Question 3-44F1

In a superheterodyne receiver, an image frequency is a spurious signal that, when mixed with the Local Oscillator (LO) frequency, also produces the Intermediate Frequency (IF). To find the image frequency, first determine if the LO is operating above or below the desired RF frequency. Given: * Normal channel ($f_{RF}$) = 151.000 MHz * Intermediate Frequency ($f_{IF}$) = 11.000 MHz * Local Oscillator ($f_{LO}$) = 140.000 MHz Since $f_{LO}$ (140 MHz) is less than $f_{RF}$ (151 MHz), this is a low-side injection system. We can verify this: $f_{RF} - f_{LO} = 151.000 \text{ MHz} - 140.000 \text{ MHz} = 11.000 \text{ MHz}$, which matches the given $f_{IF}$. For a low-side injection system, the image frequency ($f_{image}$) is calculated using the formula: $f_{image} = f_{LO} - f_{IF}$ Substituting the given values: $f_{image} = 140.000 \text{ MHz} - 11.000 \text{ MHz} = 129.000 \text{ MHz}$ Alternatively, for low-side injection, the image frequency can be calculated as $f_{RF} - (2 \times f_{IF})$: $f_{image} = 151.000 \text{ MHz} - (2 \times 11.000 \text{ MHz}) = 151.000 \text{ MHz} - 22.000 \text{ MHz} = 129.000 \text{ MHz}$. Both methods confirm that the image frequency is 129.000 MHz. Options A, C, and D are incorrect as they do not correspond to the correct image frequency calculation for this system.