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Designing an Auto Match Unit for RF supply

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radiotech

Thu Nov 25 2010, 07:26PM

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

There is a series in the QEX magazine on the use of QUICKSMITH
which is free for now on the net.

Seen on the pantry door of the radio school;

Y=0

Steve Conner

Thu Nov 25 2010, 09:03PM

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

No admittance?

Ash Small

Fri Nov 26 2010, 07:33AM

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

Geordieboy, I think I've got it now. Ct=(C1xC2)/(C1+C2).

Radiotech, I've run into the same problem with Quicksmith as I did with the Smith charts, namely, if the load is varying you don't have a figure to input.

I've run into a similar problem with the capacitive divider network, ie, if C1 is a variable capacitor and the probes are connected accross C2, you can't simply adjust C1 until the voltage accross C2 is at a maximum.

I've been reading this article on measuring inductance:

I'm planning to start with this circuit and modify it one step at a time until I get to where I want to be. What puzzles me, though, is why is this circuit earthed? (earth symbol at the bottom)

Once again, thanks for your help.

Nicko

Fri Nov 26 2010, 11:01AM

Registered Member #1334 Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615

Ash Small wrote ...

I'm planning to start with this circuit and modify it one step at a time until I get to where I want to be. What puzzles me, though, is why is this circuit earthed? (earth symbol at the bottom)

There is an earth symbol because its a SPICE model, not a pure circuit diagram - SPICE needs a "node 0" (ground) to work from, so every SPICE schematic must have at least one. In real life (is there such a thing?), depending on the circuit, it may not be needed - its a SPICE convenience.

Cheers

radiotech

Sat Nov 27 2010, 01:43AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Ash Small --" if the load is varying you don't have a figure to input.
how about a bogey value range to start with. Do these
rules have any value

Ash Small

Sat Nov 27 2010, 11:23AM

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

Radiotech, That's an interesting article, I take it that (+ or - lambda/4) = (phase shift is never more than 90 degrees in either direction).

Again, I assume this is about when the load is an antenna, the load in my case is the inductor itself (but there is a minimum value for this ie an equivalent 'no load' situation).

I've done some calculations using a value of 2 micro Henries for the minimum value (the approximate value of most of the coils used for induction heaters in the projects I've read, which are similar to the coil I will initially be using), this gives a maximum value for a parallel tank capacitor of around 0.7 micro Farads. so a variable cap of 1 microfarad sounds like a good starting point.

I've then calculated that, in order to set up a test circuit using a 1 meg signal generator (500 KHz starting point) I need an inductor of 0.15 micro Henries. (if I use a cap of 0.7 micro Farads)

Next step is to sort through my junkbox.

Proud Mary

Sat Nov 27 2010, 03:38PM

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

Ash Small wrote ...

a variable cap of 1 microfarad sounds like a good starting point.

It's very unusual to find variable capacitors larger than 2nF, but you could switch by increments, I suppose.

Ash Small wrote ...

I've then calculated that, in order to set up a test circuit using a 1 meg signal generator (500 KHz starting point) I need an inductor of 0.15 micro Henries. (if I use a cap of 0.7 micro Farads)

The resonant frequency of an LC tuned circuit of 0.15Î¼H & 0.7Î¼F is 491kHz, or were you intending something else?

Ash Small

Sat Nov 27 2010, 05:24PM

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

Proud Mary, I did mention that I'm going to experiment with a signal generator first, I also mentioned that it's a 1 MHz unit and that 500 KHz was the starting point as this is half the maximum, so I've plenty of scope either side to play with.

This is just in order to try and understand how these circuits work, etc.

491 KHz is pretty close to 500 KHz (my calculations were only approximate). I'm actually quite relieved that your calculations agree with mine, as it confirms that my method was correct.

I'll probably try and get a small 13.56 MHz source from somewhere later to run some tests with.

I want to get on top of things before I start 'playing' with the 600 watt 13.56 MHz generator that I have as firstly, I don't want to damage it and, secondly, I'm assumint that 600 watts of RF power could cause some serious RF burns, damage my eyesight, etc.

Thanks for your comments, btw.

Proud Mary

Sat Nov 27 2010, 05:33PM

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

Design aspects of 13.56MHz, 1kW, CW-RF oscillator for plasma production

2010 J. Phys.: Conf. Ser. 208 012012

which you download free here:

radiotech

Sat Nov 27 2010, 06:14PM

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

Ash Small "I've then calculated that, in order to set up a test circuit using a 1 meg signal generator (500 KHz starting point) I need an inductor of 0.15 micro Henries. (if I use a cap of 0.7 micro Farads)

There are 'existological' limits on the use of resonance
, in this case we cant make a capacitor of that size that
could resonate 100 nanoheneries.
A good source for consideration is the NBS pepers on
measurement teqniques.
An interesting exercise it to go back to the old 'Jars and
Mics' system to get a feel of these dimentions.
Its a real pity that our units go back to gravity for force.
Calculate what plate spacing would be needed to make
a capacitor to resonate a 100 nH inductance, then see what
shape the metals would have to be. And then find out how
the electrical signal could get into the device to be measured.

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