GROL Test Equipment and Measurements 2 — Questions and Answers
Question 1: What is the input impedance of most RF test instruments and transmission systems?
- 25 ohms
- 50 ohms (Correct answer)
- 75 ohms
- 300 ohms
Correct answer: 50 ohms
The standard impedance for most RF test equipment and coaxial transmission systems is 50 ohms, chosen as a compromise between power handling and low loss.
Question 2: What does noise figure (NF) measure in a receiver?
- The amount of harmonic distortion at the output
- The degradation of signal-to-noise ratio caused by the receiver itself (Correct answer)
- The receiver's selectivity at the IF frequency
- The maximum input power the receiver can handle
Correct answer: The degradation of signal-to-noise ratio caused by the receiver itself
Noise figure quantifies how much the signal-to-noise ratio degrades as a signal passes through the receiver, with lower NF indicating better sensitivity.
Question 3: What is the decibel (dB) expression for a power ratio of 2:1?
- 3 dB (Correct answer)
- 6 dB
- 10 dB
- 20 dB
Correct answer: 3 dB
A power ratio of 2:1 corresponds to approximately 3 dB because 10 × log₁₀(2) ≈ 3.01 dB.
Question 4: When using a vector network analyzer (VNA), what is S11 (return loss) a measure of?
- Forward gain of an amplifier
- Power reflected back from the device input (impedance mismatch) (Correct answer)
- Phase noise of an oscillator
- Noise floor of a receiver
Correct answer: Power reflected back from the device input (impedance mismatch)
S11 measures the ratio of reflected to incident power at the input port, indicating how well the device's input impedance matches the reference impedance.
Question 5: What is intermodulation distortion (IMD) in a receiver or amplifier?
- Distortion caused by insufficient bias voltage
- Unwanted signals produced when two or more signals mix in a non-linear stage (Correct answer)
- Noise generated by thermal effects
- Distortion from excessive supply ripple
Correct answer: Unwanted signals produced when two or more signals mix in a non-linear stage
IMD occurs when two or more signals interact in a non-linear device, producing spurious signals at sum and difference frequencies (and their harmonics).
Question 6: What does a Bird-type directional wattmeter measure in a transmission line?
- Only total power at the antenna
- Forward (incident) and reflected power separately, allowing SWR calculation (Correct answer)
- Signal frequency and harmonic content
- Cable loss in dB
Correct answer: Forward (incident) and reflected power separately, allowing SWR calculation
A directional wattmeter (such as the Bird 43) uses a slug-and-meter system to measure forward power in one direction and reflected power in the other direction on the transmission line.
GROL Test Equipment and Measurements 2
What is the input impedance of most RF test instruments and transmission systems?