06-08-2013, 12:22 PM
Radio Frequency Circuit Design
Radio Frequency.pdf (Size: 3.29 MB / Downloads: 41)
BASIC TRANSMITTER–RECEIVER CONFIGURATION
The design of radio frequency (RF) circuits borrows from methods used in low-
frequency audio circuits as well as from methods used in design of microwave
circuits. Yet there are also important departures from these techniques, so the
design of radio frequency circuits requires some specialized techniques not found
in these other frequency ranges. The radio frequency range for present purposes
will be taken to be somewhere between 300 MHz and 3 GHz. It is this frequency
range where much of the present day activity in wireless communication occurs.
In this range of frequencies, the engineer must be concerned about radiation,
stray coupling, and frequency response of circuit elements that from the point
of view of lumped, low-frequency analysis might be expected to be indepen-
dent of frequency. At the same time the use of common microwave circuit
elements such as quarter wave transformers is impractical because of the long
line lengths required. The use of monolithic circuits have enabled many high-
frequency designs to be implemented with lumped elements, yet the frequency
response of these “lumped” elements still must be carefully considered.
DEPENDENT STATES
The definitions of the preceding section imply that the voltage level in each time
interval, , is independent of the voltage level in other time intervals. However,
one simple example where this is not the case is the transmission of the English
language. It is known that in the English language the letter e appears more
frequently than the letter z. It is almost certain that the letter q will be followed
by the letter u. So, in transmitting a typical message in English, less information
is actually sent than would be sent if every letter in the alphabet were equally
likely to occur. A way to express this situation is in terms of probability. We
are interested in the total number of signal combinations that could occur in
a message T seconds long if each interval that is independent from the others
is nT / .
Resistors, Capacitors, and Inductors
INTRODUCTION
At radio frequencies passive circuit elements must be considered more care-
fully than in lower-frequency designs. The simple resistor, capacitor, or inductor
cannot be counted on to provide pure resistance, capacitance, or inductance
in high-frequency circuits. Usually the “lumped” element is best modeled as
a combination of these pure elements. In addition, when the size of the element
becomes larger than 0.1 wavelength in the circuit medium, the equivalent circuit
should include the transmission lines.
RESISTORS
Integrated circuit resistors can be classified into three groups: (1) semiconductor
films, (2) deposited metal films, and (3) cermets (a mixture of metal and dielectric
materials). Of these, only the first two have found widespread use in high-
frequency circuits. Semiconductor films can be fabricated by diffusion into a host
semi-insulating substrate, by depositing a polysilcon layer, or by ion implanta-
tion of impurities into a prescribed region. Polysilcon, or polycrystalline silicon,
consists of many small submicron crystals of silicone with random orientations.
CAPACITORS
Some of the most important parameters that need consideration in choosing
a capacitance are (1) the capacitance value, (2) capacitance value tolerance,
(3) loss or Q, (4) temperature stability, (5) mechanical packaging and size,
and (6) parasitic inductance. These criteria are interdependent, so often the
appropriate compromises depend on the constraints imposed by the particular
application. This section will consider both hybrid and monolithic capacitor
designs.
INDUCTORS
Inductors operating at radio frequencies have a variety of practical limitations
that require special attention. A tightly wound coil in addition to providing a self
inductance also has heat loss due to the nonzero wire resistance, skin effect losses,
eddy current losses, and hysteresis losses when a magnetic material is used.
Furthermore two conductors close together at two different voltages will also
exhibit an interelectrode capacitance. At radio frequencies these effects cannot
be neglected as easily as they could at lower frequencies. The equivalent circuit is
shown in Fig. 2.9. In this figure the series resistance, Rs , represents the conductor
loss as well as the skin effect losses. The parallel resistance, Rp , represents
the effect of eddy current losses and the hysteresis loss in magnetic materials
when present.
Magnetic Materials
A recurring problem is the need for a large value of inductance. An obvious
solution is to increase the flux density within an inductor coil with the addition of
a magnetic material with high permeability r . Most magnetic materials introduce
losses that are unacceptable at radio frequencies. A variety of ferrite materials
however have been found to have low loss at radio and microwave frequencies
in comparison with most other magnetic materials.
Monolithic Spiral Inductors
Lumped monolithic inductors have been used in circuit designs as tuned loads
for amplifiers, filters to reduce out of band signals and noise, and as a means
of enhancing stage gain by tuning out device or parasitic capacitances at the
center frequency. Planar inductors have been implemented in practical systems
for many years using a variety of different substrates. They were examined early
in the development of silicon integrated circuits, but were abandoned because of
process limitations and losses in the series resistance and substrate that effectively
reduced their operating frequency. Now, however, technological improvements
have made them available for mobile communications systems.
Impedance Matching
INTRODUCTION
A major part of RF design is matching one part of a circuit to another to provide
maximum power transfer between the two parts. Even antenna design can be
thought of as matching free space to a transmitter or receiver. This chapter
describes a few techniques that can be used to match between two real impedance
levels. While some comments will be made relative to matching to a complex
load, the emphasis will be on real impedance matching. The first part of this
chapter will discuss the circuit quality factor, Q. The Q factor will be used with
some of the subsequent matching circuit designs.