Page 66 - Complete Wireless Design

Modulation

Modulation 65

output is the baseband signal, which is amplified and sent on to headphones
or speakers.
Depending on the number of modulating tones and their amplitude, differ-
ent time-domain outputs can be viewed on an oscilloscope. If a single baseband
tone is injected into an SSB modulator, a steady RF signal in both amplitude
and frequency will be created, as in Fig. 2.20. This is simply a CW signal.
However, a two-tone baseband signal will generate the consummate SSB mod-
ulation envelope display of Fig. 2.21, with the amplitude of the modulation
envelope dependent on the baseband modulation level. The two-tone RF signal
will start to “flat-top” (Fig. 2.22) if overmodulation occurs, causing extreme
distortion and spurious outputs.

2.3.4 Output power
The measurement of output power in SSB is the same as in AM, the peak enve-
lope power (PEP) being the measurement of the average peak power of the
transmitted signal with 100 percent modulation. PEP can be calculated by
V 2 /R, or V I , or I 2 R, where V and I are those of the maximum
RMS RMS RMS RMS
modulated peak.

Figure 2.20 A single-tone SSB signal in the time
domain.

Figure 2.21 A two-tone SSB signal showing its
modulation envelope.

Figure 2.22 An overmodulated two-tone SSB signal.

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