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Designing an Auto Match Unit for RF supply

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radiotech

Fri Nov 19 2010, 07:01PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546

" But, the 10 turn one has 100 times the inductance, so it takes 10 times the voltage to force 1/10th of the current through it. "

What relates the current to the amount of voltage is soley
the resistance. and a coil with 100 Henries would have the
same current as one with 1 Henry if there were no resistance
no matter what the voltage was.

The plasma is transforming your Joules/sec into a basket
of radiations of various frequencies and that thermal radiation
resistance will draw maximum power when it equals the resistance
of the generator,if the plasma forms a conjugate load.

So, it your coil able to "write a coil" into the gas?

The space that the plasma occupies had an impedance
of 377 ohms before you struck the ark, if it think of it as a vacuum.

Ash Small

Fri Nov 19 2010, 11:06PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Steve, there must be something in this due to the fact that electrostatic shields are used on some commercial plasma systems. I'm assuming the electrostatic shiels operates in a similar way to a faraday cage, blocking the electric field, but allowing the RF/electromagnetic field to pass through the slits. This would suggest that the field generated by the coil is not purely electromagnetic and that there is something in what you say.

For some reason I'm unable to get into the hyperphysics site, along with some others. I think this is a due to a problem with internet explorer on my laptop. (I've not been able to get into hyperphysics for months.)

Steve Conner

Sat Nov 20 2010, 09:57AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Proud Mary: Yes. 13.56MHz is an "ISM Band" that you don't need a license to transmit in, which is why some RFID systems operate there. 2.4GHz is another one, shared by Wi-fi, cordless phones and microwave ovens.

Radiotech: Surely you're not going to deny that an inductor has a reactance of 2*pi*F*L ohms? (j if you like) And if space has an impedance of 377 ohms, why doesn't every ohmmeter read 377 just sitting on the table? (What reading would you get if the test leads were infinitely long?)

Ash: It could be that in those systems, they used a multiturn coil driven by high voltage because that was most suitable for matching to the generator, and the electric field from the coil was too much, and caused plasma to appear in unwanted places.

Try Firefox.

Proud Mary

Sat Nov 20 2010, 10:13AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Steve McConner wrote ...

And if space has an impedance of 377 ohms, why doesn't every ohmmeter read 377 just sitting on the table?

Ash Small

Sat Nov 20 2010, 12:23PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Steve, I've a hunch (and this is a bit of 'reverse engineering') that these systems, which have large gaps between the wingings, presumably so that the volume of plasma is large, also use electrostatics (charged electrodes) to accelerate/decelerate the ions and to 'steer' the ions to the workpiece (ie negative charge applied to workpiece). I imagine they want to eliminate the electrostatic field from the coil so that it doesn't interfere with the electrostatic fields that they intentionally create inside the chamber. This is just an 'educated guess', though.

Ash Small

Sat Nov 20 2010, 08:55PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

On the other hand, I suppose it could be to stop the electrostatics inside the shield from affecting the coil (current flow inside generating electricity in the coil) but I'm no expert. some advice here would be appreciated.

Ash Small

Wed Nov 24 2010, 10:25AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I've come to the conclusion, after studying Smith Charts and and reading everything I can find on LC circuits, that I need to do some experimentation based on the circuits here:

I'll have to get another 'scope as mine is only good for 10MHz, but I might do some initial tests using this and a 1MHz signal generator that I bought for another project I'm working on (HV power supply for electrostatic grids). At least this should give me some understanding of how these circuits behave.

While I can visualise how a parallel tank circuit behaves, I'm still having trouble visualising how these more complex circuits behave, so I'll just have to dig out some inductors and capacitors (and maybe a 50 Ohm resistor and some co-ax), connect them up to the signal generator and 'scope, and see what happens.

GeordieBoy

Wed Nov 24 2010, 10:26PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

> I'll just have to dig out some inductors and capacitors (and maybe a 50 Ohm resistor and some co-ax), connect them up to the signal generator and 'scope, and see what happens.

Whilst you can do this with a small-signal RF drive, you need to be very careful scoping matching networks in RF power circuits! This is particularly so when the range of load impedance is not well controlled.

Large RF voltages can develop across components in matching networks, and the voltage rating of most oscilloscope probes derates rapidly with increasing frequency. Direct connection of a x10 probe to a matching network is likely to cause a puff of smoke and a blown scope channel. Been there, done that, exhibited the red face

The best way to measure high voltage RF waveforms at the 13.56 and 27.12 MHz ISM bands is to make a capacitive divider to reduce the HF component down to a safe level for measurement. If the output capacitor in say an L-match circuit is 1nF, then you can easily put an additional 100nF capacitor in series with the grounded end of the L-match capacitor to form a capacitive divider. The output voltage being measured across the bottom 100nF capacitor and being equal to approximately 1V per 100V at the output of the matching network.

-Richie,

Ash Small

Wed Nov 24 2010, 11:19PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I think I follow what you are saying, Richie. A series capacitance 100 times the capacitance it is in series with acts like a series resistance 100th of the resistance it is in series with.

I assume that even using the output from a signal generator (up to 12 volts), the voltage could increase in a tank circuit (I was aware that the current could increase, but not that the voltage could increase).

While my 'scope only cost Â£20 from eBay, and I will need one suitable for more than 10MHz, I won't learn much if I blow a channel as soon as I connect it to this circuit.

Can I connect a high value resistor into the circuit to reduce the voltage I'm measuring? (eg in series with the probe? I think I saw an article somewhere about making your own 10X probes that involved something like this)

GeordieBoy

Thu Nov 25 2010, 03:31PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

It's best to use a capacitive divider because it is the high-frequency AC features of the signal that you want to preserve.

A resistive divider is always going to form a low-pass filter with the cable capacitance and input capacitance of your scope. At frequencies in the tens of MHz this can give significant amplitude and phase error.

The neat thing about the capacitive divider is that the inherently large capacitor at the bottom of the divider effectively shunts any cable capacitance and input capacitance of the scope that is connected to it. This minimises measurement error at high frequencies.

The schematic that Proud Mary posted earlier in this thread even exhibits an example of a capacitive divider for high-voltage RF measurement:

In this case it is used to aid the adjustment of the PA MOSFET's drain voltage for correct Class-E operation. You can see the capacitive divider next to the measurement port at the bottom marked "J4 VDS_sample."

-Richie,

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