Question

A double conversion superheterodyne receiver is designed with $f_{IF\left(1\right)}=30MHz$ and $f_{IF\left(2\right)}=3MHz$. LO frequency of each mixer stage is set at the higher of the two possible value. When the receiver is tuned to a carrier frequency of 300 MHz, insufficient filtering by the RF and first IF stages results in interference from three image frequencies. Those three image frequencies are

• A. 294 MHz, 360 MHz, 426 MHz
• B. 354 MHz, 360 MHz, 366 MHz
• C. 357 MHz, 360 MHz,363 MHz
• D. 294 MHz, 360 MHz, 366 MHz

Answer 1

The correct option is D 294 MHz, 360 MHz, 366 MHz

Let us consider a simple block diagram of a double conversion receiver as follows:

$f_{LO\left(1\right)}=f_c+f_{IF\left(1\right)}=300+30=330MHz$
$f_{LO\left(2\right)}=f_a+f_{IF\left(2\right)}=3+30=33MHz$

When there is insufficient filtering by RF and IF stage -I, the possible value of $f_a$ at the input of IF stage for the selected $f_{LO\left(2\right)}$ are ,
Desired , $f_a=f_{LO\left(2\right)}-f_{IF\left(2\right)}=33-3=30MHz$

Unwanted$f_a^{\prime }=f_{LO\left(2\right)}+f_{IF\left(2\right)}=33+3=33MHz$

The input frequencies that produce $f_a=30MHz$ are

Desired$f_c=f_{LO\left(1\right)}-f_a=330-30=300MHz$

Unwanted$f_c^{\prime }=f_{LO\left(1\right)}+f_a=330+30=360MHz$

The input frequencies that produce ${f^{\prime }}_a=36MHz$

Unwanted${f_c}^{\prime \prime }=f_{LO\left(1\right)}-f_a^{{}^{\prime }}=330-36=294MHz$

Unwanted${f_c}^{\prime \prime }=f_{LO\left(1\right)}+f_a^{{}^{\prime }}=330+36=336MHz$

So, the three unwanted image frequencies are 294 MHz, 360 MHz , 366 MHz