# Help with LTSPICE on a high frequency circuit

• artis
In summary, the conversation is discussing how to determine the effectiveness of a specific circuit, which includes a self-generating coil, two series capacitors, and a high frequency low voltage AC signal. The circuit has been simulated, but the results are not fully understood. The conversation also includes information about the components and the importance of accurately modeling them to get accurate simulations. The conversation also mentions the potential losses for the low voltage AC signal to pass through the circuit and the need for an energy input to properly model the circuit.
artis
Hi, can you please tell me how would I be able to determine the effectiveness of a specific circuit, see in the attached spice file I tried to simulate a circuit , my secondary side is a self generating coil which in spice I had to simulate with mutual inductors aka a transformer, my basic idea is I want to understand what would be the effectiveness of two series capacitors at passing through high frequency low voltage AC.
I sort of simulated my circuit but got some weird results that I don't understand.I have also attached a paint image where I have only the real physical parts of my real circuit, a generator coil which is connected into a loop with two capacitors in series.
I need to determine whether I will be able to have any useful energy in the loop, my frequency is high it starts from 1Mhz and I don't have a clearly defined upper limit but let's assume 500Mhz. I just want to understand how effective the capacitor capacitive reactance will be at these frequencies and will any signal get through.
Any advice how to simulate or calculate this?

https://failiem.lv/u/kqbhugcn#sign_up
here is the asc LTspice file as I couldn't upload it here.

#### Attachments

• kontuurs.png
25.5 KB · Views: 325
You can attach a text file to a post on this forum. LTspice uses ASCII files, so add .txt as an extra extension to the file before posting. Remove the extra extension before running with LTspice.
Attached are a couple of examples to get you started.

AC 1.0 defines a source of excitation.
Kt couples L1 to L2, to make a transformer with coupling coefficient = 1.0
.ac directs analyse frequency response with 1000 points per decade, from 1 MEG to 100MEG.

#### Attachments

• gen_caps_1.asc.txt
1 KB · Views: 255
• gen_caps_1.plt.txt
283 bytes · Views: 215
berkeman
artis said:
I need to determine whether I will be able to have any useful energy in the loop, my frequency is high it starts from 1Mhz and I don't have a clearly defined upper limit but let's assume 500Mhz.
When you say "high frequency" simulation, that raises important considerations. To get accurate simulations (especially up to 500MHz), you will need to model the components (and the PCB layout) more completely. Inductors have parasitic parallel capacitance and series resistance, and capacitors have parasitic series inductance and parallel leakage resistance/loss. You will need to consult the datasheets for the parts you are considering to try to get values for these parasitics so that you can make your SPICE model more realistic.

Datasheets for jellybean parts may not have information about these parasitics, but datasheets for parts that are intended for higher frequency operation should have them. If not, you may need to make the measurements yourself to refine your SPICE models.

eq1
Oh I should have mentioned that it's not a pcb , it's a toroidal shaped loop where the air inductor is one part of the loop while the capacitors are another part, they are simple parallel flat surfaces much like a parallel plate capacitor, the whole energy is contained only in the E field between the plates and the B field in the toroid, (the toroid loop is the inductor)

Now I have no problems of understanding how the loop could perform but I haev problems of understanding how the capacitor capacitive reactance would perform and how much energy would be wasted. So I specifically would like to understand the behavior of the capacitor.
Since it's not an ordinary capacitor with leads but rather a sheet of metal attached to the loop at all points of the side of the sheet I think it's ESR should be low , the question I get is mostly about the potential losses for a low voltage (2-5v amplitude) AC sine to pass through.

You have specified the 1uF capacitance; also that an inductor is present; but not the series resistance.

You have a circuit without an input or an output. It looks like an isolated Pi low pass filter element. To model that circuit requires an energy input. Where does that energy come from and how is it coupled into the circuit?

As an isolated circuit, when the positive reactance of the inductor, plus the negative reactance of the two series capacitors, sums to zero, the circuit will be resonant. That circuit resonance will be damped by the unspecified series resistance.

artis said:
... , the question I get is mostly about the potential losses for a low voltage (2-5v amplitude) AC sine to pass through.
Amplitude is irrelevant for linear circuits. What do you mean by “pass through”. As a series circuit the energy stored in the capacitor will pass through the inductor twice per cycle. The voltage on one capacitor will be opposite the voltage on the other, they in anti-phase.

The shape and construction of the inductor will be most important in determining the Q of the resonant circuit, the number of cycles or the time it takes for any resonance to decay.

Your flat plate capacitors with low ESR are specified as 1uF, which is a very big plate with a small separation from what must be a bigger common ground plane. With plates that large there will probably be transmission line effects at HF. Geometry and materials are very important.

artis said:
So I specifically would like to understand the behavior of the capacitor.
Then specify the size, shape, separation and material of the plates.

Tom.G

## 1. How do I simulate a high frequency circuit in LTSPICE?

To simulate a high frequency circuit in LTSPICE, you will need to use the .AC analysis option. This will allow you to specify the frequency range and step size for your simulation.

## 2. What is the maximum frequency that can be simulated in LTSPICE?

The maximum frequency that can be simulated in LTSPICE is determined by the .AC analysis option and the length of your simulation. The default maximum frequency is 100GHz, but this can be changed by modifying the Max Freq parameter in the .AC analysis setup.

## 3. How do I model high frequency components in LTSPICE?

To model high frequency components in LTSPICE, you will need to use the appropriate models for those components. These can usually be found on the manufacturer's website or by searching for LTSPICE models online. You can then import these models into your LTSPICE simulation.

## 4. Can I use LTSPICE to design a high frequency circuit?

Yes, LTSPICE can be used to design a high frequency circuit. It allows you to simulate your circuit and analyze its performance at different frequencies. You can also use LTSPICE to optimize your circuit by adjusting component values and seeing the effect on the overall performance.

## 5. How accurate are the results from LTSPICE for high frequency circuits?

The accuracy of the results from LTSPICE for high frequency circuits depends on the accuracy of the models and parameters used in the simulation. It is important to use accurate models and to ensure that your simulation setup is appropriate for the frequency range you are interested in. Additionally, it is always recommended to verify the results with physical measurements or other simulation tools.

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