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100+ Free SBE CBRE Practice Questions
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Sample SBE CBRE Practice Questions
Try these sample questions to test your SBE CBRE exam readiness. Each question includes a detailed explanation. Start the interactive quiz above for the full 100+ question experience with AI tutoring.
1In an FM broadcast station, what peak frequency deviation is defined by the FCC as 100% modulation?
A.±75 kHz
B.±25 kHz
C.±100 kHz
D.±53 kHz
Explanation: FCC rules define a peak carrier deviation of ±75 kHz as 100% modulation for FM broadcast stations. Total modulation must not exceed 100% on peaks of frequent recurrence. Subcarriers may permit limited increases up to 110%.
2For an AM broadcast station, FCC rules limit modulation to no more than what value on negative peaks of frequent recurrence?
A.125%
B.100%
C.85%
D.150%
Explanation: FCC Part 73 limits AM modulation to a maximum of 100% on negative peaks of frequent recurrence to prevent carrier pinch-off and splatter. Positive peaks may reach 125%. Modulation should generally not fall below 85% on peaks.
3What is the standard FM pre-emphasis time constant used in the United States?
A.50 microseconds
B.25 microseconds
C.75 microseconds
D.100 microseconds
Explanation: The United States uses a 75-microsecond pre-emphasis/de-emphasis time constant for FM broadcasting, which boosts high frequencies before transmission to improve signal-to-noise ratio. Europe and much of the world use 50 microseconds. The receiver applies complementary de-emphasis.
4An FM transmitter feeds 1 kW into an antenna with a power gain of 6 dB. Ignoring line loss, what is the approximate ERP?
A.2 kW
B.6 kW
C.8 kW
D.4 kW
Explanation: A gain of 6 dB represents a power ratio of 4 (10^(6/10) ≈ 4). Multiplying the 1 kW transmitter output by 4 gives an ERP of about 4 kW. Each 3 dB doubles power, so 6 dB quadruples it.
5In a high-level (plate) modulated AM transmitter, where is the audio modulation applied?
A.To the final RF power amplifier
B.To the oscillator stage
C.To the first RF buffer
D.To the antenna feed point
Explanation: High-level modulation applies the audio signal to the final RF power amplifier, typically through its plate/collector supply, so the full carrier power is modulated. This requires a high-power audio modulator but yields good efficiency and linearity. Low-level modulation modulates an earlier stage and then linearly amplifies.
6What does the term 'composite' refer to in an FM stereo STL or transmitter exciter context?
A.The unprocessed mono microphone feed
B.The full multiplexed baseband signal including L+R, pilot, and L-R subcarrier
C.The DC power supply rail
D.The 10 MHz reference oscillator output
Explanation: The composite (multiplex or MPX) signal is the complete FM stereo baseband: the L+R sum, the 19 kHz pilot tone, the 38 kHz double-sideband suppressed-carrier L-R signal, and any subcarriers such as RDS. A composite STL carries this entire signal to the transmitter for direct application to the exciter.
7A solid-state FM transmitter is generally more efficient than a vacuum-tube transmitter primarily because of what design feature?
A.Higher operating plate voltages
B.Larger physical heat sinks
C.Use of switching-mode RF amplification and power supplies
D.Operation in pure Class A
Explanation: Modern solid-state FM transmitters achieve high efficiency through switching-mode (Class D/E/F-style) RF amplification and high-efficiency switching power supplies, often exceeding 70-75% AC-to-RF efficiency. Tube transmitters using Class C grounded-grid amplifiers are robust but typically less efficient overall and require high-voltage supplies.
8The carrier frequency of a U.S. AM broadcast station must be maintained within what tolerance of its assigned frequency?
A.±2 kHz
B.±100 Hz
C.±10 Hz
D.±20 Hz
Explanation: FCC Part 73 requires AM broadcast stations to maintain carrier frequency within ±20 Hz of the assigned frequency. This tight tolerance prevents heterodyne whistles between co-channel and adjacent stations. Frequency is typically held by a crystal or synthesizer reference.
9What is the channel bandwidth (spacing) between adjacent FM broadcast channels in the U.S. band?
A.200 kHz
B.150 kHz
C.100 kHz
D.250 kHz
Explanation: The U.S. FM broadcast band (88-108 MHz) uses 200 kHz channel spacing, giving 100 possible channel center frequencies. Each station's center frequency ends in an odd tenth of a MHz (e.g., 88.1, 88.3). This spacing accommodates the ±75 kHz deviation plus guard band.
10The 19 kHz pilot tone in an FM stereo signal serves what purpose?
A.It carries the RDS data
B.It tells the receiver a stereo signal is present and provides a phase reference for demodulating the 38 kHz subcarrier
C.It is the SCA subcarrier audio
D.It is the de-emphasis reference
Explanation: The 19 kHz pilot tone signals the receiver that the transmission is stereo and provides the coherent phase/frequency reference; doubling it yields the 38 kHz suppressed carrier needed to recover the L-R subchannel. Without the pilot, the receiver decodes the signal as mono.
About the SBE CBRE Practice Questions
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