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Registered Member #1623
Joined: Tue Aug 05 2008, 03:31PM
Location: The Netherlands
Posts: 39
Hi,
I'm building a SSTC with Steve Conners PLL driver, but there are some problems with the output of the PLL. For some reason the PLLs duty cycle changes when there is feedback from the coil. When no power is going to the coil the output is a nice 50-50 duty cycle but when I crack up the voltage on the coil the output shifts to 60-40 or even 70-30. I've tried to put a signal generator on the PLL circuit and if I do the pll will follow just fine with no problems. Now from what I know of pll's is that they try to keep a nice Vdd/2 Voltage on the input of the filter (am I right?), when the coil is out of tune there is a massive 10V on that point (12V supply). I've made some images from my scope showing the effect: Normal functioning Odd duty cycle (something like 60-40) with a coil voltage of only 15V Odd dutycyle and antenna feedback (I've tried to use feedback from a CT too but the same thing happens) together. And even though the duty cycle is off my coil does produce a little output, only the ringing on the gates is massive (not when I feed the bridge directly from signalgenerator)
I have tried playing with the phase angle (potmeter in Steve's schematic), reducing the antenna's size (I thought that a close frequency might produce enough output on the antenna that the PLL might lock on that signal instead of the real Fres), as mentioned above using a CT instead of an antenna, but none of these solutions work. Does someone know what might be the cause of this, and maybe how to fix it?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
This is probably caused by the RF field feeding back into the VCO (through the resistors). I have had this happen. The only way around this is to shield the circuit into a grounded metal case.
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Problem with duty cylce 4046 feedback.
