Download - SSB TE4201-08
1TE4201-Communication Electronics
8. SSB Generation and Transmitters
Types of Sideband Transmission
Generation of Suppressed Carrier DSB
SSB Filters
SSB Generation
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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
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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
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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
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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
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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
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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
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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
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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
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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.
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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
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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
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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
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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