HP 8901B Operation And Calibration Manual page 31

Model
8901B
General Information
BASEBAND SIGNAL
W W
BASEBAND SIGNAL
W
SIGNAL FROM FREQUENCY MODULATOR
SIGNAL FROM FREQUENCY MODULATOR
SIGNAL FROM PHASE MODULATOR
SIGNAL FROM PHASE MODULATOR
SIGNAL FROM AMPLITUDE MODULATOR
(c) SINE WAVE BASEBAND SIGNAL
SIGNAL FROM AMPLITUDE MODULATOR
(d) SINE WAVE BASEBAND SIGNAL: 3/2 THE RATE OF
(c)
~~
Amplitude Modulators for Various Baseband Signals
Other Considerations
In practice, it is difficult to produce an FM or cPM signal which does not also have a small amount
of AM-called
incidental AM or AM-on-FM. Likewise, an AM signal usually contains a small
amount of incidental FM and cPM. In order to accurately measure this incidental modulation, the
Modulation Analyzer itself must not contribute to it. This contribution is specified as AM rejection
and FM rejection.
A typical CW signal also contains a
small
amount of residual AM, FM, and ckM. The residual
modulation is generated by such things as line hum, noise, and microphonics. The residual AM and
FM specifications quantify the residual modulation internal to the Modulation Analyzer.
Residual modulation affects the modulation readings in a manner which depends on the detector used,
the nature 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 discussed under Residual Noise Effects in the Detailed Operating Instructions in Section
3. When the average detector is used, the residuals add approximately in an rms manner, that is, the
square root of the sum of the squares of the noise and the signal. The effect of 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.
1-17