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
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