Analyzing Amplitude Modulation On Spectrum Analyzer
Analyzing Amplitude Modulation On Spectrum Analyzer
Analyzing Amplitude Modulation On Spectrum Analyzer
Introduction
A spectrum analyzer is an essential measurement tool for an engineer and technician to study
signals in real time. The purpose of this lab is to analyze amplitude modulation on a spectrum
analyzer. In electronics and telecommunications, modulation is the process of varying one or
more properties of a periodic waveform, called the carrier signal, with a modulating signal which
typically contains information to be transmitted.
Objectives
Equipment
Required equipment for the lab
Qty Equipment
Calculate the error between time domain measurement and frequency domain measurement of
the modulation index.
Specifications:
Amax - Amin
The modulation index in time domain can be computed as h=
Amax + Amin
h= (3.89-1.55/3.89+1.55) = (43%)
Modulated Signal in frequency domain on spectrum analyzer:
To measure the modulation index ‘m’ in frequency domain, measure the difference of peak
amplitude in dB between the carrier peak and any side lobe. The following relation is then used
to calculate the modulation index.
1db=30dbm
ESB m
(dB) = 20 log( )
EC 2
m=0.417
Error between time domain measurement and frequency domain measurement of the modulation
index:
Error=0.43-0.417=0.013
Task#2
To measure the modulation index of a highly under modulated AM wave in frequency domain.
Calculate the error between time domain measurement and frequency domain measurement of
the modulation index.
Specifications:
Amax - Amin
The modulation index in time domain can be computed as h=
Amax + Amin
h= (7.02-4.68/7.02+4.68) = (20%)
Modulated Signal in frequency domain on spectrum analyzer:
To measure the modulation index ‘m’ in frequency domain, measure the difference of peak
amplitude in dB between the carrier peak and any side lobe. The following relation is then used
to calculate the modulation index.
ESB m
(dB) = 20 log( )
EC 2
m=0.21
Error between time domain measurement and frequency domain measurement of the modulation
index:
Error=0.21-0.2=0.01
Task#3
To measure the modulation index of an over-modulated AM wave in frequency domain.
Observe the AM wave on spectrum analyzer and measure the modulation index.
Specifications:
Conclusion:
Interestingly, besides measuring in time domain, the modulation index can also be measured in
the frequency domain. This is a powerful method that enables one to calculate modulation
indexes more than 1 and modulations index as low as 0.01. And 1% modulation percentage is
virtually impossible to measure in time domain using a standard oscilloscope.