DSB Modulation Introduction

DSB modulation, which stands for Double Sideband modulation, is a type of amplitude modulation (AM) technique used to transmit analog signals such as voice, music, or video over radio frequency (RF) communication channels. In DSB modulation, the amplitude of the carrier signal is varied in proportion to the instantaneous amplitude of the modulating signal, resulting in a modulated signal that has two identical sidebands, one above and one below the carrier frequency.

DSB Modulation Introduction

The main advantage of DSB modulation is that it is a simple and efficient modulation technique that requires only a single frequency conversion in the transmitter and receiver. However, it also has some drawbacks such as a high bandwidth requirement and susceptibility to noise and interference. To overcome these issues, other modulation techniques such as single-sideband modulation (SSB) and frequency modulation (FM) have been developed.

DSB modulation can be implemented using either an analog or digital modulation scheme. In analog DSB modulation, the carrier and modulating signals are both continuous waveforms, while in digital DSB modulation, the modulating signal is a digital data stream that is used to modulate the carrier signal.

Overall, DSB modulation is a fundamental technique in RF communications, and understanding its principles is essential for anyone working in this field.

Objective:

The theory of AM showed that a carrier and two side bands are produced in AM generation. This chapter will show that it is not necessary to transmit all those signals to provide the receiver with enough information to reconstruct the original signal. Thus, it will be seen, the carrier signals may be removed or attenuated, and so can one of the two sidebands. The resulting signals will require less transmitted power and will occupy less bandwidth, and yet perfectly acceptable communications will be possible. This chapter introduces Several SSB AM systems (i.e. SSBSC, VSB),
Generation of SSB and VSB waves, Detection of SSB and VSB waves, bandwidth requirements, and advantages & disadvantages of choosing them over conventional DSB AM. After studying this chapter the student should be familiar with the following

1. SSB- Spectral characteristics
2. SSB Generation –filter method, Phase shift method
3. Demodulation of SSB wave
4. Effects of frequency and phase errors in synchronous detection DSB-SC, SSB-SC cases.
5. Comparison of AM systems.
6. VSB: generation, spectra, demodulation
7. Application of different AM systems.

DOUBLE SIDEBAND-SUPPRESSED CARRIER (DSBSC) MODULATION

Double sideband-suppressed (DSB-SC) modulation, in which the transmitted wave consists of only the upper and lower sidebands. Transmitted power is saved through the suppression of the carrier wave, but the channel bandwidth requirement is same as in AM (i.e. twice the bandwidth of the message signal). Basically, double sideband-suppressed (DSB-SC) modulation consists of the product of both the message signal m (t) and the carrier signal c(t),as follows:
S (t) =c (t) m (t)
S (t) =Ac cos (2 fct) m (t)
The modulated signal s (t) undergoes a phase reversal whenever the message signal m (t) crosses zero. The envelope of a DSB-SC modulated signal is different from the message signal. The transmission bandwidth required by DSB-SC modulation is the same as that for amplitude modulation which is twice the bandwidth of the message signal, 2W.
Assume that the message signal is band-limited to the interval –W ≤f≤ W

Spectrum of Baseband signal

 

Spectrum of DSBSC wave

Single-tone modulation:-

In single-tone modulation modulating signal consists of only one frequency component where as in multi-tone modulation modulating signal consists of more than one frequency components.
The standard time domain equation for the DSB-SC modulation is given by
S (t) =Ac cos (2 fct) m (t)………………… (1)
Assume m (t) =Amcos (2 fmt)……………….. (β)
Substitute equation (2) in equation (1) we will get
S (t) =Ac Am cos (2 fct) cos (2 fmt)
S (t) = Ac Am/β[cos βπ (fc-fm) t + cos βπ (fc+fm) t]…………… (γ)
The Fourier transform of s (t) is
S (f) =Ac Am/4[δ (f-fc-fm) + δ (f+fc+fm)] + Ac Am/4[δ (f-fc+fm) + δ (f+fc+fm)]

Power calculations of DSB-SC waves:-

Total power PT = PLSB+PUSB
Total power PT =Ac2Am2/8+Ac2Am2/8
Total power PT =Ac2Am2/4