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59
Selection
and Operation
of Wireless Microphone Systems
R EFERENCE I NFORMATION
Glossary
Field
a distribution of energy in space, ie. electric,
magnetic, sound
Field strength
the amplitude of a field at a given point, measured
in volts per meter for electric energy
FM
frequency modulation
Frequency
a measure of the rate of variation of a wave or signal
Frequency agile
having the ability to change frequencies: tuneable
Frequency response
variation in amplitude of a signal over
a range of frequencies
Front end
initial filter stage of a receiver
Gain
amplification
Ground plane
electrical approximation of a zero-potential reflective
surface at the base of an antenna
Handheld
transmitter type which can be held in the hand
Hash
a term for audible radio interference
Heterodyne
to combine signals of various frequency in a
manner that produces additional signals at
frequencies which are sums and differences of the
original frequencies
IEM
in ear monitor
IF
intermediate frequency, a lower radio
frequency (typically 10.7 MHz) found in the middle
stages of a receiver
IM
intermodulation, frequencies produced by combina-
tions of other frequencies in non-linear devices
IMD
intermodulation distortion (another name for IM)
IM rejection
ability of a receiver to reject IM products
Image
an interfering frequency which differs
from the desired frequency by twice
the intermediate frequency (IF)
Image rejection
the ability of a receiver to reject interference from an
image frequency (determined by the front end)
Impedance
a measure of the resistance to current
flow in a circuit (may vary with frequency)
Indirect
reflected or diffracted
Intercept (third order)
a measure of the ability of a radio input stage to han-
dle high signal levels without overload or distortion
Inverse square law
mathematical relationship in which one quantity is
inversely proportional to the square of another
quantity, ie. signal strength decreases according
to the distance squared
Level
the amplitude or strength of a signal
LF
low frequency
Limiter
a circuit which limits the maximum level of a signal
LO
local oscillator, in a receiver it is tuned to a
frequency which is offset from the operating
frequency by an amount equal to the intermediate
frequency (IF)
Loss
decrease in signal strength during
transmission, propagation or reception
Medium
substance through which a wave propagates;
for radio it may be vacuum, gas, liquid or solid;
wave speed is affected by medium
Mixer
circuitry in a receiver that combines the
received signal with the local oscillator to
produce the IF signal
Modulating frequency
the frequency of the audio signal used for
modulation of the radio wave
Modulation
variation of a wave parameter (such as amplitude
or frequency) to carry information
Modulation noise
low level noise which accompanies the audio signal
in a companded wireless microphone system
Multipath
reflection of radio waves that causes
fluctuation in received signal strength