Correct Answer - Option 4 : 138.6
Concept:
The image rejection ratio of the superheterodyne receiver is defined as:
\(IRR = \alpha = \sqrt {1+Q^2 ρ ^2}\)
Q: Quality factor
\(ρ = \frac{f_{si}}{f_s}-\frac{f_s}{f_{si}}\)
fsi: Image frequency
\(f_{si}=f_s+2IF\)
IF: Intermediate frequency
Calculation:
Given fs = 1000 kHz and Q = 100, IF = 455 kHz
Image frequency is:
\(f_{si}=1000kHz + 910kHz = 1910kHz\)
\(ρ = \frac{1910}{1000}-\frac{1000}{1910}\)
ρ = 1.91 - 0.52
ρ = 1.39
\(IRR = \alpha = \sqrt {1+100^2\times 1.39 ^2} \)
\(IRR = \alpha = \sqrt {1+19321} \)
IRR = 139.00
