DSB-SC signal

It is power efficient system

B.W – 2fm

Spectrum only contacts two sidebands carrier is not present, hence some power saving takes place.

B.W is same as that of DSB-FC

DSB – SC using balanced modulator: (BM)

BM is used to suppress the unwanted carrier in AM wave

The carrier and mod. sig are applied to i.p of BM and we get DSB sig with suppressed carrier at the o/p if B.M

o/p – upper and lower sidebands only.

⇒ Using diodes

Analysis:

Diode current $i_1$ and $i_2$

$i_1=av_1+bv_1^2$

$=a[x(t)+\cos⁡wct ]+b(x(t)+\cos⁡wct )^2$

$=ax(t)+a \cos⁡wct+bx^2 (t)+2bx(t) \cos⁡wct+b cos^2⁡wct$

Similarly,

$i_2=av_2+bv_2^2 \\ =a[\cos⁡wct-x(t)]+b[\cos⁡wct-x(t) ]^2 \\ =a \cos⁡wct-ac(t)+bx^2 (t)-2bx(t) \cos⁡wct+b cos^2 wct \\ ∴o/p vtg is ; \\ V_0=V_1-V_2 \\ V_0=i_1 R-i_2 R$

Substituting

$V_0=R[2aa(t)+4bx(t) \cos⁡wct ] \\ ∴V_0=2a Rx(t)+4b Rx(t) \cos⁡wct \\ \hspace{1cm} ↓ \hspace{4cm} ↓ \\ \text{Mod signal } \hspace{2cm} \text{DSB-SC signal}$

o/p contains a modulating term and DSB –SC signal. The modulating term is eliminated by bandpay filter and 2nd term is allowed to pa nthru LC bandpan filters.

Final o/p 4 $= 4bRx(t) \cos wct \\ = K x(t) \cos wct$

Thus balanced modulator with diode produces DSB-SC signal at o/p