Question
What are the principal frequencies that appear at the output of a mixer?
Answer Options
- A) Two and four times the input frequency
- B) The square root of the product of input frequencies
- C) The two input frequencies along with their sum and difference frequencies
- D) 1.414 and 0.707 times the input frequency
Correct Answer: C
Explanation
A mixer circuit is the heart of a superheterodyne receiver or transmitter, designed to combine two input signals to produce a new set of output frequencies. The two input frequencies are the signal frequency (F_S) and the local oscillator frequency (F_{LO}). The output frequencies are governed by the physics of non-linear signal mixing.
The principal frequencies that appear at the output of a mixer are the two input frequencies along with their sum and difference frequencies (F_S, F_{LO}, F_S + F_{LO}, \text{ and } F_S - F_{LO}). All other output products are harmonic and intermodulation products that must be filtered out. The desired output is almost always the difference frequency (F_S - F_{LO}), which becomes the Intermediate Frequency (\text{IF}) in a receiver.
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