Page 17 - Complete Wireless Design

Wireless Essentials

16 Chapter One

Figure 1.19 Capacitance versus the applied reverse
voltage for a varactor diode.

Varactors are found in circuits that require a voltage-variable capacitance,
such as tunable resonant filters and voltage-controlled oscillators (VCOs).
They are available in many diverse capacitance values for almost any RF
application.

PIN diodes. PIN diodes are constructed of a thin intrinsic layer sand-
wiched between a positive and a negative doped layer. They can be operat-
ed as RF switches and attenuators. PIN diodes, above certain frequencies
(greater than 50 MHz), do not act as normal PN junction rectifier diodes,
but as current-controlled resistors (the carrier lifetime rating will decide
the diode’s low frequency limit, under which the PIN begins to function as
a normal PN junction diode). PINs will also have a much lower on resis-
tance than do normal PN junction rectifier diodes, which can be changed
1
over a range of ohm to over 10,000 ohms with the application of a DC
2
control current. When employed as a switch, this control current is
switched on or off, thus going from a very low resistance (on), to a very high
resistance (off). When used as an attenuator, this control current is
changed continuously, normally in nondiscrete steps, allowing the PIN to
alter its resistance from anywhere between its lowest to its highest resis-
tance values. Figure 1.20 displays a typical PIN diode’s forward-bias cur-
rent and resultant RF resistance.

Schottky diode. The Schottky diode is constructed of a metal that is deposited
on a semiconductor material, creating an electrostatic boundary between the
resulting Schottky barrier. These diodes can be found in microwave detectors,
double-balanced modulators, harmonic generators, rectifiers, and mixers.
Some Schottky diodes can function up to 100 GHz, have a low forward barrier
voltage, and are mechanically sturdy.

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