
Joined: Wed Mar 05 2008, 08:31AM
Location:
Posts: 49
Hello,
I have attached the circuit diagram to include the Resonance Indicator Circuit. Would this work or are there obvious problems with the circuit, perhaps an unstable signal on pin 14?
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PLL lock indicator?
