Receivers and Transmitters Flashcards
6 cards from real GROL practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 6 Receivers and Transmitters flashcards as text
In a superheterodyne receiver, what is the primary function of the mixer stage?
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.
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?
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.
In a transmitter's RF power amplifier stage, what is the purpose of a neutralization circuit?
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.
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?
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.
Which of the following receiver characteristics best describes its ability to differentiate between a desired signal and signals on adjacent frequencies?
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.
In the generation of a single-sideband (SSB) signal, what is the primary output of a balanced modulator circuit?
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.