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Designing an Auto Match Unit for RF supply

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Ash Small

Fri Dec 03 2010, 08:43AM

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

Mattski, the probe giving the 15 volt reading is a X10 probe.

The signal generator was definitely set to around 1.5 volts.

The peak is at 210 KHz, the voltage drops away either side.

There is also a phase shift of around 20-30 degrees. (I'll try and measure this as accurately as possible)

I've been charging the batteries for my digital camera overnight, I'll post a photo later.

I will also try swapping the probes over to make sure it is the probe that is causing the problem.

(I think I read somewhere that a series tank circuit tends to boost the voltage and a parallel tank circuit tends to boost the current, I was planning to investigate this phenomenon as part of this project, as I obviously want as much current flowing in the tank circuit as possible(EDIT: at least, I think I do.))

Ash Small

Fri Dec 03 2010, 03:52PM

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

It's not the probe, it is channel 2 on the 'scope.

I've attached a schematic of the input circuit, any advice would be appreciated.

In the meantime, I'll have to try and make do with channel 1.

1291391546 3414 FT100535 D61a Ch2 Input Circuit

Sulaiman

Fri Dec 03 2010, 06:44PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

If you put a signal into Ch2 you can fault-find using Ch1

Ash Small

Fri Dec 03 2010, 08:30PM

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

Thanks Sulaiman,

I'll try that over the weekend.

Ash Small

Sat Dec 04 2010, 06:46PM

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

Well, I've tried increasing the capacitance in the tank circuit a few times and I'm now confident that the inductance of the coil is around 3 microhenries (lowest value measured is 2.8 and highest is 3.13, but I don't know how accurate my DMM and signal generator are, so 3 is an approximate figure).

I've ruled out the 0.1 microhenry value measured previously, as I seem to be having problems with the signal generator and/or the 'scope above 100 KHz, and I'm now measuring at lower frequencies due to the increased capacitance. I'm assuming the problems are probably related to 30 year old electrolytic capacitors. Other websites suggest I should replace all the electrolytics in the 'scope as a matter of course. It would make sense to replace them in the signal generator as well, as that is around 40 years old.

This means a resonant cap at 13.56 Meg will be around 46 picofarads.

This gives me a starting point for the Smith charts, impedance calculators, etc.

Ash Small

Wed Dec 08 2010, 01:36AM

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

Ok, since the weekend I've been contemplating impedance. The impedance calculator here:

is what I used to calculate that for a circuit consisting of a parallel tank circuit, L=3 microhenries, C=46 picofarads, resonant at 13.56 MHz, connected to a series capacitor, the value of the series cap needs to be approx 1 nanofarad for an impedance of 50 Ohms.

I've been googling, trying to find the maths.

The resonant tank circuit has an impedance of zero, so I'm led to believe. A capacitor of 235 picofarads has an impedance of 50 Ohms at 13.56 MHz. A series impedance is Z1-Z2 (50 -0, or 50 Ohms). I appreciate this is incorrect but this is all I'm able to find on the subject.

Can anyone point me towards the maths that I need for a parallel tank circuit in series with a capacitor (or, for that matter, a series tank circuit in parallel with a capacitor)

The Impedance Calculator in the above link gives a value of approx 1 nanofarad for a cap in parallel with a series tank circuit as well, I had to extrapolate a bit, as it only goes up to 1 Meg but it's a logarithmic scale so it's not a big problem.

radiotech

Wed Dec 08 2010, 02:57AM

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

you said :

Can anyone point me towards the maths that I need for a parallel tank circuit in series with a capacitor (or, for that matter, a series tank circuit in parallel with a capacitor)

Try out B and Y succeptance and admittance. they make
parallel calculations easier.

when |Xc| = |Xl| |X| = 0 series

when |Bc| = |Bl| |B| = 0 parallel

1/X = B Y= 1/Z

Ash Small

Fri Dec 10 2010, 01:20AM

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

It appears I've been following the wrong approach here.

I assumed the priority was to obtain resonance, then adjust the impedance of the circuit to 50 Ohms. A resonant parallel circuit has an infinite impedance (apparently) and a series capacitor with a resonant parallel circuit will also give an infinite impedance (apparently). A series resonant circuit has no impedance (apparently) and a parallel capacitor with a series resonant circuit will also result in a zero impedance (apparently).

It looks like I can rule out a parallel circuit with a series capacitor as the impedance of a 3 microhenry inductance has an impedance of approx 255 Ohms at 13.56 MHz (more than 50 Ohms).

It looks like I need a capacitor in series with the coil, and another capacitor in parallel with them. It also looks like I need to tune the circuit to 50 Ohms impedance, then, maybe, adjust for maximum resonance.

Thanks for all the advice I've received so far. Any further suggestions will be appreciated (am I now on the right track?).

Ash Small

Sat Dec 11 2010, 08:47AM

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

Am I where I want to be?

I've been playing around with QuickSmith, and enclose a screen-shot below.

I've put a high resistance in series with the load to simulate an 'open circuit'. This effectively eliminates the load and just gives me two capacitors and an inductor (coil).

Input impedance equals R plus X (50 Ohms).

Can someone please take a look at this and either confirm that this is what I want, or tell me why it is wrong?

(I've previously obtained R=50, X=very high positive value and R=50, X=very high negative value. I've also obtained R+X=-50. I've decided this is not what is required and that R+X should be 50 Ohms. Am I correct?)

Ash Small

Sat Dec 11 2010, 09:18AM

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

Capacitor values are much closer to resonance in this screenshot:

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