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The next variation, "antenna switching diversity," again
consists of a single receiver with two antennas. The receiver
includes circuitry that selects the antenna with the better
signal according to an evaluation of the radio signal.
Switching noise is possible but this system avoids the
possibility of phase cancellation between antennas
because the antennas are never combined. Range is the
same as for a single antenna system. Cost is relatively low
and setup is convenient. (See Figure 2-22.)
In both of these active antenna diversity approaches, the
switching decision is based on the received signal quality of
a single receiver section. When the signal quality falls below
some preset threshold, switching occurs immediately. If the
new antenna (or antenna combination) doesn’t improve the
reception, the receiver must switch back to the original state.
The lack of "predictive" ability often causes unnecessary
switching, increasing the chance of noise. The switching
speed is also critical: too fast and audible noise occurs, too
slow and a dropout may occur.
A recent antenna switching method offers predictive
diversity capability using a microcontroller to optimize
switching characteristics. A running average signal level
and a maximum signal level are calculated by analyzing
the change in signal level over time. Comparing the
current average signal level to the most recent maximum
signal level determines the switch action, based on typical
dropout characteristics. Small declines at high signal
levels indicate impending dropout, causing a switch to
occur. At moderate signal levels, larger decreases are
allowed before switching. At very low signal levels
switching is curtailed to avoid unnecessary noise. Of
course, if the signal level is increasing, no switching
occurs. The onset of dropout can be more accurately
recognized and countered, while eliminating switching
when there is little likelihood for improvement.
"Receiver switching diversity" is a widely used diversity
system. It consists of two complete receiver sections, each
with its own associated antenna, and circuitry that selects the
audio from the receiver that has the better signal. Switching
noise is possible but when properly designed these systems
can have very good dropout protection with little chance of
other audible effects due to incorrect selection. This is
because the system compares the signal condition at each
receiver output before audio switching occurs. Range is the
same as with single antenna systems. Cost is higher, but
setup is convenient. (See Figure 2-23.)
"Ratio combining diversity" also uses two complete
receiver sections with associated antennas. This design
takes advantage of the fact that, most of the time, the
signal at both antennas is useable. The diversity circuitry
combines the outputs of the two receiver sections by
proportionally mixing them rather than switching between
them. At any given moment, the combination is
proportional to the signal quality of each receiver. The
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Selection
and Operation
of W
ireless Microphone Systems
C
H A P T E R
2
Basic Radio Systems
Figure 2-22: antenna switching
Figure 2-21: antenna phase switching
Figure 2-20: passive antenna combining
Figure 2-23: receiver switching
Figure 2-24: receiver combining