Page 57 - Complete Wireless Design

Modulation

56 Chapter Two

modulated by the baseband, the antenna current will rise because of the power
added by the increasing sidebands.
An enormous amount of sidebands are created during normal voice modula-
tion, and they are located at many different frequencies and amplitudes. As
shown in Fig. 2.10, a spectrum analyzer display of a voice amplitude-modu-
lated signal can become quite complex. This is why we must employ single- or
dual-tone baseband input signals for testing purposes.
A majority of AM voice transmitters confine their modulation frequencies to
between 300 and 3000 Hz to limit transmitted bandwidth. Limiting the base-
band frequencies is easily accomplished by the use of a bandpass filter located
just after the first audio (microphone) amplifier. An amplitude limiter circuit
can also be employed in order to limit the maximum audio baseband amplitude
to prevent AM overmodulation, which causes an unwelcome increase in trans-
mitted bandwidth due to spectral “splatter,” as well as distortion. Splatter is
the harmonic production in the original baseband frequencies created by clip-
ping of the signal’s modulation envelope. This action further modulates the RF
carrier, producing adjacent channel interference (ACI). The distortion level is
increased because now part of the AM signal is not actually present at the
demodulator (see Fig. 2.1), so intelligibility of the received signal is degraded.

2.1.3 Power measurement
The power of an AM signal can be measured as the peak envelope power (PEP),
which is utilized to gauge the average peak power, with 100 percent modula-
tion applied, of the transmitted signal:
PEP V 2 /R V I I 2 R
RMS RMS RMS RMS
The carrier power can also be calculated with these same formulas, but with
zero transmitter modulation.

Figure 2.10 A voice signal, as
viewed in the frequency domain,
is composed of many sidebands
at various frequencies and
amplitudes.

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