HP 8901B

General Information Model 8901B
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BASEBAND SIGNAL
BASEBAND SIGNAL
SIGNAL FROM FREQUENCY M 0 D U LATO R
SIGNAL FROM PHASE MODULATOR
SIGNAL F RO M F REQU ENCY MO DU LATO R
SIGNAL FROM PHASE MOOULATOR
SIGNAL FROM AMPLITUDE MODULATOR
(a) SQUARE WAVE BASEBAND SIGNAL
SIGNAL FROM AMPLITUDE MODULATOR
(b) TRIANGLE WAVE BASEBAND SIGNAL
Figure 1-7. Signals from Requency, Phase, and
Now note the sine wave of Figure 1-7(c). The signals from the frequency and phase modulators look
the same except for the 90' phase shift between the two. For the frequency modulated signal, the
frequency is highest when the baseband signal
is most positive and lowest when most negative. For the
phase modulated signal, the frequency
is highest when the slope of the baseband signal is steepest in
a positive direction. This occurs at the positive-going zero crossing. Similarly, the frequency is lowest
when the slope
is most negative.
If in the last example, the rate, but not the amplitude, of the baseband signal is increased, the highest
and lowest frequencies
of the signal from the frequency modulator stay the same-they just occur more
often. However, for the signal from the phase modulator, not only do the frequency peaks occur more
often, but the excursions are large because the slopes
of the baseband signal are steeper at the zero
crossings. See Figure 1-7(d).
The maximum frequency deviation which can be measured is 400 kHz. The maximum phase deviation is
400 rad or 400 kHz divided by the modulation rate, whichever is smaller. As with AM, an asymmetrical
baseband waveform will result in different readings in
PEAK+ than PEAK-.
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