Amateur Extra E4C: Receiver performance
These are the 14 official questions in group E4C of the 2024–2028 Amateur Extra question pool, with the correct answer marked and a plain-English explanation of why it's right. The real exam asks one question from this group.
- E4C01Question 1
What is an effect of excessive phase noise in an SDR receiver’s master clock oscillator?
- AIt limits the receiver’s ability to receive strong signals
- BIt can affect the receiver’s frequency calibration
- CIt decreases the receiver’s third-order intercept point
- DIt can combine with strong signals on nearby frequencies to generate interference Correct
Why: Clock phase noise can mix with strong nearby signals and cause interference.
- E4C02Question 2
Which of the following receiver circuits can be effective in eliminating interference from strong out-of-band signals?
- AA front-end filter or preselector Correct
- BA narrow IF filter
- CA notch filter
- DA properly adjusted product detector
Why: A front-end filter or preselector blocks strong out-of-band signals.
- E4C03Question 3
What is the term for the suppression in an FM receiver of one signal by another stronger signal on the same frequency?
- ADesensitization
- BCross-modulation interference
- CCapture effect Correct
- DFrequency discrimination
Why: The capture effect lets the stronger FM signal suppress the weaker one.
- E4C04Question 4
What is the noise figure of a receiver?
- AThe ratio of atmospheric noise to phase noise
- BThe ratio of the noise bandwidth in hertz to the theoretical bandwidth of a resistive network
- CThe ratio in dB of the noise generated in the receiver to atmospheric noise
- DThe ratio in dB of the noise generated by the receiver to the theoretical minimum noise Correct
Why: Noise figure compares the receiver's noise with the theoretical minimum, in dB.
- E4C05Question 5
What does a receiver noise floor of -174 dBm represent?
- AThe receiver noise is 6 dB above the theoretical minimum
- BThe theoretical noise in a 1 Hz bandwidth at the input of a perfect receiver at room temperature Correct
- CThe noise figure of a 1 Hz bandwidth receiver
- DThe receiver noise is 3 dB above theoretical minimum
Why: −174 dBm is the theoretical noise in 1 Hz for a perfect receiver at room temperature.
- E4C06Question 6
How much does increasing a receiver’s bandwidth from 50 Hz to 1,000 Hz increase the receiver’s noise floor?
- A3 dB
- B5 dB
- C10 dB
- D13 dB Correct
Why: 10 log(1,000 ÷ 50) = 10 log 20 ≈ 13 dB.
- E4C07Question 7
What does the MDS of a receiver represent?
- AThe meter display sensitivity
- BThe minimum discernible signal Correct
- CThe modulation distortion specification
- DThe maximum detectable spectrum
Why: MDS stands for minimum discernible signal.
- E4C08Question 8
An SDR receiver is overloaded when input signals exceed what level?
- AOne-half of the maximum sample rate
- BOne-half of the maximum sampling buffer size
- CThe maximum count value of the analog-to-digital converter
- DThe reference voltage of the analog-to-digital converter Correct
Why: An SDR overloads when the input exceeds the ADC's reference voltage.
- E4C09Question 9
Which of the following choices is a good reason for selecting a high IF for a superheterodyne HF or VHF communications receiver?
- AFewer components in the receiver
- BReduced drift
- CEasier for front-end circuitry to eliminate image responses Correct
- DImproved receiver noise figure
Why: A high IF makes images easier for the front end to reject.
- E4C10Question 10
What is an advantage of having a variety of receiver bandwidths from which to select?
- AThe noise figure of the RF amplifier can be adjusted to match the modulation type, thus increasing receiver sensitivity
- BReceiver power consumption can be reduced when wider bandwidth is not required
- CReceive bandwidth can be set to match the modulation bandwidth, maximizing signal-to-noise ratio and minimizing interference Correct
- DMultiple frequencies can be received simultaneously if desired
Why: Matching bandwidth to the mode maximizes signal-to-noise and cuts interference.
- E4C11Question 11
Why does input attenuation reduce receiver overload on the lower frequency HF bands with little or no impact on signal-to-noise ratio?
- AThe attenuator has a low-pass filter to increase the strength of lower frequency signals
- BThe attenuator has a noise filter to suppress interference
- CSignals are attenuated separately from the noise
- DAtmospheric noise is generally greater than internally generated noise even after attenuation Correct
Why: Low-band atmospheric noise stays above the receiver's own noise even after attenuation.
- E4C12Question 12
How does a narrow-band roofing filter affect receiver performance?
- AIt improves sensitivity by reducing front-end noise
- BIt improves intelligibility by using low Q circuitry to reduce ringing
- CIt improves blocking dynamic range by attenuating strong signals near the receive frequency Correct
- DAll these choices are correct
Why: A roofing filter knocks down strong nearby signals, improving blocking dynamic range.
- E4C13Question 13
What is reciprocal mixing?
- ATwo out-of-band signals mixing to generate an in-band spurious signal
- BIn-phase signals cancelling in a mixer resulting in loss of receiver sensitivity
- CTwo digital signals combining from alternate time slots
- DLocal oscillator phase noise mixing with adjacent strong signals to create interference to desired signals Correct
Why: Reciprocal mixing is LO phase noise mixing with strong nearby signals.
- E4C14Question 14
What is the purpose of the receiver IF Shift control?
- ATo permit listening on a different frequency from the transmitting frequency
- BTo change frequency rapidly
- CTo reduce interference from stations transmitting on adjacent frequencies Correct
- DTo tune in stations slightly off frequency without changing the transmit frequency
Why: IF shift moves the passband away from adjacent-frequency interference.