Question

An AM superheterodyne receiver is designed to receive the signals with carrier frequency ($f_c$) in the range (550 to 1650) kHz. The intermediate frequency (IF) of the receiver is 450 kHz and the local oscillator frequency $f_{LO}$ of the mixer is selected. Such that $f_{LO}$>$f_c$ always. For proper reception of the signals, the frequency ${}^{\prime \prime }{f_{LO}}^{\prime \prime }$ should be in the range

• A. (1650 to 2100) kHz
• B. (900 to 2000) kHz
• C. (1000 to 2100) kHz
• D. (1450 to 2550) kHz

Answer 1

The correct option is C (1000 to 2100) kHz

Given that IF =450 kHz
range of $f_c$ = (550 to 1650) kHz
$f_{LO}$ > $f_c$
For proper reception of signals,
$f_{LO}-f_c=IF$

$f_{LO(min)}=f_{c(min)}+IF=550+450=1000kHz$

$f_{LO(max)}=f_{c(max)}+IF=1650+450=2100kHz$

So, the range of $f_{LO}$ is (1000 to 2100) kHz.