lc resonance

Can some people help me with this please?

I want to model the coil on the left and what I currently have is what is on the right.

I probably should add some switches to model the ac voltage applied to the coil but lets just say the coil is closed and the L's & C's should resonant at 64 MHz and cause some current to be flowing around in the loop, correct?

From an LC calculator, @ 64 MHz and a C value of 30 pF, the L value required for resonance is 206 nH. As you can see the values of the C's, when you set the value of the inductors, 2 for each transformer, I set them all to 206 nH. Was that correct?

How can I see that they are resonating at 64 MHz? Did I place my GNDs and that 1 Ohm resistor in the right place?

I only put those 100 Meg Ohm resistors because PSpice kept telling me that 8 nodes were floating. Did I even need those?

Some serious and quick help on this would be appreciated.

Thanks.

To decide, if your equivalent circuit is correct, you have to know, what you want to simulate. I guess, that the wiring of the voltage sources to the "transformers" is not according to the simulated system, but you didn't specify the type of coupling. An arrow isn't a specific symbol in this regard.

If you want to simulate he voltage induced my a magnetic field, a single series connected voltage source would be sufficient and no transformers needed in the circuit.

The coil on the left is exposed to 250V @ 64 Mhz of RF voltage. To get current running through the coil, the coil is tuned to resonate at 64 MHz. When it is de-tuned then the coil is "invisible". I'm just looking at the tuned aspect.

I figured that this would be a good way to model it in Pspice but I guess I'm wrong. Right?

What I wanted to be able to do later was add a switch in the "coil" loop. That adds some high impedance, isolation, and de-tunes the "coil" and no current runs through the loop.

How can I make this transformer setup work? The traces of the coil should be modeled with inductors and the total inductance is 206 nH. One inductor in each corner yields 51.5 nH. Each Capacitor is 120 pF. This should resonate the coil at 64 MHz and cause significant current flowing through the coil, correct?

Does me setting my transformer inductances to 51.5 nH give me that? For each transformer both inductors are set to 51.5 nH to have a complete transfer of energy.

I should change those caps to 120 pF each as well.

Added after 25 seconds:

Any help would be appreciated!

Thanks!

How?

You can pretty well simulate simular circuits with PSPICE. I did e.g.with RFID reader and tag coils. But I don't exactly understand, what you want to simulate.

Basically, you have a single LC resonator (for the basic simulation, it doesn't matter, if it's splitted). Resonator losses and external coupling are unspecified. You also decided to tune it to the nominal resonance frequency. So what do you expect to see in the simulation, that isn't already known before?

I don't understand the purpose of a switch, cause it's not present in the real resonator.

What I want to see is substantial current flowing in the loop, but I get a very very small amounts of current, uA. Also, if I change the capacitor values, change the resonance of the coil, I shouldn't see any current. But I still see the same small current.

Do you think it would just be better to get a series source, inductor and capacitor, have the series LC resonant at the frequency of the source and make up my loop that way? So when the LC doesn't resonate then it is not a "short" anymore and causes no current to flow. I am just trying to see what I am to expect.

I am running transient simulations and using the current prode in spice to see the current.

Thanks for the replies. I appreciate it.

1. I didn't calculate the values. Actually, the capacitors must be 120 pF each for 64 MHz resonance with 51.5 nH. I understood, you are simulating a real circuit, so you should know the real value.

2. With transformers, the inductors are shorted by the voltage sources. That's not according to the real system. A series source should rather fit it. The source voltage can be possibly much lower than 250V to achieve 250 V resonant voltage.

So I did a series version and an AC sweep. How can I increase the Q of this circuit while still have it simulate? The impulse I get at 63.75 MHz only has a Q of about 6. I am expecting a Q of 230.

Thanks for the help.

A Q of 6 seems to be a simulation error. A Q around 80 is set by the 1 ohm resistor, it has to be reduced for a higher Q.

I calculated Q as the center frequency divided by the 3 dB bandwidth or the bandwidth at half the peak(center) energy/power.

When I reduce the R nothing happened?

How are you calculating Q to be 80?

Thank you so much for your help.

I get a 3 dB bandwith of 770 kHz respectively a Q of 83, as expected. I used ZL/R as an estimation, resulting in exactly same value.

As I mentioned before:

I have to add more points in my simulation profile. Stupid me.

Thanks again for the help.

Added after 15 minutes:

So when I change this from AC simulation to transisent simulation using a Sine Wave with 250V Peak @ 64 MHz I get a seriously huge current running through this loop. Why so big? There shouldn't be more than 130 mA of current flowing in this loop.

Is it possible to keep this coil with a Q of 200 but have 130mA flowing throughout the loop? If so, how?

Thanks again.

Which parameter of the simulated real system defines the magnitude of the stimulating voltage source? I would expect, that 250V is a voltage measured in resonance, e.g.at a capacitor. This would already require capacitors with considerable voltage rating. The driving voltage source (simulating e.g. an external field) can be much lower, effectively decreasing with increasing quality factor.

Hi FvM, I have been search for some ideal switches in spice but couldn't find any.

Are there any switches that I can use where I just tell it when to open and close? And maybe I can give it Ron and Roff?

The voltage and current controlled switch are generic built-in PSPICE parts (S respectively W), I wonder, how you managed to miss them. Take a look to the reference manual.

I didn't want to use those switches actually. I saw those but found some others. You can see them in the attached image.

I finally got my coil model working. The first one. The series on would give me a problem when I want to remove the applied voltage to the coil.

You can see that all sources provide 75V (for pspice testing) @ 64 MHz. They are applied for 1 us and then turned off.

Note: The coil is open for 50 us.

I am expecting that at 1 us the inductors in the loop would ring down at a different frequency, about 750 MHz because when it is open (Switch U5) that is the resonant frequency.

But I get this huge spike of voltage across all the inductors when the switches open up.

Why is that happening? It should just start from the +/- 250 V (approx) and die down. Die down slowing that is.

Any help would be appreciated.

Thanks.

Circuit

Response

Doen't look like a real world problem to me.