Some help with a PLL circuit?

[Things]

Hi all, I've got this plasma speaker here I'm trying to debug. It uses a PLL in the following circuit:



The problem is, the arc output is meant to be silent, but there's this weird high-ish frequency buzz. There also seems to be a point at around 1/3 power where there is still an arc, but it's silent, like it's meant to be, but above or below that, it makes that noise. Anyone got any ideas what it could be?

Here's a quick video of the noise:

Cheers,
Dan

[w2aew]

I'm not sure what you are phase to, so don't know if that's stable.  Two things to look deeper into:

1) Arcs radiate a lot of RF noise. Make sure your circuit is well shielded to avoid RFI, particularly in the phase detector and VCO control input.

2) Check to see if the loop is stable and well damped. Check to see if the VCO control voltage is well behaved ( noise, oscillations, RF pickup, etc.).

[Things]

I don't know a whole lot about PLL's, which is kind of why I'm asking here

As for what it's phased to, I believe it forms kind of a self-tuning circuit, as I believe it gets phasing from the coil itself through the antenna on the left of the diagram.

[w2aew]

I don't know a whole lot about PLL's, which is kind of why I'm asking here

As for what it's phased to, I believe it forms kind of a self-tuning circuit, as I believe it gets phasing from the coil itself through the antenna on the left of the diagram.

Since you said you don't know a lot about PLLs, you might be interested in viewing this video I did a few months ago on the basics of PLLs. It features the 4046.

[Things]

Hmm, nice, will give that a look, thanks

[SeanB]

It is a TV set, check the electrolytics there are still working well. Check as well D2, D3 and C3, C10 and C11, and at last resort change the 4046 along with the 3 transistors, they might have suffered from damage due to arcing. Use a exact same part, not a similar one, this is one of those times where the differences between different manufacturers does make a difference.

[Things]

They are all brand new parts, they came in kit form. It seems to be a recurring problem with the kits, and we can't figure out where the noise is coming from.

[Things]

I just did a frequency analysis on that annoying buzzing noise, and it seems to have fundamentals at 775Hz, 1549Hz, 2323Hz, 3882Hz and a bunch more at higher inaudible frequencies. Anyone have any idea where this may be coming from? Could it be related to trace lengths on the PCB etc?

Dan

[SeanB]

Component values for the circuit would be nice, but it is probably coming from the feedback side of the PLL, probably the transformer is resonating with C11, it probably needs a series resistor in series with C11 and a bias resistor from C11 and pin14 to ground. Try 10k for the series resistor and 1M as a pull down for starters.

[Things]

Oops, forgot the component values. Have updated the image in the OP with them.

I tried your idea, and it does seem to help a bit, although the coil's output is no where near as powerful as it used to be. I tried replacing the 10K series with like 3.9K and it didn't really help. Think it might be the PLL having trouble locking on as I've had it happen before.

I have also tried reducing the track length between C10 and the 4046 which didn't seem to help.

As a side note, I did a frequency analysis of the whine last night, and it has fundamentals at 775Hz, 1549Hz, 2323Hz, 3882Hz and some other inaudible frequencies.

I have a feeling that this may actually be coming from the PWM used for adjusting the power (Using P1, a 10K potentiometer)

Is there anything on that circuit that would actually generate a frequency around those ranges? It's possible it could also be a beat frequency, but it's pretty difficult to analyze.

[w2aew]

Well, the frequencies are all harmonically related, so I would expect its related to your PWM signal as you suggested.

[Things]

Hmm, well as I mentioned before, power control is being done by P1 in that image, and audio fed in through the jack (however it does it with and without the audio jack connected).

Being the noob I am to PLL's (Your video was great BTW ), what is actually generating the PWM in that circuit? The combination of P1, R2 and R3?

EDIT: I've done a bit of playing around, and I actually tried removing C14, just in case maybe it was forming a resonant circuit. The whining noise actually reduces dramatically. It's still there, but it's a lot softer than with it connected. Problem is, without it, you have no audio input.

Anyone got any ideas?

[Things]

Just made a breakthrough!

I was mucking around with the coil a bit, and I accidentally knocked the primary coil off center from the secondary. It's a helical coil that is usually perfectly centered around the secondary coil, with about 5 turns of wire. It was sitting off center quite a fair bit from the secondary, and the annoying whining noise disappeared! If I put the primary back centered again, it comes back.

I have heard that maybe the phases being slightly offset might be causing these harmonics in the coil? Is there a way to correct for this in the circuit? What do you think it could be effecting?

[holko]

Just made a breakthrough!

I was mucking around with the coil a bit, and I accidentally knocked the primary coil off center from the secondary. It's a helical coil that is usually perfectly centered around the secondary coil, with about 5 turns of wire. It was sitting off center quite a fair bit from the secondary, and the annoying whining noise disappeared! If I put the primary back centered again, it comes back.

I have heard that maybe the phases being slightly offset might be causing these harmonics in the coil? Is there a way to correct for this in the circuit? What do you think it could be effecting?

It could have something with the coupling between your primary and secondary. Try using a small shunt resistor between source and ground and measure the voltage accros.

I would also include a RF-choke from VCC to Drain so the FET "sees" a smoother current, instead of a switching one. And IRFP450 has a rather high gate capacitance so I think I would try a better FET driver. (Or you could switch to Infineon MOSFET as it has very low gate capacitance)

About your switching topology: I see you use a simple fet switch. With some math and a few more components you could try running it in class-E.

I have included a few links:
http://www.richieburnett.co.uk/hfsstc.html
http://4hv.org/e107_plugins/forum/forum_viewtopic.php?67496.post
http://4hv.org/e107_plugins/forum/forum_viewtopic.php?19970.post
http://uzzors2k.4hv.org/index.php?page=4MHzclassE1