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General Information
Model
8902A
I
BASEBAND SIGNAL
BASEBAND SIGNAL
SIGNAL FROM FREQUENCY MODU LATD R
SIGNAL FROM FREQUENCY MODULATOR
SIGNAL FROM PHASE MODULATOR
SIGNAL FROM PHASE MODULATOR
SIGNAL FROM AMPLITUDE MODULATOR
(a) SQUARE WAVE BASEBAND SIGNAL
SIGNAL FROM AMPLITUOE MODULATOR
(b) TRIANGLE WAVE BASEBAND SIGNAL
modulation is generated by siich things as line hum, noise, and microphonics. The residual AM and
FM
specifications quantify the residual modulation internal t o the Measuring Receiver.
Residual modulation affects the modulation readings in a manner which depends o n the detector
used, the natiire of the residuals, and the signal-to-noise ratio. If the residual is predominately noise,
when the peak detector is used, the residuals add in a way that is statistically related to the signal-
to-noise ratio. This is disciissed under Re,idiiul Noiw
I$]>C[S
in the Dc>(uilcd Opcwlrng
/n$(riic(iwi)
in
Section
3.
When the average detector is used, the residuals add approximately in an rms manner,
that is, the square root o f the sum o f t h e squares o f t h e noise and the signal. T h e effect o f this noise
becomes insignificant, however, when the signal-to-noise ratio rises above a few dB. Noise can be
further reduced by filtering the demodulated signal.
In
FM
broadcasting and communications, the signal-to-noise ratio is improved by giving the baseband
signal a high-frequency boost before applying it t o the modulator. This is called pre-emphasis. T h e
boost is a simple 6 dB per octave with the
3
dB corner specified by a time constant: for example, 75
p s
(which corresponds t o a
3
dB corner of 2.12
kHz)
for commercial broadcast FM. If desired, the
demodulated FM can be de-emphasized t o equalize the signal at the modulation outpiit and at the
display.
1-18
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