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Registered Member #3595
Joined: Mon Jan 10 2011, 04:46AM
Location:
Posts: 26
I am putting together a tiny audio modulated 1.5mHz class-e SSTC. It's up and running on the bench with a final PCB and etc, I am just making final adjustments to tune the circuit correctly. ZVS is perfect, etc.
I am using a 4046 PLL to keep the coil in tune. Feedback is taken from a base current transformer capacitively coupled into a inverter, voltage limited by two schottkys.
My problem is this: When the power is turned up too high (controlled by a thrown together linear DC PSU and a variac) the PLL goes out of lock. I have found out why it goes out of lock but not why the effect happens. My waveform on the inverter input is a pretty decent square wave with 10-20VDC B+, but as the power is increased there is a notch in the top of the square wave (looks like a sine wave peak pointing down). This notch increases in depth with an increase in B+ until the notch hits the inverter threshold and the inverter starts outputting a doubled frequency. Then the lock is hopeless.
The output of the base current transformer unloaded is a very nice sine wave when in lock, maybe +- 5V or so... (forgot exact). Where is this notch coming from?
Thanks in advance. I'll try to post some pics soon.
EDIT: Also, an interesting relationship - the sine notch on the inverter input is in the exact position & shape of the drain voltage on the MOSFET.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Sounds like a great project. Definitely post some pics! Anyways, i'm not a PLL expert, but if you contact Geordie Boy (Richie Burnett) or Steve McConner (sp?) through this forum, they have lots of experience with PLLs and will definitely be able to point you in the right direction.
Registered Member #3595
Joined: Mon Jan 10 2011, 04:46AM
Location:
Posts: 26
Alright, all it needed was a resistor tacked on to the output of the current transformer. Easy enough, no more notch. Perfect square.
Except I still lose lock when the power is increased too high (though I have a little more.. headroom). SIGH. Troubleshoot, troubleshoot, troubleshoot. Loss of lock always seems to occur around the D-S peak of 100V. Maybe some unintentional noise on the wrong part of the circuit.... More scoping is in order, I suppose.
As soon as I clean my desk I'll take some pics. It's amazing how fast my desk becomes unusable! (1 day of tinkering...)
Annnd solved the problem again. Really bad ground/5v supply noise. The 5v line was dropping to less than a volt at peak D/S a high voltage!
Separating the ground connections almost totally eliminated it. Might try adding some more bypass capacitance to help things even more.
Now to get the audio a little louder...
Pics!
Board mounts inside metal base upside down. Tight fit. Like I said: A mess. Transformer in the middle is the quick and dirty DC supply, variac controlled for adjustable output. Best sparks so far, decent audio but not loud enough. Little bit of hiss that obscures audio, may experiment with different discharge point styles to lessen this?
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1.5mHz Audio SSTC
