HP 8901B Operation And Calibration Manual page 28

General Information Model 8901B
I
r
(a) BASEBAND SIGNAL
When the baseband signal is symmetrical, the modulation index can
also
be expressed in terms of the
average carrier level, A , and the envelope peak,
T ,
relative to the carrier. Then P = A + r , and T
=
A-
r ,
and the expression for modulation index becomes
27
r
x
100%
=
-
x 100% =
-
x 100%.
2A A
A + r - A + r
m =
A + r + A - r
This is the expression which the Modulation Analyzer evaluates when making an AM measurement,
Referring back
to Figure 1-3, it is apparent that A = 1 and
T
=
0.5 so, as before
m
=
-
0.5
x
100%
= 50%
1
The Modulation Analyzer makes an AM measurement by forcing the average carrier level,
A, to
a
known, fixed level by means of an automatic level control (ALC) circuit. The signal is then demodulated,
and the amplitude of the recovered baseband signal is measured with a peak detector. The output of
the detector is
r ,
which is (in effect) multiplied by the constant 1OO/A and displayed as the 9% AM.
(b)
AMPLITUDE MODULATED CARRIER
I
Figure
1-5.
AM with an Asymmetrical Baseband Signal
The range of modulation indexes for AM measurements by the Modulation Analyzer is essentially 0 to
100%. There are, however, types of modulation that produce modulation indexes greater than 100%.
An example of such is suppressed-carrier AM. The Modulation Analyzer is not intended for measuring
such signals. Nevertheless, there are cases, when the Modulation Analyzer will display a modulation
index that exceeds 100%. This can occur, for example, on an asymmetrical waveform where a narrow
peak
is
greater than the average carrier level. This is illustrated in Figure 1-6.
1-14