ssb te4201-08

1TE4201-Communication Electronics

8. SSB Generation and Transmitters

Types of Sideband Transmission

Generation of Suppressed Carrier DSB

SSB Filters

SSB Generation

TE4201-Communication Electronics 2

Types of Sideband TransmissionStandard SSB (only one sideband is transmitted)

SSSC – Single Sideband Suppressed Carrier (one sideband and a trace of carrier is transmitted)

ISB – Independent Sideband (suppressed carrier and two independent sidebands are transmitted)

VSB - Vestigial Sideband (a trace or vestige of the unwanted SB , carrier, and a full SB are transmitted as in TV transmission)

ACSSB – Amplitude Compensated Single Sideband (it is a SSSC with pilot carrier and the information signal is compressed to maintain the S/N ratio at all signal levels)

trace of carrier

Suppressed carrier

USB

USBLSB

USBcarrier

Compressed signal


Generation of Suppressed Carrier DSB

eS

eSeS eCes

ec

2. FET 1 input is ec + es and therefore id1 is a function of ec+es

3. FET 2 input is ec - es and therefore id2 is a function of ec-es

4. id1 will produce in-phase currents for ec and es

5. id2 will produce out-of-phase currents for ec and es

1. Common ec and differential es are applied to the input of two FET

6. Due to id1 and id2 the output will cancel the currents for ec ( no carrier ) and sum the currents for es (DSB – sum and diff. of fs and fc )

Balanced Modulator


Draw the Balanced Modulator circuit diagram using JFET and show how a Suppressed Carrier AM wave is obtained at the output of the Balanced Modulator. Let the waveform of the modulating signal be vm = Vm sin mt and the carrier wave be vc = Vc sin ct.

Example on DSB generators

2gs2cvgs2bvaD2iand2

gs1cvgs1bvaD1ibygivencurrentDrainFETAssume

VDD

vm

vm

vm

vc

vc

v GS

v gs1

v gs2

i D1

i D2

vo

fm

fc

DSB suppressedcarrier output

Balanced Modulator

tunedtofC

( )f fC m ( )f fC m

no fC

LSB USB

i D1 i D2

mC1gs vvv1JFETofGate

mC2gs vvv2JFETofGate

)1(vcv2cvcvbvbva

vvcvvbai

mc2m

2CmC

2mCmC1D

)2(vcv2cvcvbvbva

vvcvvbai

mc2m

2CmC

2mCmC2D

tsinVvandtsinVvIfvcv4bv2ihavewe)2(and)1(ngsubstitutiandiiithen

mmmCCC

mCmo2D1Do

tcosktcosktsink

tcostcosVcV24tsinbV2tsinVtsinVc4tsinVb2i

mC2mC2m1

mCmCmCmmmmCCmmo

opo2Cp iRvthenrQR tcosktcosktsinkR mC2mC2m1p


Ring ModulatorAt positive signal cycle

diode A = carrier + signal current

diode B = signal current

diode C = open

diode D = carrier - signal current

as A conduct more than D it will produce unbalanced current in the two windings of output transformer


Ring ModulatorAt negative signal cycle

as D conduct more than A it will produce unbalanced current in the two windings of output transformer

• Carrier current alone will cause balanced current at output transformer = No carrier output

• Signal current alone will cause AB or CD short according to it’s polarity = No signal output

• Unbalanced currents cause both carrier + signal = modulated signal of the carrier

At complete signal cycle

diode D = carrier + signal currentdiode C = signal currentdiode B = opendiode A = carrier – signal current


SSB Filters After Suppressed carrier modulator, output is Double Sideband Suppressed Carrier Select LSB and filter USB and the rest of all. Then SSB is obtained. In the diagram below has a freq. Band from fc-3k to fc-100 the information band 100 to 3k is there !

SSB is obtained

f4

)20/dB(logfQ21

1C

When filtering out SSB from suppressed-carrier DSB signal, the required Q depends upon the center frequency fc, the separation between the two sidebands f and the desired attenuation level of the unwanted sideband. It can be calculated from:

SSB Filters

dB suppression


Example on SSB Filters

Calculate the required Q for the situation depicted in the figure on last page for

(a) A 1MHz carrier and 80dB sideband suppression(b) A 100kHz carrier and 80dB sideband suppression

000,125Hz2004

)20/80(log10f4

)20/dB(logfQ)a(21

1621

1C

500,12Hz2004

)20/80(log10f4

)20/dB(logfQ)b(21

1521

1C

High carrier requires high Q


Crystal Filters

The crystal filter is commonly used in SSB system to attenuate unwanted sideband. Because of requirement of high Q, the crystal filter is most suitable than the best of LC filters. Crystals with Q > 50,000 are available.

L

R

Cs

Cp

Equivalent circuit of the crystal

fp fs

Z

0

Crystal is low impedance at series frequency and high impedance at parallel frequency

Tuned to center freq. = crystal freq.

Working crystal freq. is the series frequency

Crystal

C1

CC

I nputDSB

SSBoutputCP

Crystal holder capacitance CP

Counter Crystal holder capacitance C1


Ceramic Filters

The ceramic filter is in the same family of Piezoelectric effect as Crystals. Because of lower cost, ruggedness, and smaller size they are not only used for SSB filters, but also as 445kHz IF filters in superhet receivers. Ceramic filters with Q > 2,000 are available.

Note that the BW of 60dB is only 8kHz which is not very different from 6dB BW of 6.8kHz. Which means a very sharp (steep) slope.


BalancedModulator

Primaryf requencyOscillator

Audio f requencyBand

Mixer

Secondaryf requencyOscillator Linear Amp. only

Class-A orClass-B

LSB Filter

SSB outusing USB

Linear Amp. onlyClass-A or

Class-B

SSB outusing LSB

fc

fm = 0 tofm

fm

f cfm

f c fmDSB, no f c

LSB only

f c fm

fm

no f c

f c fmno f c

fm fm

f cfmf c fm

fO

f c fmMixer output

fO

fm fm

f o- (f c fm)

Bandpass Filter

f o+ (f c fm)= fo+ fc - fm

f o+ (f c fm)

f o and (f c fm)

f o- (f c fm)

f o+ (f c fm)

f o+ (f c fm)

Bandpass Filter

f o+ f c tof o+f c - f m

f o- (f c fm)= fo - fc + fmf o- f c tof o- f c + f m

f o- (f c fm)

SSB Generation

Draw the block diagram of a Single Side Band

transmitter and show the flow of signal frequencies,

if only upper side-band ( USB ) is to be

transmitted. Let modulating frequency fm

and a primary crystal frequency fC be entered

into the Balanced Modulator.

Example on SSB generators


Optimizing Q value by using two balanced modulator

Audioamplifier class

A

Low-f reqBalanced

Modulator

RF carrieroscillator

Filter andamplifier

Medium f req.Balanced

ModulatorLinear power

amplifier

Medium f req.oscillator

100kHz Crystal 3MHz variable

2.898MHz 3.102MHz98kHz 102kHz

2kHzSignal

102kHz

3.102MHz

3MHz

If only one balanced modulator is used to generate 3MHz carrier for 2kHz signal required Q will be 750

104103

kHz22MHz3

BWfQ 3

6o

If only two balanced modulators are used to generate 3MHz carrier for 2kHz signal required Q will be

15102.0

103MHz9.21.3

MHz3BWfQ 6

6o


tcostcos21

icic

tcostcos21

icic

BalancedModulator

90 deg phaseshif ter

Linear poweramplifier

BalancedModulator

cos i t +

90 deg phaseshif ter

Audio

Carriercos c t

cos i tcos i t

cos c t

cos c t

sin c t

(sin i t)(sinc t)

sin i t

(cosi t)(cosc t)

cos c -i)t

LSB onlycos c -i)t

SSB transmitter by Phase Method


Compressed signal

MixerAmplitudeSensor

Amplifier-40 dBm to 0 dBm

0 dBmPass through

0 dBm to +15dBm Attenuator

Amplitude Compandor to drive SSB input of information signal

I nput signal of variousamplitudes

S/ Nratiopoor

S/ Nratio

normal

S/ Nratiogood

Output signal of compressedamplitudes

S/ Nratio

normal

S/ Nratio

normal

S/ Nratio

normal

ssb te4201-08

Documents

carrier dsb signal

carrier signal currentas

carrier output signal

khz carrier

carrier modulator

pilot carrier

information signal

modulated signal