HAM Practical Circuits and Components 3 — Questions and Answers
Question 1: In a superheterodyne receiver, what is the purpose of the IF (intermediate frequency) filter?
- To amplify the detected audio signal
- To convert the RF signal to a lower fixed frequency for improved selectivity
- To provide selectivity by passing only a narrow band of frequencies centered on the IF (Correct answer)
- To eliminate the local oscillator signal
Correct answer: To provide selectivity by passing only a narrow band of frequencies centered on the IF
The IF filter provides the receiver's primary selectivity, rejecting adjacent channels while passing the desired signal at the fixed intermediate frequency.
Question 2: What type of transistor configuration provides voltage gain greater than unity, current gain approximately equal to unity, and low output impedance?
- Common emitter
- Common base
- Common collector (emitter follower) (Correct answer)
- Cascode
Correct answer: Common collector (emitter follower)
The common-collector (emitter follower) configuration has near-unity current gain, voltage gain slightly less than one, high input impedance, and low output impedance.
Question 3: Which component characteristic is most important when selecting a capacitor for use in a high-Q resonant circuit?
- High ESR (equivalent series resistance)
- Large physical size
- Low ESR and low dielectric loss (high Q) (Correct answer)
- High temperature coefficient
Correct answer: Low ESR and low dielectric loss (high Q)
Low ESR and low dielectric loss ensure minimal energy dissipation, maintaining the high Q of the resonant circuit.
Question 4: A PIN diode is used as an RF switch. What controls its RF resistance?
- The peak RF voltage applied to it
- The DC bias current through it (Correct answer)
- The frequency of the RF signal
- The temperature of the junction
Correct answer: The DC bias current through it
The RF resistance of a PIN diode is controlled by the DC bias current — higher forward current reduces resistance, making it conductive at RF.
Question 5: What is the function of an automatic level control (ALC) circuit in an SSB transmitter?
- To maintain constant audio frequency response
- To limit the RF output power to prevent overdrive and distortion (Correct answer)
- To automatically select the correct sideband
- To compensate for antenna SWR variations
Correct answer: To limit the RF output power to prevent overdrive and distortion
The ALC circuit senses the RF output level and reduces the drive to the final amplifier to prevent flat-topping and intermodulation distortion on peaks.
Question 6: What is the primary disadvantage of using a switching (switch-mode) power supply in a ham radio station compared to a linear supply?
- Lower efficiency under full load
- Generation of broadband RF interference across many frequencies (Correct answer)
- Inability to provide regulated output voltage
- Higher output ripple at 60 Hz
Correct answer: Generation of broadband RF interference across many frequencies
Switching power supplies generate harmonic noise across a wide frequency range due to their high-frequency switching action, potentially causing interference in receivers.
Question 7: In a phase-locked loop (PLL) synthesizer, what component compares the VCO output frequency to the reference and generates an error signal?
- The loop filter
- The voltage-controlled oscillator
- The phase-frequency detector (Correct answer)
- The programmable divider
Correct answer: The phase-frequency detector
The phase-frequency detector (PFD) compares the phase and frequency of the divided VCO output to the reference oscillator and produces a correction voltage.
HAM Practical Circuits and Components 3
In a superheterodyne receiver, what is the purpose of the IF (intermediate frequency) filter?