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Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Steve and Proud Mary, there seems to have been some confusion on Saturday.
The 700 nanohenry capacitor was only for measuring the value of my prototype coils using a 1 Meg signal generator.
I don't need variable caps that size for the 13.56 MHz circuit, I need to measure some coils similar to the one I'll use. Looks like from the above post I need variable caps of around 50 picofarads if it is a 3 microhenry coil up to around 1000 picofarads if it is around 1.5 microhenries.
Once I've determined the inductance of the coil and the capacitance I require, I can choose an inductor to resonate with the capacitor at around 500 KHz for test purposes (playing with the circuit), at least, I assume this approach will work.
Googling all the maths on Saturday scrambled my brain.
I'll have to decide on the final dimensions of the coil before I can start constructing the variable capacitors, I had initially been planing to make sliding vane caps, but it now looks like conventional rotary caps might do the job if I only need a few picofarads. In the past I've wired four fifty nano home brew HV caps in parallel for 200 nanofarads at up to 100KV (safety factor of 2), so making lower voltage variable caps of less than 1 nanohenry will be a piece of cake.
I reckon I'll need an inductor of around 2 millihenries for the test circuit at 500KHz with a 50 nano cap.
In the meantime I'm still puzzling over the 'scope traces in the above pics.....
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Ash Small wrote ...
Steve and Proud Mary, there seems to have been some confusion on Saturday.
The 700 nanohenry capacitor was only for measuring the value of my prototype coils using a 1 Meg signal generator.
I don't need variable caps that size for the 13.56 MHz circuit, I need to measure some coils similar to the one I'll use. Looks like from the above post I need variable caps of around 50 picofarads if it is a 3 microhenry coil up to around 1000 picofarads if it is around 1.5 microhenries.
Once I've determined the inductance of the coil and the capacitance I require, I can choose an inductor to resonate with the capacitor at around 500 KHz for test purposes (playing with the circuit), at least, I assume this approach will work.
Googling all the maths on Saturday scrambled my brain.
I'll have to decide on the final dimensions of the coil before I can start constructing the variable capacitors, I had initially been planing to make sliding vane caps, but it now looks like conventional rotary caps might do the job if I only need a few picofarads. In the past I've wired four fifty nano home brew HV caps in parallel for 200 nanofarads at up to 100KV (safety factor of 2), so making lower voltage variable caps of less than 1 nanohenry will be a piece of cake.
I reckon I'll need an inductor of around 2 millihenries for the test circuit at 500KHz with a 50 nano cap.
In the meantime I'm still puzzling over the 'scope traces in the above pics.....
The longer this thread has gone on, the less I understand what it's all about!
"around 1000 picofarads if it is around 1.5 microhenries." - fo for this combination is 4.1MHz
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
That's a typo, Proud Mary, should be 0.15.
Maybe I should start a new thread, as it is going off topic a bit.
The question I'm asking now is why do I get two peaks when I try to measure the inductance of a coil, one which corresponds to an inductance of 0.1 microhenries, the other which corresponds to 3 microhenries?
The 3 microhenry trace has much greater amplitude, so do I assume this one is correct?
(before I can design a matching network I need to have an idea of what the coil inductance is under no load, at the moment I seem to have two figures, see the 'scope traces above.)
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
I've found quite a useful little impedance calculator that displays a graph of impedance over frequency.
The only drawback is it only goes up to 1 Megahertz, but as it's a logarithmic scale I only have to extrapolate a quarter inch or so off the end to get to 13.56 Meg.
Anyway, I've been playing around with different values, with series and parallel capacitors, along with a coil, but it seems I'm unable to get 50 ohm impedance with either cap resonant with the coil.
I assumed I'd need a parallel capacitor in resonance with the coil, and an overall circuit impedance of 50 ohms, but this appears to be impossible, at least so far anyway. (certainly with caps of less than 2 nanofarads and a coil of 3 microhenries).
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Radiotech, I'm sure you're not as dull as you make out.
I've taken the circuits here:
and put them into the calculator here:
with a coil inductance value of 3.13 microhenries (resonant cap at 13.56 Meg of 50 picofarads)
and tried several different combinations of inductance and capacitance to obtain the impedance graph, and while I can get 50 ohm impedance, it is not with inductance and capacitance values that resonate at that frequency.
While I understand that the experts here are more used to tuning antenna circuits, and these circuits have no antenna, (the circuits supplied by Proud Mary both had antennas, etc) , and I'm aware that this thread has gone off-topic a few times and I've got confused once or twice, I'm certainly making some progress.
What surprises me is that while I can tune the circuit to 50 ohms, it isn't any where near resonant frequency for either of the capacitors.
Do I just need to tune it to 50 Ohms, or do I need it to resonate as well?
It's late here 2.40am and I need some sleep. Maybe it will be clearer in the morning.
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
wrote ... The question I'm asking now is why do I get two peaks when I try to measure the inductance of a coil, one which corresponds to an inductance of 0.1 microhenries, the other which corresponds to 3 microhenries?
If you have two peaks then you must have more than more than just one capacitor and one inductor in the full model for your circuit, so there's a parasitic element somewhere that you're missing. If there is a lot of inductance in your test setup that could resonate with the capacitor, or maybe it's something to do with the parasitic inter-turn capacitance of the inductor. Those are my two guess.
How about trying just signal generator to inductor to a load resistor to ground, measure the amplitude of the voltage on the load resistor using your oscilloscope. It forms a low-pass filter where the pole frequency is given by (Rout,signal-generator+Rload)/(2pi*L). The output voltage should stay fairly constant with frequency and then start dropping. The point where amplitude has dropped by half is the pole frequency. (If I've done my math right). See if this L matches up with either one you found with the resonator method.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Well I've found the parasitic capacitance.
I connected up the circuit as Mattski described, with the same 1 K Ohm resistor I used before, and was getting a voltage rise, so, after a bit of playing around, I ended up with two probes connected accross the resistor. Out put from the signal generator was 1.5 volts approx, and at 210KHz I had around 1.5 volts on channel 1 of the scope, using my 'cheap' probe, and around 15 volts on channel 2, (my 'expensive' probe).
It looks like my expensive probe, (£20, as opposed to a fiver for the cheap one) is the source of the parasitic capacitance.
I'm going to start again from scratch tomorrow using the cheap probe, and see what results I get then.
(I've looked at the equation Mattski provided and I should be able to sort out some suitable values for resistors myself)
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
Ash Small wrote ...
I connected up the circuit as Mattski described, with the same 1 K Ohm resistor I used before, and was getting a voltage rise, so, after a bit of playing around, I ended up with two probes connected accross the resistor. Out put from the signal generator was 1.5 volts approx, and at 210KHz I had around 1.5 volts on channel 1 of the scope, using my 'cheap' probe, and around 15 volts on channel 2, (my 'expensive' probe).
It is funny that you get two voltages a factor of 10 apart, makes me suspicious of an X10 issue. Is there an X10 probe setting on your scope? Is either probe an X10 probe or switchable from X1 or X10? Try swapping the two probes putting the expensive probe on channel 1 and vice versa, make sure the problem follows the probe and not the channel. Are you pretty sure that your signal generator is putting out 1.5V and not 15V?
edit: I should also ask, was this factor of 10 separation only at 210kHz?
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Designing an Auto Match Unit for RF supply
