Receivers

Explanation of question 7

In a superheterodyne receiver the output from the RF amplifier is at 3.75MHz, and the output from the local oscillator at 4.215MHz. Both signals are fed into a mixer. The output from the mixer could be at

1. 465kHz
2. 3.75MHz
3. 4.215MHz
4. 7.5MHz

The answer is :- 4 It has previously been stated that when two frequencies f₁ and f₂ are mixed, the output is xf₁ ± yf₂. where x and y are whole numbers.

465 kHz is the difference frequency (4.215 MHz - 3.75 MHz). The i.f. could equally be on 7.965 MHz (4.215 MHz + 3.75 MHz) if the mixer output circuit was tuned to the frequency. The output frequency would be determined by the mixer output tuned circuit.