User Guide

PAMS
Technical Documentation
NHD–4
System Overview
Page 3–8
Original 11/97
Frequency Modulation
Unmodulated Carrier
1 kHz
Mod
Tone
+8 kHz
–8kHz
AMPS_5
Figure 5. FM Modulation
The figure above represents the effect of a 1 kHz audio modulating tone on an RF
carrier. The RF carrier in Figure 5 has a deviation of 8 kHz. A carrier deviation of
8 kHz means the frequency swings from plus 8 kHz to minus 8 kHz about the center
frequency.
When using Frequency Modulation (FM) how far from the center frequency the
carrier is deviated relates to how strong (loud) the modulating signal is. For
example if you whispered at the microphone the carrier might deviate only 1 or 2
kHz. On the other hand someone shouting at the microphone of their cellular
phone might cause it to deviate the maximum amount of 12 kHz. The standard
convention is that the positive part of the modulating signal will cause the carrier to
deviate to a higher frequency. While a negative modulation signal will cause the
carrier to go lower in frequency.
The rate, how fast the carrier frequency changes from high to low frequency, is
determined by the modulating signal’s frequency.
AMPS_6
Advanced Mobile Phone System ––– AMPS
Full Duplex Operation
TX to RX Spacing = 45 MHz
Channel Width = 30 kHz
832 Channels total for the 800 MHz band
Figure 6. AMPS: Specifications
The above information describes what AMPS stands for along with a “Nuts and
Bolts” description of the 800 MHz AMPS cellular system