Electromagnetic Compatibility
Grzegorz Kosobudzki; Jerzy Leszczyński
Spectrum Analyzer
Frequency Domain vs Time Domain
time
2
Voltage
y
c
n
e
u
q
e
r
f
Swept Tuned Spectrum Analyzer
Tunable
Bandpass Filter
f1
Detector
Input
f2
Scan
Generator
f1
f2
3
Real Time Spectrum Analyzer
Filter Detector
1 1
Filter Detector
2 2
Signal
Scan
Input
Switch
Filter Detector
3 3
Scan
Filter Detector
Generator
n n
Frequency range
Frequency
resolution
bandwith
1 2 3 n
Frequency
4
Amplitude
Vector signal spectrum analyzer
Mixer
Signal
Attenu- Low Pass IF
AD
Input
ator Filter Filter
Local
DSP
Oscillator
Memory
5
Super heterodyne spectrum analyzer
Attenu- IF Video
Detector
ator Filter Filter
Signal
Input
AD
Local
Oscillator
Sweep
AD CPU memory
generator
finput fLO fIF
6
Frequency Resolution
a) IF filter Bandwidth used
b) Shape of the IF filter
c) Sideband noise of the IF filter
3dB
Band-
width
60 dB Bandwidth
Center frequency frequency
7
3dB
60dB
Amplitude
Frequency Resolution
3dB
Bandwidth
Real Spectrum
of signal
IF Filter 2
IF Filter 1
frequency
8
Amplitude
Effect of too fast scanning for given IF bandwidth filter
Center frequency frequency
9
Amplitude
Amplitude modulation
1.5
1
0.5
0
0.5
1
1.5
x(t) U cos( t)
c(t) Ec cos( t)
c
10
max
E
Ec
min
E
voltage
Amplitude modulation
2 3
1.5
2
1
1
0.5
0
0
0.5
1
1
2
1.5
time
30
20
5 10 15
5 10 15
s(t) Ec (1 m cos( t)) cos t
E max E min
c
m
E max E min
11
voltage
voltage
Amplitude modulation
Carrier Ec
Modulating
frequency
Upper
Sideband
Lower
Sideband
frequency
12
Amplitude
References
" Guide to Spectrum Analyzers  www.anritsu.com
" Fundamentals of Real Time Spectrum Analysis 
www.tektronix.com
" Agilent Spectrum Analysis Basics, aplication note 150
- www.agilent.com