Am Concepts - Keysight N9030A Measurement Manual

Concepts

AM Concepts

AM Concepts

Figure 2-1 AM waveform
In AM (Amplitude Modulation), the instantaneous amplitude of the modulated
carrier signal changed in proportion to the instantaneous amplitude of the
information signal.
Figure 2-2 Calculation AM index in time and frequency domain
The modulation index "m" represents the amount of the modulation or the degree to
which the information signal modulates the carrier signal. The index for an AM
signal can be calculated from the amplitudes of the carrier and either of the
sidebands by the equation:
Equation 2-1
m =
For 100% modulation, the modulation index is 1.0, and the amplitude of each
sideband will be one-half of the carrier amplitude expressed in voltage. On a decibel
power scale, each sideband will thus be 6 dB less than the carrier, or one-fourth the
power of the carrier. Since the carrier power does not change with amplitude
modulation, the total power in the 100% modulated wave is 50% higher than in the
unmodulated carrier. The relationship between m and the logarithmic display can be
expressed as:
E – E E – E
max c max
---------------------- - = --------------------------- - =
E E + E
c max
26
E + E
min USB LSB
----------------------------- =
E
min c
2E
SB
-----------
E
c