1. AM Modulation – Types of AM Methods (SSB and VSB)
CHAPTER 2 – Part 3
AM Modulation – Types of AM Methods
(SSB and VSB)

2. Why SSB?
Shortage of AM and DSB signals is basic information is transmitted TWICE (2X).
Once in each sideband
Both sideband have identical information
No solid reason to transmit both!!
One sideband may be suppressed, the remaining sideband is SSB signal.
Bandwidth SSB signal, BWSSB = fm(max)

3. Benefits of SSB Signals
All power transmitted (one-half of power in DSB method) represents intelligence.
Occupy half spectrum space than AM and DSB signals and less interference.
Transmit stronger information and more reliable over longer distances.
Susceptible to less noise due to restricted signal BW and receiver BW (major adv for long-distance comm.)
Experience less fading due to ionosphere.

4. How Fading Affect Signal?
Fading means signal alternately increase and decrease in strength as pick-up by the receiver.
The ionosphere bends all signals compositions back to earth at slightly different angles resulting canceling out or adding up of the signals.
Fading does not occur in SSB since only one sideband transmitted.

5. Characteristic of SSB
When no information is present, no RF signal is transmitted (unusual condition) – e.g voice pause
A carrier modulated by a single tone audio signal generates a constant frequency pure sin wave at the rate of either the upper or lower frequency.
Modulating voice (varying amplitude and freq) will create a complex RF SSB which varies in freq and amplitude over narrow spectrum bounded by voice BW.

6. SSB Power Content
In practice an SSB transmitter generates both sidebands and a switch is used to select either USB or LSB transmission.
Since SSB transmitter sends no carrier the carrier power, PC = 0.
The transmitter output is expressed in peak envelope power (PEP) – maximum power produced on voice amplitude peaks.

7. PEP Computation The PEP output power is computed by,
The PEP input power is computed by,
V = Voltage across tx antenna
R = antenna chac. impedance
Eq (2.17)
Vs = dc supply voltage of final
amplifier stage
Imax = maximum final amplifier peak
current
Eq (2.18)

8. Contd… PEP occurs only occasionally.
Voice amplitude peaks are produced only when very loud sound generated or words emphasized.
During normal speech levels, the average power is typically only one-fourth or one-third of the PEP value with typical human speech.
Typical SSB transmitters designed to handle the average power level on continuous basis not the PEP.

9. Example 1
Calculate the input and output PEP assuming that a voice signal produces a 120-Vpp signal across a 50- transmitting antenna and the dc supply of the final amplifier stage is 300 V with a peak current of 0.6 A. Hence evaluate the power handling capability of a typical SSB transmitter for this system.

10. Solution 1 PEP (in) = VsImax = (300V)(0.6A) = 180 W
PEP (out) = Vp2/2R = (60)2/[2x50]
= 36 W
Pavg = 1/4[PEP(out)] to 1/3[PEP(in)]
= 1/4[36W] to 1/3[36 W]
= 9 W to 12 W

11. Example 2
A voice signal with a frequency range of 300 to 3000 Hz modulates a 14.3-MHz RF carrier. Determine the frequency limit for the output signal produced by SSB AM modulator and its BW.
Draw the time-domain wave and spectrum using a constant 2 kHz sine wave tone.

12. Solution 2 The SSB signal may use either USB or LSB.
If chosen USB: Frequency band extends from [14.3M + 300] to [14.3M ] i.e. 14,300,300 Hz to 14,303,000 Hz.
If chosen LSB: Frequency band extends from [14.3M – 3000] to [14.3M – 300] i.e. 14,297,000 Hz to 14,299,700 Hz.
BW = fUSB(highest) –fUSB(lowest) or
fLSB(highest) – fLSB(lowest) = 2.7 kHz

13. Solution 2

14. Solution 2

15. Variations of SSB Operations
Single sideband suppressed-carrier (SSBSC or simply SSB) – used for HF mobile communications.
Vestigial sideband (VSB) – used worldwide for television (picture) transmissions.
Single sideband full-carrier (SSBFC) – used in a low-distortion, compatible AM broadcasting system whose transmitted signal can be received by standard AM and SSB receivers.

16. Contd…
Single sideband reduced-carrier (SSBRC) – a pilot carrier sys widely used in transmarine point-to-point radiotelephony.
Single sideband controlled-carrier (SSBCC) – another pilot carrier sys used in which power is independent of modulation and output Pavg is constant.
Independent sideband (ISB) also called twin-sideband suppressed-carrier (TSBSC) – transmit two independent sidebands, each containing different intelligence, frequently used for HF point-to-point radiotelephony and primarily used in military communications.
We will discuss on the first THREE only.

17. Comparing Spectrum of AM Methods
Figure 23 (a) & (b)

18. Contd…
Figure 23 (c) and (d)

19. Two Basic Methods for SSB Generation
Filter method.
Phase cancellation method.
Both methods use some form of balanced modulator to suppress the carrier but differ in the suppression of the unwanted sideband.
Commonly used SSB generators consist of balanced modulator followed by an extremely selective crystal, ceramic or mechanical filter or phasing circuit.

20. Balanced Modulators
The balanced modulator (BM) is a symmetrical circuit that delivers an output signal containing the sum and difference frequencies of the two inputs.
In theory the injected carrier is balanced out so that it disappear in the output.
Practical design characteristics determine the extent to which the carrier can be balanced out.
Existing BM suppress the carrier from 30 dB to 40 dB below the PEP.
The SSB filter further suppresses the carrier by an additional 20 dB.
Total carrier suppression of from 50 dB to 60 dB can be reasonable be expected from SSB transmitter systems.
Refer Figure 24

21. Simplified Block Diagram of a Filter-type SSB generator
Figure 24

22. Filter Method
The widely used technique is filter method which uses a BP filter to pass one sideband while reject the other.
The filters used in SSB transmitters must have very fast rise and fall slopes and flat BW characteristics.
Crystal filters are commonly used in SSB systems because they have very high Q (selectivity) values.
This method is not suitable for high frequency signals (VHF and above).

23. Simplified Block Diagram of a Filter-type SSB Transmitter
Figure 25

24. SSB Transmitter Action
Figure 26

25. Sideband Filters
Voice transmission for adequate speech intelligibility – audio frequency from 100 Hz to 3000 Hz
After carrier suppression, one sideband rejected without affecting the other.
Closeness between sidebands require high Q-filters for sharply defined skirts and narrow pass band and flat band-pass characteristics to pass the desired sideband and reject the other.

26. Phase-cancellation Method
The phase-cancellation method uses two matched balanced modulators, both of which suppress the carrier and have a double-sideband output.
The two sideband signals are combined by a summing circuit producing only the desired sideband.
Advantage of this method is SSB signal at the operating frequency can be generated without the frequency conversion.
Disadvantage is the phase shifts and amplitudes of the signals to the summing circuit must be maintained.

27. Contd…
This method can be used at higher frequency band (VHF and above).
Reconfigurable for any sideband output i.e. upper or lower-generated SSB signal.
In case of upper sideband modulation, the modulated output is phase-shifted by /2.

28. Block Diagram of Phase-shift SSB Transmitter
Figure 27

29. Upper Sideband SSB Modulator
Figure 28

30. Lower Sideband SSB Modulator
Figure 29

31. Mathematical Analysis of SSB Signals
Recall from previous part that the output of balanced modulator (DSB signal) is
The expression is DSB signal contains two sideband frequencies.
Eq (2.19)

32. Contd…
If one sideband is removed either by filtering or canceling method, taking USB to remain hence
Since all the transmitted power goes into USB wave, yield
Eq (2.20)
Eq (2.21)

33. Contd…
Demodulation of a single sideband signal is achieved by multiplying it with a locally generated synchronous carrier at the receiver.
Detectors using this principle are product detectors which use balanced modulator circuit.
The carrier must be as closely synchronized in frequency (≤100Hz) and phase with the original carrier in the transmitter to avoid distortion in the modulated output.

34. Contd…
To demonstrate the demodulation process, multiply the received signal with Vccosct yields,
Low-pass filtering easily remove the 2nd term which is the upper side freq at the second harmonic of the local carrier freq, leaving only the 1st term which is required information.
Eq (2.22)

35. Demodulation using carrier reinsertion
Figure 30

36. SSB Receiver (demodulator)
SSB receivers are superheterodyne but differ from standard AM receivers in that additional circuitry must be used to restore the suppressed carrier.
Since SSB transmission is used extensively in HF communications systems, SSB receivers must be capable of receiving signal in HF band (3 MHz to 30 MHz).

37. Block Diagram of SSB Receiver
Figure 31

38. SSB Applications
SSB Signals are popularly used in telephone systems as well as in two-way radio.
Two-way SSB communications is used in the military, CB radio and radio amateurs.

39. Disadvantage of SSB
SSB transmitters are more complicated hence more expensive than standard AM transmitters – due to added cost and system complexity involved in providing better frequency stability in the transmitter and receiver.

40. VESTIGAL SIDEBAND
(VSG)

41. Vestigal Sidebands (VSB)
Overcomes the complexity problem of coherent demodulation.
Obtained by passing one pass band mostly and a vestige, just a trace, of the other sideband of an AM/DSB-SC modulation.
Easy to generate.
Bandwidth is ~ 25% greater than SSB signals.

42. Contd… Also called asymmetric sideband system.
Compromise between DSB & SSB.
Low frequency including DC components can also be transmitted and received.
Average transmitted power equals SSB + small power of carrier.

43. Contd…
AM wave is applied to a vestigial sideband filter, producing a modulation scheme – VSB + C
Mainly used for television video transmission.
VSB allows recovery of the base band signal with a receiving system (envelope detector) intended for AM signal.
Avoid the necessity of coherent demodulation.
Introduce some distortion.

44. VSB Spectrum
Figure 32
Eq (2.23)

45. Demodulation of VSB Signal
Using envelope detector.
Mathematically written as
SVSB(t) = Ac cos ct + (mAc / 2) cos(c+m)t
= Ac (1 + (m/2)cos m t) cos c t –
(mAc /2) sin c t sin m t
Eq (2.24)

46. COMPARISON
Table 1

47. Summary of Various AM Methods
DSB-SC
Less transmitted power – all the transmitted power is useful.
Wide bandwidth.
Requires a coherent carrier at the receiver – increased complexity.
Suited for point to point communication involving one Tx and one Rx which would justify the use of increased Rx complexity.

48. Contd…
Conventional
Sidebands are transmitted in full with the carrier.
Simple to demodulate ( envelope detector).
Wide bandwidth.
Used in commercial AM radio broadcasting - one TX, many receivers

49. Contd…
SSB
Good bandwidth utilisation (information signal BW = modulated signal BW)
Good Power efficiency.
Demodulation is harder compares to conventional – exact filter design & coherent demodulation.
Widely use for long distance transmission.
Preferred in systems requiring a minimal BW – multi channel carrier telephony.

50. Contd… VSB Compromise solution between DSB & SSB.
Has lower power, less bandwidth than DSB.
Higher power, slightly greater bandwidth than SSB.
Standard for transmission of TV (low frequency component).