PLL around Resonant LC Circuit

Only at a much lower frequency - around 100Hz - to drive a solenoid being used to excite a reciprocating stirrer at its mechanical resonance.

It worked, but the loop didn't settle on the frequency which gave the maximum mechanical displacement, which didn't correspond to a 0 degree phase shift.

I hypothesise that if I'd chosen to lock at a different phase shift (and I could have varied that in steps of 3 degrees) I could have got the loop to settle very close to the point that did give maximum amplitude, but I was on holiday in Australia when the circuit was being tested on real live stirrers, so I didn't get the chance.

-------- Bill Sloman, Nijmegen

1. Your PLL idea should work.

1. Why not an oscillator with the coil as the frequency-determining element?

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Klaus,

If the receive pcb energy requirement were low enough you might get away with a high-efficiency LED/photodiode transmission arrangement.

Stepping down in frequency, how about a microwave transmitter driving pcb (tx/rx) antennas?

At 10Mhz you might consider using diode phase detectors rather than the

4046's exclusive-or device.

Good luck, Mike

Why?

It should. Sounds like a fun project.

Edge-matching? Since Ron Treadway and I invented the scheme I DO think it will work ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Klaus is trying to find the "sweet spot" for best power transfer.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

You can adjust the coupling coefficient by adjusting the Q of the resonance. At the peak, the coupling will go through a maximum and then start to decline, like a double-tuned IF transformer. If you make the coupling a shade too strong in the normal position, that'll make the coupling nearly independent of distance over a reasonable range. How far were you planning to go?

Cheers,

Phil Hobbs

Are you sure that it is not better to put a capacitor across the primary winding and then run it at the resonant frequency? If not, why not? If you have found a frequency better than the resonant frequency, then how much better is it? Would it be tolerable to just run it at the resonant frequency?

Ok, but according to what?

Ok, this makes sense to me, I would run it at resonance by making a LC oscillator around the primary. This doesn't seem consistent with what you said before, but it seems more sensible.

If you want it to lock at 0 degrees, you need the phase detector with two flip-flops, but it seems too complicated for my liking. Perhaps it will work but first I think you should try an oscillator. I suggest the following MOSFET circuit, it oscillates particularly well: Sorry about the drawing, I don't have software to do this for me.

• = junction

--+-- 5V | .| . nnnnn.nnnnn Centre-tapped inductor | | +---| |---+ Cap, or can use two caps to positive supply instead, | | +------ | | ---|---+ | | | | D | | D Small n-mos fets, try to get ones with low capacitance G-+ +--G S S | | +----+----+ | | ( ( Optional choke, not really needed for basic operation ( | \\ / \\ Optional resistor or current sink, if you want to control / the amplitude of oscillation, otherwise short to gnd | GND

Not me.

One thing I would worry about, either with a free running oscillator or with a PLL circuit, is that the variable frequency might cause the device to fail EMC testing. I would be inclined to try forcing the thing to run at

13.56MHz or whatever the ISM band is around there. That way you would have less difficulty in getting it approved I would think. You might have to tune the capacitance on the primary of the transformer to keep it resonating at 13.56MHz under all circumstances, but that might not be too hard. One option might be to make a self-oscillating VCO, and then phase lock it to a 13.56MHz crystal oscillator. That way it would be both (a) at 13.56MHz and (b) at resonance all the time.

Chris

Phil Hobbs skrev:

I'm planning to transmit power over a barrier of approx. 10mm

Thanks

Klaus

Well, beause I'm moving through the resonant point and the phase is then in once instance postive and in the other negative. I'm just worrying about the lock of the PLL circuit when the phase is negative and the function of the phase comparator in that case

What is "Edge-matching"?

Thanks

Klaus

I should have said that from the get-go - the barrier is through a plastic cabinet - so no LED option :-(

Thanks

Klaus

Good point. I have a model of a transformer that transmits power from one layer in a PCB to another layer. And in that circuit I added a parallel capacitor on the secondary side to reduce the resonant frequency to a tolerable level. I will do the same in the this circuit where the distance is greater - to reduce the frequency down to one that is manageable by the PLL and associated circuits (bytheway - isn't there a drawback in adding this capacitance?)

According the 0 phase difference (resonance point)

Very insteresting - I will look into this idea of using a self-tuning LC circuit

I have thought about this and the conclusion is that for sure I need to very soon perform a radiated emission test to confirm I don't have a show-stopper. Other people has made layer to layer PCB transformers with no shield with both theoretical and experimental results that show no problems. But these circuits operate with low size, so I might run into problems. If I do, one thing I looked at was to modulate the freuquency with a low frequency source to shape the noise outside the reciever bandwidth of the EMC test reciever (120kHz). I have tried this in the lab and it can easily give an improvement of 10dB

Thank you very much for your good inputs :-)

Klaus

Will look out for that ;-)

I was aiming for a simple voltage measurement of the voltage and a small current sense resistor in the common ground connection returning from the LC circuit to measure the current

Have checked them out - they are too expensive and too difficult to modify

Thanks

Klaus

Hello Klaus,

The usual way this is done is series resonance. Put a cap in series with the primary coil and then ride on the resonance. This cap should be able to stomach the expected maximum RF currents. I had a ceramic cap turn into green glass on a project like that, so not all of them like that.

I don't think the 4046 is going to cut it at 10MHz but you could use other chips. Precise measurement of V&I going into the coil would require an RF bridge, basically a couple of little toroids. Or only one and a resistive divider for V. All you'd really need is reflected power which would be minimized, unless you also want to know how much the remote board is drawing.

There are some TI chips (the former UC Unitrode series) that have the series resonant control guts in them but you can't directly use their outputs. They are meant to be used in series resonant power converters around 100kHz. But maybe they could be 'abused' here.

Please mind the legal stuff, you don't want to have the laws come in and shut it down because you were outside ISM (10MHz is outside). Trouble is, the next ISM band at 13.56MHz is too narrow to do series resonance sliding.

Regards, Joerg

The phase doesn't "snap", it smoothly transitions, tightness determined by "Q".

PFD... Zero phase at lock, as opposed to XOR which has 90° phase at lock.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

How much power are you trying to tramsit to the other side?

Are both sides shielded?

Sounds like you are making an interference generator.

Mark

Hello Klaus,

You could. For voltage sensing that's also done in RF gear when the toroid coupler costs too much. The problem with current sensing is that even with Schottkys it's tough to demodulate unless you can allow a Volt or two of drop. Germaniums would be best but can be hard to obtain. Or you could sample since you'll have a clean reference of the 10MHz going in. Sometimes stepping up the current signal with an RF current transformer is easier, it's how I usually do that.

Regards, Joerg

Hello Mark,

I don't know about Klaus but I have done contactless power transfer up to 100W. RF measurements up to a kilowatt.

At 10MHz hopefully they are.

That's why I hinted that it might be good to move to an ISM band.

Regards, Joerg

Could you get the correct phasing by self-oscillating and, (depending on the method of driving the tuned circuit), switch the drive current when the voltage zero-crosses, or switch the drive voltage when the current zero-crosses?

--
Tony Williams.

Tony Williams skrev:

I have actually though about this. Sort of a short voltage pulse on the LC circuit and when the current in the inductor rises to a certain level - switch off the pulse. Then I would wait for the voltage to cross zero from negative and that would be the point of another pulse. So effectively just letting the circuit ring at the natural frequency of the LC resonant point. The reason I haven=B4t pursued this is that I worried if the Q is bad that there will not be enough voltage to detect the zero crossing.

But now you mentioned it - I think I'll give it another shot :-)

Thanks

Klaus