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is not tuneable. Filter circuits of various types ranging from
simple coils to precision "helical resonators" are used in front
end filters. The second receiver section is the "local
oscillator" (usually abbreviated as "LO"). This circuit
generates a constant radio frequency that is related to the
frequency of the received radio signal but differs by a
"defined amount." Single frequency receivers have a fixed
frequency local oscillator (LO), again using a quartz crystal.
Tuneable receivers have an adjustable LO, which generally
uses a frequency synthesis design. (See Figures 2-11 a & b.)
Next, the (filtered) received signal and the local
oscillator output are input to the "mixer" section. The mixer,
in a radio receiver, is a circuit that combines these signals
in a process called "heterodyning." This process produces
two "new" signals: the first new signal is at a frequency
which is the sum of the received signal frequency and the
local oscillator frequency, while the second is at a frequency
which is the difference between the received signal
frequency and the local oscillator frequency. Both the sum
and the difference signals contain the audio information
carried by the received signal. It should be noted that the
LO frequency can be above or below the received
frequency and still yield the same difference frequency
when combined in the mixer. When the LO frequency is
lower than the received frequency the design is called
"low-side injection." When it is above it is called "high-side
injection." The sum and difference signals are then sent to
a series of filter stages that are all tuned to the frequency
of the difference signal. This frequency is the
"intermediate frequency" (IF), so-called because it is lower
than the received radio frequency but still higher than the
final audio frequency. It is also the "defined amount" used
to determine the local oscillator frequency of the previous
section. The narrowly tuned IF filters are designed to
completely reject the sum signal, as well as the LO
frequency and the original received signal, and any other
radio signals that may have gotten through the front end.
The IF filters allow only the difference signal to pass
through. (See Figure 2-12.) This effectively converts the
received radio frequency (RF) signal to the much lower
intermediate frequency (IF) signal and makes subsequent
signal processing more efficient. This overall process is
called "downconversion."
If only one LO and one mixer stage are used then only
one intermediate frequency is produced and the receiver
is said to be a "single conversion" type. In a "double
conversion" receiver the incoming signal is converted to
the final IF in two successive stages, each with its own LO
and mixer. This technique can provide increased stability
and interference rejection, though at significantly higher
design complexity and cost. Double conversion is more
common in UHF receiver designs where the received
signal frequency is extremely high. (See Figures 2-13 a & b.)
The IF signal is finally input to the "detector" stage
which "demodulates" or extracts the audio signal by one of
several methods. One standard technique is known as
"quadrature." When two signals are out of phase with each
other by exactly 90 degrees they are said to be in
quadrature. When such signals are multiplied together
and low-pass filtered the resulting output signal consists
13
Selection
and Operation
of W
ireless Microphone Systems
C
H A P T E R
2
Basic Radio Systems
Figure 2-11a: single conversion, crystal-controlled receiver
Figure 2-11b: single conversion, frequency-synthesized receiver
Figure 2-10: general receiver block diagram
Figure 2-12: receiver, filter characteristic