Subelement F: Receivers— Topic 44: Mixers
Question 3-44F5
Element 3 (GROL)If a receiver mixes a 13.8 MHz VFO with a 14.255 MHz receive signal to produce a 455 kHz intermediate frequency signal, what type of interference will a 13.345 MHz signal produce in the receiver?
Explanation
A superheterodyne receiver mixes a radio frequency (RF) signal with a local oscillator (LO) to produce a fixed intermediate frequency (IF). Here, the 14.255 MHz RF mixes with the 13.8 MHz VFO (LO) to create a 455 kHz IF (14.255 - 13.8 = 0.455 MHz).
An image response occurs when an unwanted signal, different from the desired RF, also mixes with the LO to produce the same IF. The image frequency is typically LO ± IF. In this case, where the LO is below the RF (LO + IF = RF), the image frequency is LO - IF.
Calculating this: 13.8 MHz (LO) - 0.455 MHz (IF) = 13.345 MHz.
The 13.345 MHz interfering signal perfectly matches this image frequency. It will mix with the 13.8 MHz VFO (13.8 - 13.345 = 0.455 MHz), creating an identical IF signal and thus an image response. Other options describe different interference mechanisms or are not specific terms for this superheterodyne phenomenon.
Related Questions
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