Page 5 - Complete Wireless Design

Wireless Essentials

4 Chapter One

polarized. However, they possess a very large amount of capacitance per unit
volume, with this value ranging from greater than 22,000 F down to 1 F for
the aluminum types. Aluminum electrolytics have a limited life span of
between 5 to 20 years while tantalums, with their dry internal electrolyte,
have a much longer lifetime—and less DC dielectric leakage. Unfortunately,
tantalums have less of a range of values (between 0.047 F and 330 F) and
a lower maximum working voltage rating.
Metallized film capacitors are commonly good up to about 6 MHz and are
adopted for low-frequency decoupling. These capacitors are available in capac-
itance ranges from 10 pF to 10 F, and include the polystyrene, metallized
paper, polycarbonate, and Mylar™ (polyester) families. Metallized film capac-
itors can be constructed by thinly metallizing the dielectric layers.
Silver mica capacitors are an older, less used type of high-frequency capaci-
tor. They have a low ESR and good temperature stability, with a capacitance
range available between 2 and 1500 pF.
Ceramic leaded capacitors are found in all parts of RF circuits up to a maxi-
mum of 600 MHz. They come as a single-layer type (ceramic disk) and as a
stacked ceramic (monolithic) structure. Capacitance values range from 1.5 pF to
0.047 F, with the dielectric available in three different grades: COG (NPO) for
critical temperature-stable applications with tight capacitance tolerance values
of 5 percent or better (with a capacitance range of 10 to 10,000 pF); X7R types,
with less temperature stability and a poorer tolerance (±10 percent) than COG
(with available values of 270 pF to 0.33 F); and Z5U types, which are typically
utilized only for bypass and coupling because of extremely poor capacitance tol-
erances (±20 percent) and bad temperature stability (with a range of values from
0.001 to 2.2 F). However, the dominant microwave frequency capacitors today
are the SMD ceramic and porcelain chip capacitors, which are used in all parts
of RF circuits up to about 15 GHz. Nonetheless, even for these ultra-high-quali-
ty RF and microwave chip capacitors, the capacitance values must be quite small
in order for them to function properly at elevated frequencies. Depending on the
frequency, a maximum value of 10 pF or less may be all that we can use in our
circuit because of the increasing internal inductance of the capacitor as its own
capacitance value is raised. These leadless microwave chip capacitors are also
available in multilayer and single-layer configurations, with the multilayer types
normally coming in a basic SMD package, while single-layer capacitors are more
difficult to mount on a board because of their nonstandard SMD cases.
Nonetheless, single-layer capacitors can operate at much higher frequencies—up
to tens of GHz—than multilayer; but they will also have a much lower capaci-
tance range. In addition, some ceramic and porcelain microwave SMD capacitors
will have a microstrip ribbon as part of their structure for easier bonding to the
microstrip transmission lines of the printed circuit board.

1.1.4 Inductors
A significant, real-world high-frequency effect in an inductor is undesired dis-
tributed capacitance—which is a capacitance that is in parallel with the actual

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