6-5. Actual characteristics of capacitors
This section discusses why the noise reduction properties of a
simple bypass capacitor differ from the basic characteristics.
Knowing why this is can help you to construct filters that provide
excellent noise reduction at lower costs, and to select parts with
good cost-effectiveness.
6-5-1. Review of bypass capacitor operation
(1) Bypass the noise current to ground
Some noise reduction filters built from capacitors use bypass
capacitors. As shown in Fig.
1, bypass capacitors eliminate noise by bypassing the noise current
to the ground.
(2) The smaller the impedance, the greater the noise reduction
The smaller the impedance of the bypass capacitor, the easier it
will be for the current to flow ( (1) in Fig. 1) . This means that
more noise will be eliminated. In other words, insertion loss is
increased.
For example, if the insertion loss and impedance of the 1,000pF
capacitor introduced in Section 6-4 are compared, the shape of the
graphs will be just about the same, as shown in Fig.2. This is
because an insertion loss of 3dB occurs at frequencies at which the
impedance is 25 ohms or lower, while the insertion loss will be
greater the lower the impedance is in frequencies above that range.
(3) The noise reduction effect of a capacitor is expressed by its
impedance
The noise reduction effect of a capacitor can therefore be expressed
with impedance, in the attenuation range. To keep the explanation
simple, this discussion will only take impedance into
consideration.
You may be aware that the impedance of a capacitor is inversely
proportional to the frequency and electrostatic capacitance. This is
why, when graphed, impedance forms a simple downward sloping line,
as shown by the theoretical values in Fig. 2 (a) . These theoretical
values will be referred to as the "ideal capacitor" in future
graphs, and will be used for comparison purposes.
(4) Examples of actual capacitor impedance measurements
Fig. 3 shows examples of the actual impedance measured from several
capacitors. A film capacitor, some MLCCs, and an electrolytic
capacitor are shown in the graph.
The MLCCs and film capacitor look similar as they all form rough
V-shaped curves. The electrolytic capacitor forms a round U-shaped
curve at the bottom. This shows that the trend shown by the 1,000pF
capacitor in Fig. 2 is common to all capacitors. The reason for this
shape is explained below.
Note however that the measured values used here are just some
examples to demonstrate trends, and that values may differ depending
on the product.
(5) The greater the electrostatic capacitance, the smaller the
impedance
The following describes a case where the electrostatic capacitance
is changed for a given type of capacitor.
Fig. 4 shows what happens when the electrostatic capacitance of an
MLCC (1608 size SMD) is changed from 1,000pF to 1μF by a factor of
ten each step (nominal values). The impedance of an ideal capacitor
is shown with a dotted line for comparison.
As shown in the figure, the impedance of the capacitor forms a
V-shaped curve that is very close to that of the ideal capacitor on
the left side, and the lines for each electrostatic capacitance
lined up cleanly in order. The capacitor can be seen as a simple
electrostatic capacitance element at these frequencies.