GROL - General Radiotelephone Operator License Receivers and Transmitters Questions and Answers 1 — Questions and Answers
Question 1: In a superheterodyne receiver, what is the primary function of the mixer stage?
- To amplify the weak incoming RF signal before any other processing.
- To demodulate the audio information from the intermediate frequency.
- To combine the incoming RF signal with the local oscillator signal to produce a fixed intermediate frequency (IF). (Correct answer)
- To provide the main filtering and selectivity for adjacent channel rejection.
Correct answer: To combine the incoming RF signal with the local oscillator signal to produce a fixed intermediate frequency (IF).
The mixer is a non-linear circuit that combines, or heterodynes, the received RF signal with a signal from the local oscillator (LO). This process generates sum and difference frequencies. The circuit is designed to select the difference frequency, which is a fixed lower frequency called the Intermediate Frequency (IF), for subsequent amplification and filtering.
Question 2: A receiver with an intermediate frequency (IF) of 455 kHz is tuned to a station at 1500 kHz. The local oscillator (LO) is operating using high-side injection. What is the image frequency that could potentially interfere with reception?
- 1045 kHz
- 1955 kHz
- 2410 kHz (Correct answer)
- 455 kHz
Correct answer: 2410 kHz
With high-side injection, the LO frequency is the desired frequency plus the IF (1500 kHz + 455 kHz = 1955 kHz). The image frequency is an unwanted signal that is also 455 kHz away from the LO, but on the opposite side. It can be calculated as F_signal + (2 * F_IF). Therefore, the image frequency is 1500 kHz + (2 * 455 kHz) = 1500 kHz + 910 kHz = 2410 kHz.
Question 3: In a transmitter's RF power amplifier stage, what is the purpose of a neutralization circuit?
- To match the amplifier's output impedance to the antenna system.
- To cancel the effects of inter-electrode capacitance and prevent self-oscillation. (Correct answer)
- To filter out harmonic frequencies from the final output signal.
- To regulate the DC supply voltage for the amplifier tube or transistor.
Correct answer: To cancel the effects of inter-electrode capacitance and prevent self-oscillation.
At high frequencies, the inherent capacitance between the output (plate/collector) and input (grid/base) of an amplifying device can feed energy back from the output to the input. If this feedback is in phase, it can cause the amplifier to oscillate. A neutralization circuit provides an opposing, out-of-phase feedback path to cancel this unwanted effect and ensure stability.
Question 4: A technician testing an SSB transmitter with a two-tone audio signal observes multiple new, unwanted signals on a spectrum analyzer located near the desired output frequency. What is the most likely cause?
- Over-deviation of the frequency modulator.
- Poor carrier suppression in the balanced modulator.
- Non-linearity in a power amplifier stage causing intermodulation distortion. (Correct answer)
- Parasitic oscillations in the oscillator stage.
Correct answer: Non-linearity in a power amplifier stage causing intermodulation distortion.
When two or more signals pass through a non-linear circuit, such as an overdriven amplifier, they mix and create new frequencies called intermodulation distortion (IMD) products. These products, which appear as unwanted sidebands, are a direct result of the amplifier's inability to linearly reproduce the input signal and are a critical performance metric for SSB transmitters.
Question 5: Which of the following receiver characteristics best describes its ability to differentiate between a desired signal and signals on adjacent frequencies?
- Fidelity
- Sensitivity
- Selectivity (Correct answer)
- Stability
Correct answer: Selectivity
Selectivity is the measure of a receiver's ability to reject unwanted signals on adjacent frequencies while receiving the desired signal. This is primarily determined by the quality and sharpness of the filters, especially in the IF (Intermediate Frequency) stages of a superheterodyne receiver.
Question 6: In the generation of a single-sideband (SSB) signal, what is the primary output of a balanced modulator circuit?
- The original carrier signal, amplitude-modulated by the audio.
- The upper and lower sidebands, with the carrier signal suppressed. (Correct answer)
- A single sideband, with both the carrier and other sideband removed.
- The original audio signal, amplified to an RF level.
Correct answer: The upper and lower sidebands, with the carrier signal suppressed.
A balanced modulator is a circuit that takes a carrier signal and a modulating audio signal as inputs and produces an output where the original carrier is canceled or suppressed. The output consists only of the sum and difference frequencies, which are the upper and lower sidebands. This Double-Sideband Suppressed-Carrier (DSB-SC) signal is the first step in creating an SSB signal.
