vesigial signals

8/11/2019 Vesigial Signals

http://slidepdf.com/reader/full/vesigial-signals 1/16

1

Vestigial Sideband (VSB)

Modulation



2

VSB Modulation

Basic idea

Signal generation

VSB Demodulation

Heterodyning

Vestigial Sideband (VSB) Modulation



3

Vestigial Sideband Transmission

• Vestigial sideband (VSB) transmission is a compromise betweenDSB and SSB

• In VSB modulation, one passband is passed almost completelywhereas only a residual portion of the other sideband is retained insuch a way that the demodulation process can still reproduce theoriginal signal.

f c− f c 0 f

XDSB( f )

f c− f c 0 f

XVSB( f )

• VSB signals are easier to generate because some roll-off in filter

edges is allowed. This results in system simplification. And theirbandwidth is only slightly greater than that of SSB signals (-25 %).



4

Generation of VSB Signal

The filtering operation can be represented by a filter H (f ) that

passes some of the lower (or upper) sideband and most of the

upper (or lower) sideband.

)()]()([)(21

21 f H f f M f f M f X ccVSB ++−=

m(t)



6

Demodulation of VSB Signal

The spectral density of the received vestigial-sideband signal is

)()]()([)(21

21 f H f f M f f M f X ccVSB ++−=

The output of the synchronous detector is

LPcVSB t f t xt m ]2cos)([)( π =

[ ] [ ]

[ ] LPcc

LPc LPc D

f f H f f H f M

f f H f M f f H f M f M

)()()(

)()()()()(

41

41

41

−++=

−++=

For faithful reproduction of x(t), we require that

. constant,)]()([ m LPcc f f f f H f f H ≤=++−

LPF mD(t)

cos 2π f ct

xVSB(t)

Synchronous Demodulation



7

Synchronous Demodulation of

VSB Signal

ω c−ω c 0 ω

XVSB(ω )

ω c−ω c 0 ω

Xd(ω )

−2ω c 2ω c

ω c−ω c 0 ω

Xo

(ω )

AfterLPF

After

mixer

Received VSBsignal



8

. constant,)]()([ m LPcc f f f f H f f H ≤=++−

By letting the constant be 2H( f c) :

mc LPcc f f f H f f H f f H ≤=++− ),(2)]()([

Thus, H(ω ) exhibits odd symmetry around the carrier frequency

ω c.The sum of the values of H(ω ) at any two frequencies equally

displaced above and below ω c is unity.

H(ω

)

H(ω – ω c)

H(ω +ω c)



9

Television picture signal has nominal bandwidth of 4.5MHz

If DSB modulation is used, it requires at least 9MHz for each TV

channel.

So, VSB modulation is used so that the whole TV signal is

confined to about 6MHz.

Example of VSB Signal: TV Signal

See pp. 122



10

VSB Modulation

Basic idea

Signal generation

VSB Demodulation

Heterodyning

Vestigial Sideband (VSB) Modulation



11

Heterodyning

t LOω cos

t t m RF ω cos)(

Synchronization

Bandpass filter Center Frequency

ωIF

t t m IF ω cos)(

LocalOscillator

ωRF: Radio frequency

ωLO :Local oscillator ωLO

ωIF : Intermediate frequency ωIF

• Heterodyning means the translating or shifting in frequency.

• By heterodyning the incoming signal at ωRF with the local oscillator

frequency ωLO, the message is translated to an intermediate frequencyωIF, which is equal to either the sum or the difference of ωRF and ωIF.

• If ωIF = 0, the bandpass filter becomes a low-pass filter and the original

baseband signal is presented at the output. This is called homodyning.

t t m LO RF )cos()( ω ω ±=



12

Heterodyning: Image Response

Another channel located at ωc +2ωIF will produce a heterodyned

output that overlaps with that from ωc.



13

Heterodyning: Image Response

Methods to solve the image response in heterodyne

receiver

1. Careful selection of intermediate frequency ωIF for a

given frequency band.

2. Attenuate the image signal before heterodyning



14

Superheterodyne Receiver See pp. 121-124

Radio-

Frequency

(RF) filter and amplifier

Intermediate--

Frequency

(IF) fil ter and ampli fier

Demodulator

Output

Local

Oscillator

In superheterodyne receiver:

1. ωIF is fixed (e.g. ωIF =455 kHz in AM radio)2. Tuning is achieved in RF and LO section

0 < ωIF < ωRF



15

Superheterodyne Receiver

Attenuated image

Image attenuation

Response/transfer function of

RF amplifier and filter



16

• We have introduced linear modulation. Inparticular,

• DSB-SC, Double sideband suppressed carrier

• DSB-LC, Double sideband large carrier (AM)• SSB, Single sideband

• VSB, Vestigial sideband

Summary

vesigial signals

Documents