The coil - a passive component

Here you can examine a coil or choke in a frequency simulator. Single-layer coils are usually called coils, while multi-layer coils are called chokes. In addition to the inductance, a coil also has an internal resistance and capacitance. In constructions that work at high frequencies, these characteristics appear more clearly.

The internal capacitance becomes greater the greater the inductance (turns) the coil has. This means that the coil's range of use is limited by the resonance frequency. In some cases, you have to wind the coil wire in a special way so that the capacitance does not become too large otherwise, you can choose a core of some suitable material. With a coil core, you can obtain a large inductance with only a few turns, which means that both the internal resistance and the capacitance will be low, which in turn leads to the resonance frequency being shifted up.

The coil's internal resistance also increases with increased frequency due to the skin effect, which lowers the coil's quality factor or Q factor at high frequencies. The Q factor is a measure of how prone a coil is to self-oscillate. If the coil's internal resistance were zero, the coil could self-oscillate forever, which in turn would result in an infinitely large Q factor.

During the dimensioning process, you can choose to enter all parameters manually or you can build your inductance by letting the program calculate different coil data via four selectable options. Data regarding the legs of the inductance must always be entered otherwise, the skin effect cannot be calculated. The diameter of the legs must then correspond to the diameter of the bobbin thread!

The impedance ratio of the component can be studied in a frequency graph. This tells you what impedance a component has at a certain frequency. The impedance of the coil increases with increasing frequency until the resonance frequency is reached.

Some rules of thumb for single-layer HF coils and to obtain a high Q value

Multi-layer coils or chokes are best suited for lower frequencies. If high reliability is to be obtained in the calculations, it should be short. The internal capacitance cannot be calculated for a choke, but if you find a way to measure the choke's resonance frequency, the internal capacitance can be easily calculated.

One way to read the resonance frequency is to feed the choke with a square wave or with a square pulse train. The choke rings for each flank change. These rings are sine waves that gradually peter out. If you with an oscilloscope manage to zoom in on such a ringing, you can measure the period of a sinusoidal oscillation. This time inverted gives the resonant frequency of the choke. It is important that the frequency or pulse generator can drive the choke towards zero or down towards minus completely, i.e. that the frequency generator has low self-resistance.

HF wires are used if only a small inductance is desired. Usually, a stub of wire is enough to achieve a significant inductance. The inductance drops if there is a ground plane nearby. The closer the wire is to the ground plane, the lower the inductance becomes. What is calculated in the program is a wire that runs parallel along the ground plane and where one end of the wire is connected to this.