Anritsu spectrum master ms2711d User Manual page 125

Chapter 4 Measurement Fundamentals
band frequency and pressing the Delta (M2-M1) soft key. Figure 4-8 shows the modulating
frequency of 35 kHz as the value of delta Marker 2.
Figure 4-8. Amplitude Modulation
The markers read the frequency difference between the two signals, which is equal to the
modulating frequency. The marker also reads the difference in amplitude. This difference
in amplitude between the two signals can be used to determine the percentage of modula-
tion. Use the following formula to calculate the percentage of modulation:
E SB dB (
Where E (dB) is the energy in the sidebands in dB, E
SB
and m is the degree of modulation (multiply by 100 to get the percentage).
NOTE: Unequal amplitudes of the lower and upper sidebands indicate inciden-
tal FM on the input signal. Incidental FM can reduce the accuracy of percent-
age-of-modulation measurements.
Frequency Modulation
This section contains general information about frequency modulation, and an example of
using the MS2711D to display a FM signal.
Frequency modulations are generated when a modulating signal, fmod, causes an instanta-
neous frequency deviation of the modulated carrier. The peak frequency deviation, Dfpeak,
is proportional to the instantaneous amplitude of fmod, and the rate of deviation is propor-
tional to the frequency of the fmod.
The FM index, b, is defined as:
In general, the spectrum analyzer is a very useful tool for measuring D fpeak and b adjust-
ments of FM transmitters. FM is composed of an infinite number of sidebands. In practice,
4-14
m
) - E C dB ( ) = 20 log
2
D fpeak
b =
f mod
(dB) is the energy in the carrier in dB,
C