HAM Practical Electronics Circuits 4 — Questions and Answers
Question 1: What is the purpose of a PIN diode in an RF switching circuit?
- To detect AM signals with high sensitivity
- To act as a variable resistance controlled by DC bias current (Correct answer)
- To multiply the input frequency by two
- To filter harmonic products from a power amplifier
Correct answer: To act as a variable resistance controlled by DC bias current
A PIN diode's intrinsic region resistance is controlled by DC bias, allowing it to act as a voltage-controlled switch or attenuator at RF frequencies.
Question 2: In a phase-locked loop (PLL), what does the voltage-controlled oscillator (VCO) receive as its control input?
- The raw reference frequency directly
- The filtered error voltage from the phase detector (Correct answer)
- The divided output of the frequency counter
- A fixed DC voltage set by a potentiometer
Correct answer: The filtered error voltage from the phase detector
The low-pass filter smooths the phase detector output into a DC error voltage that steers the VCO frequency until it locks to the reference.
Question 3: Which of the following best describes the function of a diplexer at an antenna connection?
- It amplifies both transmitted and received signals simultaneously
- It separates two frequency bands so they can share one antenna (Correct answer)
- It converts a balanced feedline to an unbalanced coax
- It matches a 50-ohm transmitter to a high-impedance antenna
Correct answer: It separates two frequency bands so they can share one antenna
A diplexer uses filters to route two different frequency bands to and from a shared antenna while isolating each port from the other.
Question 4: What is the main advantage of a push-pull amplifier configuration over a single-ended amplifier?
- Lower supply voltage requirement
- Cancellation of even-order harmonic distortion (Correct answer)
- Higher input impedance at all frequencies
- Simpler bias circuit with fewer components
Correct answer: Cancellation of even-order harmonic distortion
In push-pull circuits, the two transistors handle opposite half-cycles and their even-order distortion products cancel in the output transformer.
Question 5: What happens to the Q factor of an LC resonant circuit when resistance is added in series with the inductor?
- Q increases because more current flows
- Q decreases because losses increase (Correct answer)
- Q is unchanged since only inductance affects Q
- Q doubles because the bandwidth narrows
Correct answer: Q decreases because losses increase
Q = ωL/R, so increasing series resistance reduces Q, which broadens the resonance bandwidth and reduces selectivity.
Question 6: In a superheterodyne receiver, what is the purpose of the image reject filter placed before the mixer?
- To amplify the desired signal before frequency conversion
- To prevent a signal at the image frequency from entering the mixer (Correct answer)
- To stabilize the local oscillator frequency
- To remove IF feedthrough from the antenna port
Correct answer: To prevent a signal at the image frequency from entering the mixer
The image reject filter blocks signals at the image frequency (LO ± IF) that would otherwise mix down to the same IF as the desired signal, causing interference.
Question 7: What is the effect of negative feedback on the output impedance of an amplifier?
- It raises output impedance, improving current drive capability
- It lowers output impedance, improving voltage regulation at the load (Correct answer)
- It has no effect on impedance, only on gain
- It makes output impedance frequency-dependent only
Correct answer: It lowers output impedance, improving voltage regulation at the load
Negative voltage feedback reduces the amplifier's output impedance by a factor of (1 + Aβ), making the output behave more like an ideal voltage source.
HAM Practical Electronics Circuits 4
What is the purpose of a PIN diode in an RF switching circuit?