Subelement F: Receivers— Topic 44: Mixers
Question 3-44F3
Element 3 (GROL)In what radio stage is the image frequency normally rejected?
Explanation
In a superheterodyne receiver, the image frequency is primarily rejected in the **RF (Radio Frequency) stage**.
The RF stage typically includes a tunable bandpass filter, often called a preselector, which passes the desired RF signal while attenuating unwanted signals, including the image frequency. This filtering happens *before* the signal reaches the mixer.
If the image frequency were to reach the mixer, it would mix with the Local Oscillator (LO) to produce an Intermediate Frequency (IF) signal identical to the desired signal's IF. Once an image signal is converted to the IF, it becomes indistinguishable from the desired signal, and the IF stage cannot reject it. The LO only generates a mixing frequency, and the detector extracts modulation; neither stage performs frequency rejection. Therefore, filtering at the RF stage is crucial for preventing image interference.
Related Questions
3-44F1 What is the image frequency if the normal channel is 151.000 MHz, the IF is operating at 11.000 MHz, and the LO is at 140.000 MHz?3-44F2 What is the mixing process in a radio receiver?3-44F4 What are the principal frequencies that appear at the output of a mixer circuit?3-44F5 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?3-44F6 What might occur in a receiver if excessive amounts of signal energy overdrive the mixer circuit?