18WuD2x

modulation capability • modulator-demodulator 453
modulation bars
nonlinearity occurs. Also see MODULATION LIN-
EARITY.
modulation characteristic For an amplitudemodulated
wave, the ratio of the instantaneous
amplitude of the modulated signal to the instantaneous
modulating voltage.
modulation code In a modulated transmitter, a
system of modulation in which certain signal
variations or pulses represent particular characters.
Examples are ASCII, BAUDOT CODE, and
MORSE CODE.
modulation coefficient Symbol, m. A figure expressing
the depth (extent) to which a signal is
amplitude-modulated. For a signal in which the
upward modulation is equal to the downward
modulation, m = (E m – E c )/E c , where E c is the
peak-to-peak voltage of the unmodulated signal,
and E m is the peak-to-peak voltage of the modulated
signal. For full (100%) modulation, m = 1.
modulation depth See DEPTH OF MODULATION.
modulation distortion 1. In a modulated signal,
envelope distortion introduced by the modulation
process or by the receiver circuit. 2. External
cross modulation (see CROSS MODULATION, 1).
modulation envelope See ENVELOPE, 1.
modulation-envelope distortion Undesirable distortion
in the ENVELOPE of the modulating intelligence
in an amplitude-modulated or single-sideband
signal at the output of a radio transmitter.
modulation factor See MODULATION COEFFI-
CIENT.
modulation frequency Abbreviation, f m . The frequency
of a modulating signal.
modulation linearity In a modulated signal, the
degree to which the instantaneous signal amplitude
or frequency follows the instantaneous amplitude
of the modulating signal.
modulation meter See PERCENTAGE-MODULA-
TION METER.
modulation monitor 1. A linear detector with a
pickup coil (or antenna) and headphones for listening
to an amplitude-modulated signal. 2. See
PERCENTAGE-MODULATION METER. 3. A device
that displays the modulation characteristics
of a signal (e.g., the envelope for amplitude modulation
and single sideband or the frequency-vs.
time function for frequency modulation).
modulation noise See NOISE BEHIND THE SIG-
NAL.
modulation percentage A measure of the extent
to which a signal is amplitude modulated. It is expressed
as the MODULATION COEFFICIENT
multiplied by 100.
modulation ratio For a modulated signal, the quotient
M r /M i , where M r is the percentage of radiated-signal
modulation, and M i is the percentage
of current modulation.
modulator A device or circuit for modulating a carrier.
modulator cell See KERR CELL.
modulator crystal A transparent piezoelectric crystal
to which a signal voltage can be applied to modulate
a beam of polarized light passing through it.
modulator-demodulator Also called modem. 1. A
device that converts binary data to audiofrequency-shift-keyed
(AFSK) analog signals, and
vice-versa. It is commonly used to interface computers
and terminals with telephone lines and radio
transceivers. 2. A circuit or device, such as a
Data in
Amplifier
A/D
converter
Computer
modulator-demodulator
Data out
Amplifier
D/A
converter