Ssb generation method

SSB generation method

1. SUPPRESSION OF UNWANTED
SIDEBAND (SSB-SC)
Mr.Patel J.H

2. •Three main systems are employed for the generation of SSB;
• The filter Method,
• The phase Shift Method
•The “Third method."
•They differ are from one another in the way of suppre ssing the
unwanted sideband, but all use some
form of balanced modulator to suppress the carr ier.
• The balanced modulator is the key circuit in single-sideban d
generation but each uses a different method of removin g the
unwanted sideband.
•All three systems will' remove either the upper or the low er
sideband with equal ease, depending on the specific circuit
arrangement

3. 1.The Filter System

•The filter system is the simplest system of the
three-
•After the balanced modulator the unwanted
sideband is removed by a filter.
• The filter may be LC, crystal, ceramic or
mechanical, depending on the carrier
frequency and other requirements.
•The key circuits in this transmitter are the
balanced modulator and the sideband-
suppression filter.

4. Block diagram of Filter method

5. Block diagram description
1. A crystal oscillator produces a stable carrier freq uency fc (say 100 KHz)
2. This carrier frequency is then fed to the balanc ed modulator through a
buffer amplifier.
3. The audio signal having frequency range from 300 to 2800 Hz. is applied
to balanced modulator through audio amplifier .
4. The output of balance modulator is DSB –SC i.e T he carrier is suppressed
in this stage but allows only to pass the both side bands. (USB & LSB).
5. A side band suppression filter is band pass fil ter (BPF) allows only a single
band either USB or LSB to pass through it dependin g on requirements.
6. This side band is then heterodyned in the balance d mixer stage with 12
MHz frequency produced by crystal oscillator or syn thesizer depends upon
7. the requirements of transmission. So in mixer st age, the frequency of the
crystal oscillator or synthersizer is added to SSB signal.
8. The output frequency thus being raised to the val ue desired for
transmission.

6. Advantages

The advantages of single side band SSB transmission are as
follows.
1. It allows better management of the frequency spec trum.
more transmission can fit into a given frequency ra nge
than would be possible with double side band DSB si gnals.
2. All of the transmitted power is message power no ne is
dissipate as carrier power.
3. The noise content of a signal is an exponential function of
the bandwidth: the noise will decrease by 3dB when the
bandwidth is reduced by half.
4. There fore, single side band SSB signals have les s noise
contamination than DSB double side band.

7. Disadvantages

1. The cost of a single side band SSB receiver is hi gher than
the double side band DSB counterpart be a ratio of about
3:1.
2. The average radio user wants only to flip a power switch
and dial a station. Single side band SSB receivers require
several precise frequency contro lsettings to minim ize
distortion and may require continual readjustment d uring
the use of the system.
Disadvantages Disadvantages

8. 2.Phase Shift Method

9. Block diagram description
1. The phase shift method of SSB generation causes one of the side
bands to be canceled out.
2. It uses two balanced modulators instead of one balanc ed
modulator1 & balanced modulator2 respectively . The bala nced
modulators effectively eliminate the carrier.
3. The carrier oscillator is applied directly to the upper ba lanced
modulator along with the audio modulating signal.
4. Then both the carrier and modulating signal are shifted in phase
by 90
o
and applied to the second or lower, balanced modulator.
5. The two balanced modulator output are then added to gether
algebraically.
6. The phase shifting action causes one side band to be canceled out
when the two balanced modulator outputs are combined.

10. •Let carrier signal is EcSinωct & Modulating signal (Audio Signal) EmSinωmt .
•Balanced modulator 1 produces the product of these two signals.
•Applying a trigonometric identity we get
EcSinωct * EmSinωmt = 1/2 [cos (ωc – ωm ) t - cos (ωc + ωm ) t] --- (1)
Similarly for Balanced modulator 2 carrier signal ( EcSinωct - 90
o
)= EcCos ωct
and Modulationg signal is applied with 90
o
phase shift (EmSinωmt - 90
o
)=
EmCos ωmt .
EcCos ωct * EmCos ωmt
=
1/2

[cos (ωc – ωm ) t + cos (ωc + ωm ) t ] ----- (2)
Add (1) & (2)
The output of Adder is
V0= Cos (ωc – ωm ) t .
i.e Lower Side band only .

11. Advantages
1. It can generate the SSB signal at any frequency, so the frequency up converter
stage is not required .
2. It can use the low audio frequencies as modulatin g signal .(In filter method, this
is not possible ) .
3. It is easy to switch from one sideband to other .

12. Disadvantages
1. The main drawback is that the design of the 90o p hase shifting network for the
modulating signal is extremely critical .
2. This network has to provide a correct phase shift of 90o at all the modulating
frequencies which is practically difficult to achie ve .

13. Third Method or Weaver method

14. •It consists of four balanced modulators, two carrie r signal generators , two audio
low-pass filters, and two 90 phase-shift networks.
•The low pass filter having center frequency f 0.
•The filter output is







•This output is applied to balance modulator 3 & 4 a long with RF crystal Oscillator .
•The output of balance modulator 3 & 4 is

15. •an addition operation will yield the upper
sideband, while a subtraction will yield the
lower sideband .

16. Advantages
•The third method requires neither a sideband-
suppression filter nor a wideband audio
phase-shift network.
•Correct output can be maintained simply
without critical parts or adjustments

17. Disadvantages
•Dc coupling may be needed to avoid the loss
of signal components close to the audio
carrier frequency.
•Whistle will exist at that frequency if the
balance of the low-frequency balanced
modulators deteriorates.
• This system is the most complex of the three