IF spectrum occupancy: For a 70 MHz IF carrier with a transponder bandwidth of 36 MHz, between what IF frequencies (in MHz) must the signal energy lie?

Introduction / Context:
In satellite IF processing, the signal must fit within the allocated intermediate-frequency passband. Knowing the allowable lower and upper frequency limits around the IF carrier ensures proper filtering and avoids adjacent-channel interference.

Given Data / Assumptions:

• Center IF (f_IF) = 70 MHz.
• Transponder bandwidth B = 36 MHz.
• Assume symmetric occupancy around the center: ±B/2.

Concept / Approach:

Occupied band edges are at f_IF − B/2 and f_IF + B/2. With B = 36 MHz, B/2 = 18 MHz. Therefore the passband spans 70 − 18 to 70 + 18 MHz.

Step-by-Step Solution:

Verification / Alternative check:

Filter specifications (e.g., 70 MHz SAW/crystal filters) are commonly centered at 70 MHz with 36 MHz bandwidth, giving exactly 52–88 MHz 3 dB points.

Why Other Options Are Wrong:

• 34–106 or 34–142: much wider than the stated 36 MHz bandwidth.
• 106–142: centered at 124 MHz, not 70 MHz.
• 70–106: not symmetric around 70 and not 36 MHz wide.

Common Pitfalls:

• Forgetting to divide the bandwidth by 2 when locating band edges around the center frequency.

Final Answer:

52 and 88