GROL Receivers and Transmitters 2 — Questions and Answers
Question 1: What is the primary purpose of the IF (intermediate frequency) stage in a superheterodyne receiver?
- To amplify the RF signal before mixing
- To provide fixed-frequency amplification and selectivity (Correct answer)
- To demodulate the incoming signal
- To generate the local oscillator frequency
Correct answer: To provide fixed-frequency amplification and selectivity
The IF stage amplifies and filters the signal at a fixed frequency after mixing, making high-quality filtering practical regardless of the tuned frequency.
Question 2: In a superheterodyne receiver, the local oscillator frequency is typically offset from the incoming RF signal by:
- Exactly the audio bandwidth
- The intermediate frequency (IF) (Correct answer)
- Twice the carrier frequency
- One-half the RF frequency
Correct answer: The intermediate frequency (IF)
Mixing the RF signal with the local oscillator (offset by the IF) produces a difference frequency equal to the IF, which is then amplified and filtered.
Question 3: What is the image frequency in a superheterodyne receiver?
- A harmonic of the desired carrier
- A second RF frequency separated from the desired signal by twice the IF that also produces an IF output when mixed (Correct answer)
- The mirror image of the audio output
- The local oscillator frequency reflected back to the antenna
Correct answer: A second RF frequency separated from the desired signal by twice the IF that also produces an IF output when mixed
The image frequency is separated from the desired signal by 2 × IF; it mixes with the local oscillator to produce an output at the IF just like the desired signal.
Question 4: Automatic Gain Control (AGC) in a receiver is designed to:
- Automatically tune to the strongest available signal
- Maintain a relatively constant audio output level across a wide range of signal strengths (Correct answer)
- Control the local oscillator frequency
- Increase selectivity on weak signals
Correct answer: Maintain a relatively constant audio output level across a wide range of signal strengths
AGC reduces the gain of RF and IF amplifier stages when strong signals are received, preventing overloading while keeping weak signals audible.
Question 5: The noise figure of a receiver is a measure of:
- Background atmospheric noise at the antenna
- How much noise the receiver itself adds relative to the thermal noise floor (Correct answer)
- The signal-to-noise ratio at the output
- The minimum detectable signal at the antenna
Correct answer: How much noise the receiver itself adds relative to the thermal noise floor
Noise figure (in dB) quantifies the degradation of signal-to-noise ratio as the signal passes through the receiver; a lower number means less added noise.
Question 6: What is the purpose of a pre-selector (front-end bandpass filter) in a superheterodyne receiver?
- To amplify weak signals before they reach the mixer
- To reject image frequencies and strong out-of-band signals before the mixer stage (Correct answer)
- To provide audio frequency filtering
- To stabilize the local oscillator
Correct answer: To reject image frequencies and strong out-of-band signals before the mixer stage
The pre-selector attenuates image-frequency signals and strong interferers before they enter the mixer, preventing them from producing spurious IF outputs.
Question 7: In an FM receiver, the limiter stage that precedes the discriminator is used to:
- Limit the audio output power to a safe level
- Remove amplitude variations from the IF signal so only frequency variations reach the detector (Correct answer)
- Limit the deviation of the received signal
- Prevent the squelch from activating on strong signals
Correct answer: Remove amplitude variations from the IF signal so only frequency variations reach the detector
Because FM information is encoded in frequency variations, the limiter strips unwanted amplitude noise before the discriminator detects the frequency changes.
GROL Receivers and Transmitters 2
What is the primary purpose of the IF (intermediate frequency) stage in a superheterodyne receiver?