HAM HAM Radio Extra Class Test Transmitter and Receiver Design 2 — Questions and Answers
Question 1: What is a pre-selector in an HF receiver and what does it do?
- A circuit that selects the operating mode before the audio stage
- A tunable bandpass filter ahead of the mixer that reduces image responses and intermodulation from strong out-of-band signals (Correct answer)
- An automatic antenna tuner integrated into the receiver front end
- A presettable memory for storing frequently used frequencies
Correct answer: A tunable bandpass filter ahead of the mixer that reduces image responses and intermodulation from strong out-of-band signals
A pre-selector is a tunable or switched bandpass filter placed before the mixer to reject signals outside the desired band, reducing image responses and preventing strong signals from causing intermodulation distortion.
Question 2: What are 'birdies' in a receiver and what causes them?
- External interference produced by nearby power lines at audio frequencies
- Spurious internal signals caused by intermodulation of oscillator harmonics with other internal signals (Correct answer)
- Interference from UHF bird-detection radar systems
- Audio oscillations caused by insufficient decoupling of the audio amplifier
Correct answer: Spurious internal signals caused by intermodulation of oscillator harmonics with other internal signals
Birdies are internally generated spurious signals that appear at fixed frequencies in a receiver, caused by mixing of harmonics of the local oscillator with other oscillators or clocks inside the receiver.
Question 3: What does the third-order intercept point (IP3) measure in a receiver?
- The frequency at which three separate signals combine
- A figure of merit indicating resistance to intermodulation distortion from strong signals; higher IP3 means better dynamic range (Correct answer)
- The power level at which three amplifier stages saturate simultaneously
- A measurement of three-tone audio distortion in the receiver's AF stage
Correct answer: A figure of merit indicating resistance to intermodulation distortion from strong signals; higher IP3 means better dynamic range
IP3 is a theoretical extrapolated point where third-order intermodulation products would equal the desired signal; a higher IP3 indicates the receiver can handle stronger signals before generating significant IMD products.
Question 4: What is the purpose of a noise blanker in an HF receiver?
- To provide a constant background noise reference for calibrating signal strength
- To suppress impulse-type noise such as ignition interference by briefly blanking the receiver during detected noise pulses (Correct answer)
- To reduce the overall noise floor by cooling the front-end amplifier
- To filter out audio-frequency hum from power supply noise
Correct answer: To suppress impulse-type noise such as ignition interference by briefly blanking the receiver during detected noise pulses
A noise blanker detects short-duration impulse noise pulses and mutes the receiver for the brief duration of each pulse, greatly reducing the subjective effect of ignition and other impulse noise.
Question 5: What is the 'image frequency' in a superheterodyne receiver?
- A duplicate display of the desired signal on a panadapter display
- A frequency on the opposite side of the local oscillator from the desired signal by the same IF interval, which can produce an unwanted response (Correct answer)
- The mirror-image sideband rejected during SSB demodulation
- The reflected signal from a nearby obstacle creating multipath
Correct answer: A frequency on the opposite side of the local oscillator from the desired signal by the same IF interval, which can produce an unwanted response
The image frequency is separated from the local oscillator by the same IF frequency but on the opposite side; signals at this frequency mix to produce the same IF and thus appear as interference if not rejected.
Question 6: What is the difference between receiver 'sensitivity' and 'selectivity'?
- Sensitivity applies to AM reception while selectivity applies to FM and SSB
- Sensitivity is the ability to detect weak signals; selectivity is the ability to reject adjacent-channel interference (Correct answer)
- Both terms describe the same receiver characteristic measured under different conditions
- Sensitivity measures RF performance and selectivity measures audio performance
Correct answer: Sensitivity is the ability to detect weak signals; selectivity is the ability to reject adjacent-channel interference
Sensitivity describes the minimum signal level the receiver can usefully detect (often expressed as minimum discernible signal or noise figure), while selectivity describes how well it rejects signals on adjacent frequencies.
HAM HAM Radio Extra Class Test Transmitter and Receiver Design 2
What is a pre-selector in an HF receiver and what does it do?