Advanced Qualification · A-006 Receivers · A-006-002-004
If the incoming signal to the mixer is 3 600 kHz and the first IF is 9 MHz, at which one of the following frequencies would the local oscillator (LO) operate?
- A 21 600 kHz
- B 5 400 kHz
- C 3 400 kHz
- D 10 600 kHz
Correct answer: B — 5 400 kHz. From the official ISED question bank, verified word for word.
Why
The local oscillator must run at 5 400 kHz because the mixer combines the incoming 3 600 kHz signal with the oscillator frequency to create the 9 MHz intermediate frequency, which is the difference between 9 000 kHz and 3 600 kHz. The full explanation — and one for every question — is in Pro.
Practise this section
Receivers: superheterodyne design, selectivity, sensitivity, images, AGC and noise.