Electromagnetic Compatibility

Grzegorz Kosobudzki

; Jerzy Leszczyński

Spectrum Analyzer

Frequency Domain vs Time Domain

V

o

lta

g

e

time

fre

qu

en

cy

2

Swept Tuned Spectrum Analyzer

3

f

1

f

2

Detector

Scan

Generator

f

1

f

2

Tunable

Bandpass Filter

Input

Real Time Spectrum Analyzer

4

Scan

Generator

Filter

1

Detector

1

Filter

2

Detector

2

Filter

3

Detector

3

Filter

n

Detector

n

Scan

Switch

A

m

p

lit

u

d

e

1

2

3

n

Frequency range

Frequency

resolution

bandwith

Frequency

Signal

Input

Vector signal spectrum analyzer

5

Attenu-

ator

Local

Oscillator

IF

Filter

AD

Memory

DSP

Low Pass

Filter

Signal

Input

Mixer

Super heterodyne spectrum analyzer

6

Attenu-

ator

Local

Oscillator

IF

Filter

Detector

Sweep

generator

AD

memory

CPU

AD

Video

Filter

Signal

Input

IF

LO

input

f

f

f

Frequency Resolution

7

a) IF filter Bandwidth used
b) Shape of the IF filter
c) Sideband noise of the IF filter

A

m

p

lit

u

d

e

Center frequency

60 dB Bandwidth

3dB

Band-

width

6

0

d

B

3

d

B

frequency

Frequency Resolution

8

A

m

p

lit

u

d

e

frequency

3dB

Bandwidth

IF Filter 1

IF Filter 2

Real Spectrum

of signal

Effect of too fast scanning for given IF bandwidth filter

9

A

m

p

lit

u

d

e

Center frequency

frequency

Amplitude modulation

10

1.5

1

0.5

0

0.5

1

1.5

v

o

lt

ag

e

E

m

in

E

c

E

m

a

x

)

cos(

)

(

t

U

t

x

)

cos(

)

(

t

E

t

c

c

c

Amplitude modulation

11

t

t

m

E

t

s

c

c

cos

))

cos(

1

(

)

(

min

max

min

max

E

E

E

E

m

0

5

10

15

2

1.5

1

0.5

0

0.5

1

1.5

2

v

o

lt

ag

e

0

5

10

15

3

2

1

0

1

2

3

time

v

o

lt

ag

e

Amplitude modulation

12

A

m

p

lit

u

d

e

frequency

Carrier Ec

Upper

Sideband

Lower

Sideband

Modulating

frequency

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