A 1mW video signal having a bandwidth of 100 MHz is transmitted to a receiver through a cable that has 40dB loss. If the effective one-sided noise spectral density at the receiver is 10^-20 Watt/Hz, then the signal to noise ratio at the receiver is

a) 50 dB

b) 30 dB

c) 40 dB

d) 60 dB

Answer is a)50db

 Signal Power = 1×10^-3  watts

 Total Noise power=10^-20 ×Bandwidth=10^-20 ×100 ×10⁶ 

 Now SNR at the receiver = (1×10^-3)/( 10^-20 ×100 ×10⁶ )= 10⁹

 Converting to db = 10 log  10⁹ =90db

 Now as there is a loss of 40dB

The SNR at the receiver =(90-40)db=50db