AN4169

Passive components

Doc ID 023704 Rev 1

9/14

comprised of three components: pad layout, capacitor height and power plane spreading

inductance.

The main differences between ceramic dielectric types are the temperature coefficient of

capacitance and the dielectric loss. COG and NP0 (negative-positive-zero, i.e ±0) dielectrics

have the lowest losses and are used in filtering, matching and so on.

For RF applications it is generally recommended that multi-layer (or monolithic) ceramic

capacitors with a COG dielectric material, which is a highly stable class I dielectric offering a

linear temperature coefficient, low loss and stable electrical properties over time, voltage

and frequency.

For RF decoupling purposes select a capacitor value so that the frequency to be decoupled

is close to or just above the series resonant frequency (SRF) of the capacitor. At SRF the

parasitic impedance resonates with the device capacitance to form a series tuned circuit

and the impedance presented by the capacitor is the effective series resistance (ESR).

For DC blocking or coupling applications at RF, typically a capacitor with low insertion loss

and a good quality factor is required. Since a capacitor’s quality factor is inversely

proportional to its ESR, select a capacitor with a low ESR and ensure that the SRF of the

capacitor is greater than the frequency of operation. If the working frequency is above the

SRF of the capacitor, it appears inductive.

3.2

Inductors

An inductor is a passive electrical component used to store energy in its magnetic field. Any

conductor has inductance. An inductor is typically made of a wire or other conductor wound

into a coil, to increase the magnetic field.

Inductors differ from each other in construction techniques and materials used for

manufacture. A lot of different types of inductors exist (air core inductors, ferromagnetic core

inductors and variable inductors), but this document focuses on the inductors useful for RF

only. Air core inductors are usually used in RF. The term air core describes an inductor that

does not use a magnetic core made of ferromagnetic material, but is coil wound on plastic,

ceramic, or other nonmagnetic forms. They have lower inductance than ferromagnetic core

coils, but are used at high frequencies because they are free from energy losses called core

losses.

Usually, the real circuit of an inductor is composed of a series resistance and a parallel

capacitor. The parallel capacitor is considered to be the inter-winding capacitor that exists

between the turns of the inductor. If the inductor is placed over a ground plane then this

capacitance also includes the capacitance that exists between the inductor and the ground

plane. The series resistor can be considered as the resistance of the inductor winding.

In terms of circuit performance, as already mentioned for the capacitors, the self-resonant

frequency and the quality factor are the main inductor parameters, especially for a circuit

where the losses need to be minimized. At the self-resonant frequency, the inductor

impedance is at a maximum. For a frequency above the self-resonance the inductor

behavior changes and it appears capacitive.

In general, wirewound inductors have a higher quality factor than a multi-layer equivalent.

They also reflect and radiate more energy which can give rise to higher emission levels,

especially in terms of self-coupling. Inductive coupling can give rise to undesired circuit

operation: to minimize coupling, mount the inductors in sensitive circuit areas at 90 degrees

to one another.