General Exam-Practical Circuits

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1. G7A01 What safety feature does a power-supply bleeder resistor provide?
A. It acts as a fuse for excess voltage
B. It discharges the filter capacitors
C. It removes shock hazards from the induction coils
D. It eliminates ground-loop current
• (B)
• A bleeder resistor connected across a filter capacitor discharges the capacitor when power is not supplied to the circuit. This minimizes the risk of electrical shock if the supply enclosure is opened, exposing the capacitor terminals.
2. G7A02 Which of the following components are used in a power-supply filter network?
A. Diodes
B. Transformers and transducers
C. Quartz crystals
D. Capacitors and inductors
• (D)
• A power supply filter network consists of capacitors and inductors used to smooth out the pulses of voltage and current from the rectifier. Capacitors oppose changes in voltage while the inductors oppose changes in current. The combination results in a constant dc output voltage from the supply.
3. G7A03 What is the peak-inverse-voltage across the rectifiers in a full-wave bridge power supply?
A. One-quarter the normal output voltage of the power supply
B. Half the normal output voltage of the power supply
C. Double the normal peak output voltage of the power supply
D. Equal to the normal peak output voltage of the power supply
• (D)
• In the full-wave center-tapped rectifier, when a rectifier diode is not conducting it must withstand not only the negative peak voltage from its own half of the winding but the positive voltage from the other winding. Thus the peak inverse voltage applied to the diodes is twice the normal peak output voltage of the supply. In a bridge rectifier circuit the applied PIV to either diode is only the peak ouput voltage.
4. G7A04 What is the peak-inverse-voltage across the rectifier in a half-wave power supply?
A. One-half the normal peak output voltage of the power supply
B. One-half the normal output voltage of the power supply
C. Equal to the normal output voltage of the power supply
D. Two times the normal peak output voltage of the power supply
• (D)
• The peak inverse voltage applied to the rectifier diode in a half-wave rectifier circuit is twice the supply's peak output voltage because the filter capacitor charges to the peak transformer voltage during the half-cycle when the diode is conducting. During the half-cycle when the diode is not conducting, the capacitor remains charged while the transformer output voltage reaches a peak value of the opposite polarity. Thus the peak inverse voltage is twice the normal peak output voltage of the supply.
5. G7A05 What portion of the AC cycle is converted to DC by a half-wave rectifier?
A. 90 degrees
B. 180 degrees
C. 270 degrees
D. 360 degrees
• (B)
• Since there are 360 degrees in a full cycle of ac, a half-wave rectifier converts 180 degrees of the ac input waveform to dc.
6. G7A06 What portion of the AC cycle is converted to DC by a full-wave rectifier?
A. 90 degrees
B. 180 degrees
C. 270 degrees
D. 360 degrees
• (D)
• Since there are 360 degrees in a full cycle of ac, a full-wave rectifier converts 360 degrees of the ac input waveform to dc.
7. G7A07 What is the output waveform of an unfiltered full-wave rectifier connected to a resistive load?
A. A series of DC pulses at twice the frequency of the AC input
B. A series of DC pulses at the same frequency as the AC input
C. A sine wave at half the frequency of the AC input
D. A steady DC voltage
• (A)
• A full-wave rectifier changes alternating current with positive and negative half cycles into a fluctuating current with all positive pulses. Since the current in this case has not yet been filtered, it is a series of pulses at twice the frequency of the ac input.
8. G7A08 Which of the following is an advantage of a switch-mode power supply as compared to a linear power supply?
A. Faster switching time makes higher output voltage possible
B. Fewer circuit components are required
C. High frequency operation allows the use of smaller components
D. All of these choices are correct
• (C)
• Switched-mode power supplies operate at a frequency much higher than the 60 Hz ac line current. (Oscillator frequencies of 50 kHz or more are commonly used.) This allows the use of small, lightweight transformers. While the transformer in a linear power supply capable of supplying 20 amperes might weigh 15 or 20 pounds, the transformer for a switched-mode power supply with a similar current rating might weigh 1 or 2 pounds! Switched-mode power supplies have more complex circuits than linear supplies and generally require more components than a simple linear supply.

9. G7A09 Which symbol in figure G7-1 represents a field effect transistor?
A. Symbol 2
B. Symbol 5
C. Symbol 1
D. Symbol 4
(C)

10. G7A10 Which symbol in figure G7-1 represents a Zener diode?
A. Symbol 4
B. Symbol 1
C. Symbol 11
D. Symbol 5
(D)

11. G7A11 Which symbol in figure G7-1 represents an NPN junction transistor?
A. Symbol 1
B. Symbol 2
C. Symbol 7
D. Symbol 11
(B)

12. G7A12 Which symbol in Figure G7-1 represents a multiple-winding transformer?
A. Symbol 4
B. Symbol 7
C. Symbol 6
D. Symbol 1
(C)

13. G7A13 Which symbol in Figure G7-1 represents a tapped inductor?
A. Symbol 7
B. Symbol 11
C. Symbol 6
D. Symbol 1
(A)
14. G7B01 Complex digital circuitry can often be replaced by what type of integrated circuit?
A. Microcontroller
B. Charge-coupled device
C. Phase detector
D. Window comparator
• (A)
• Microcontrollers are a special type of microprocessor designed to interact with external sensors and circuits to control or operate a piece of equipment, such as an automatic antenna tuner or electronic keyer.
15. G7B02 Which of the following is an advantage of using the binary system when processing digital signals?
A. Binary "ones" and "zeros" are easy to represent with an "on" or "off" state
B. The binary number system is most accurate
C. Binary numbers are more compatible with analog circuitry
D. All of these choices are correct
• (A)
• Binary numbers are easily represented by circuits that have two stable states--ON and OFF. The states can be used to represent either a binary 0 or 1 as the designer chooses.
16. G7B03 Which of the following describes the function of a two input AND gate?
A. Output is high when either or both inputs are low
B. Output is high only when both inputs are high
C. Output is low when either or both inputs are high
D. Output is low only when both inputs are high
• (B)
• If both inputs to an AND gate are true or “1”, the AND function output is also “1”, otherwise the output is "0".
17. G7B04 Which of the following describes the function of a two input NOR gate?
A. Output is high when either or both inputs are low
B. Output is high only when both inputs are high
C. Output is low when either or both inputs are high
D. Output is low only when both inputs are high
• (C)
• A NOR (NOT-OR) gate consists of an OR gate with its output inverted. If either or both of the inputs to a two-input NOR gate are true or “1”, the OR function is also “1”, which is then inverted to “0” at the output.
18. G7B05 How many states does a 3-bit binary counter have?
A. 3
B. 6
C. 8
D. 16
• (C)
• There are 8 states because the number of states in a binary counter is 2N, where N is the number of bits in the counter. A 2-bit counter has 22 = 4 states, a 3-bit counter 23 = 8 states, a 4-bit counter 24 = 16 states, and so forth.
19. G7B06 What is a shift register?
A. A clocked array of circuits that passes data in steps along the array
B. An array of operational amplifiers used for tri state arithmetic operations
C. A digital mixer
D. An analog mixer
• (A)
• A shift register consists of a sequence of flip-flop circuits connected output-to-input and sharing a common clock input signal. With each pulse of the clock signal, the state of the input signal (0 or 1) to the shift register is transferred to the output of the first flip-flop and each subsequent flip-flop’s output is passed to the next flip-flop’s input.
20. G7B07 What are the basic components of virtually all sine wave oscillators?
A. An amplifier and a divider
B. A frequency multiplier and a mixer
C. A circulator and a filter operating in a feed-forward loop
D. A filter and an amplifier operating in a feedback loop
• (D)
• To make an oscillator requires an amplifier and a circuit that routes some of the amplifier’s output signal back to the input (called a “feedback loop”) such that it is reinforced in the amplifier output. This is called positive feedback. A filter in the feedback loop is used so only signals at the desired frequency are reinforced. Starting with random noise, the oscillator gradually builds up an output signal at the filter frequency until it is self-sustaining.
21. G7B08 How is the efficiency of an RF power amplifier determined?
A. Divide the DC input power by the DC output power
B. Divide the RF output power by the DC input power
C. Multiply the RF input power by the reciprocal of the RF output power
D. Add the RF input power to the DC output power
• (B)
• Efficiency is defined as the total output power divided by the total input power and is measured in percent. For an RF amplifier, total output power is measured by a wattmeter. The total input power is the dc power required for the amplifier to operate. For example, if to produce 1200 watts of RF output the amplifier requires 1000 mA of plate current at a voltage of 2000 V, efficiency = 1200 watts / (1 A × 2000 V) = 1200 / 2000 = 60%.
22. G7B09 What determines the frequency of an LC oscillator?
A. The number of stages in the counter
B. The number of stages in the divider
C. The inductance and capacitance in the tank circuit
D. The time delay of the lag circuit
• (C)
• To make an oscillator requires an amplifier and a circuit that routes some of the amplifier’s output signal back to the input (called a “feedback loop”) such that is reinforced in the amplifier output. A filter in the feedback loop is used so only signals at the desired frequency are reinforced. An LC oscillator uses a parallel LC circuit (a tank circuit) as the filter in the feedback loop.
23. G7B10 Which of the following is a characteristic of a Class A amplifier?
A. Low standby power
B. High Efficiency
C. No need for bias
D. Low distortion
• (D)
• Class A amplifiers are used when low distortion is required. Class A means that current flows in the amplifying device – whether a transistor or vacuum tube – at all times. Because the amplifier is never saturated or cut off, its operation can be optimized for linear reproduction of the input signal. The tradeoff is that Class A amplifiers consume power 100% of the time, require a source of bias to keep them on in the absence of an input signal, and aren’t particularly efficient.
24. G7B11 For which of the following modes is a Class C power stage appropriate for amplifying a modulated signal?
A. SSB
B. CW
C. AM
D. All of these choices are correct
• (B)
• A Class C amplifier conducts current during less than half of the input signal cycle, resulting in high distortion. This rules out Class C amplifiers for any form of amplitude modulation, such as SSB or AM. Class C amplifiers can be used for CW since that mode requires only the presence or absence of a signal. Similarly, Class C is suitable for FM signals that only depend on signal frequency, which is not changed by the amplifier. Class C amplifiers generate significant harmonics, so they require filtering for use as transmitter outputs.
25. G7B12 Which of these classes of amplifiers has the highest efficiency?
A. Class A
B. Class B
C. Class AB
D. Class C
• (D)
• Class C amplifiers act a lot like switches--they turn on for a fraction of the input signal’s cycle and stay off the rest of the time. While these amplifiers are not linear at all and require filters to eliminate the harmonics in their output, they are quite efficient.
26. G7B13 What is the reason for neutralizing the final amplifier stage of a transmitter?
A. To limit the modulation index
B. To eliminate self-oscillations
C. To cut off the final amplifier during standby periods
D. To keep the carrier on frequency
• (B)
• Neutralization of a power amplifier is a technique that minimizes or cancel the effects of positive feedback. Positive feedback occurs when the output signal is fed back to the input in phase with the input signal, creating an oscillator. Neutralization consists of feeding a portion of the amplifier output back to the input, 180 degrees out of phase with the input, to cancel the positive feedback. This is called negative feedback. Self-oscillation creates powerful spurious signals that cause interference. Self-oscillations can also be sufficiently powerful to damage the amplifier.
27. G7B14 Which of the following describes a linear amplifier?
A. Any RF power amplifier used in conjunction with an amateur transceiver
B. An amplifier in which the output preserves the input waveform
C. A Class C high efficiency amplifier
D. An amplifier used as a frequency multiplier
• (B)
• A linear amplifier is defined as one whose output waveform is a copy of the input waveform, although larger in amplitude. Hams refer to power amplifiers as “linears”, whether they are operating linearly (for AM or SSB modes) or not (for CW or FM). It is important to understand when linear operation is important. An amplifier designed for FM will not be suitable as an SSB amplifier, for example.
28. G7C01 Which of the following is used to process signals from the balanced modulator and send them to the mixer in a single-sideband phone transmitter?
A. Carrier oscillator
B. Filter
C. IF amplifier
D. RF amplifier
• (B)
• In a single-sideband transmitter, the modulating audio is added to the RF signal in the balanced modulator. The balanced modulator also balances out or cancels the original carrier signal, leaving a double-sideband, suppressed-carrier signal. A filter then removes one of the sidebands, leaving a single-sideband signal that is sent to the mixer, where it combines with the signal from a Local Oscillator (LO) to produce the RF signal that is amplified and sent to the antenna. The LO shown in the figure is crystal-controlled for fixed-frequency operation. Replacing the crystal-controlled LO with a variable-frequency oscillator (VFO) results in a tunable transmitter similar to those in most modern transceivers.
29. G7C02 Which circuit is used to combine signals from the carrier oscillator and speech amplifier and send the result to the filter in a typical single-sideband phone transmitter?
A. Discriminator
B. Detector
C. IF amplifier
D. Balanced modulator
• (D)
• In a single-sideband transmitter, the modulating audio is added to the RF signal in the balanced modulator. The balanced modulator also balances out or cancels the original carrier signal, leaving a double-sideband, suppressed-carrier signal. A filter then removes one of the sidebands, leaving a single-sideband signal that is sent to the mixer, where it combines with the signal from a Local Oscillator (LO) to produce the RF signal that is amplified and sent to the antenna. The LO shown in the figure is crystal-controlled for fixed-frequency operation. Replacing the crystal-controlled LO with a variable-frequency oscillator (VFO) results in a tunable transmitter similar to those in most modern transceivers.
30. G7C03 What circuit is used to process signals from the RF amplifier and local oscillator and send the result to the IF filter in a superheterodyne receiver?
A. Balanced modulator
B. IF amplifier
C. Mixer
D. Detector
• (C)
• In a superheterodyne receiver, the mixer combines signals from the RF amplifier and the local oscillator (LO) then sends those signals to the IF filter, which passes the desired range of frequencies while rejecting the signals at higher and lower frequencies.
31. G7C04 What circuit is used to combine signals from the IF amplifier and BFO and send the result to the AF amplifier in a single-sideband receiver?
A. RF oscillator
B. IF filter
C. Balanced modulator
D. Product detector
• (D)
• In a superheterodyne receiver, the product detector circuit recovers the modulating audio signal by combining the output of the intermediate frequency (IF) amplifier and the beat frequency oscillator (BFO). The recovered audio signal is then passed to the audio frequency (AF) amplifier.
32. G7C05 Which of the following is an advantage of a transceiver controlled by a direct digital synthesizer (DDS)?
A. Wide tuning range and no need for band switching
B. Relatively high power output
C. Relatively low power consumption
D. Variable frequency with the stability of a crystal oscillator
• (D)
• The direct digital synthesizer or DDS replaces the analog VFO circuits with a digital circuit that creates a sine wave as a series of small steps. The duration and amplitude of each step is precisely controlled based on a crystal oscillator. This allows a DDS to act as an oscillator with stability that is comparable to that of a crystal oscillator, while still being adjustable over a wide range.
33. G7C06 What should be the impedance of a low-pass filter as compared to the impedance of the transmission line into which it is inserted?
A. Substantially higher
B. About the same
C. Substantially lower
D. Twice the transmission line impedance
• (B)
• To prevent unwanted reflected power and elevated SWR, keep all elements of the feed line and antenna system at the same impedance. A low-pass filter, designed to be installed at the transmitter output, should have the same impedance as the transmission line.
34. G7C07 What is the simplest combination of stages that implement a superheterodyne receiver?
A. RF amplifier, detector, audio amplifier
B. RF amplifier, mixer, IF discriminator
C. HF oscillator, mixer, detector
D. HF oscillator, pre-scaler, audio amplifier
• (C)
• By definition, a superheterodyne receiver must contain a mixer and a local oscillator. One additional stage, a detector, is necessary to recover the modulating audio. Thus, the simplest superheterodyne consists of a mixer, oscillator, and detector. Practical receivers add more amplifiers to improve sensitivity and filters to reject unwanted signals.
35. G7C08 What type of circuit is used in many FM receivers to convert signals coming from the IF amplifier to audio?
A. Product detector
B. Phase inverter
C. Mixer
D. Discriminator
• (D)
• A frequency discriminator converts the variations in frequency of the limiter’s output signal into amplitude variations, recovering the modulating audio signal. A quadrature detector is another type of FM demodulation circuit that converts changes in frequency into amplitude variations
36. G7C09 Which of the following is needed for a Digital Signal Processor IF filter?
A. An analog to digital converter
B. A digital to analog converter
C. A digital processor chip
D. All of the these choices are correct
• (D)
• Digital Signal Processing (DSP) is the process of converting analog signals to digital data, processing them with software programs, then converting the signals back to analog form. Digital noise reduction is but one application--filtering, demodulation, and decoding are also commonly performed by DSP circuits.
37. G7C10 How is Digital Signal Processor filtering accomplished?
A. By using direct signal phasing
B. By converting the signal from analog to digital and using digital processing
C. By differential spurious phasing
D. By converting the signal from digital to analog and taking the difference of mixing products
• (B)
• Digital Signal Processing (DSP) is the process of converting analog signals to digital data, processing them with software programs, then converting the signals back to analog form. Digital noise reduction is but one application--filtering, demodulation, and decoding are also commonly performed by DSP circuits.
38. G7C11 What is meant by the term "software defined radio" (SDR)?
A. A radio in which most major signal processing functions are performed by software
B. A radio which provides computer interface for automatic logging of band and frequency
C. A radio which uses crystal filters designed using software
D. A computer model which can simulate performance of a radio to aid in the design process
• (A)
• In a software-defined radio (SDR) nearly all of the radio's functions are performed by digital hardware. This allows the radio's operation to be changed and controlled by software without having to change the way in which the radio is physically constructed.
 Author: rledwith ID: 225600 Card Set: General Exam-Practical Circuits Updated: 2013-07-15 21:14:33 Tags: rdl gen exam g7 Folders: Description: Amateur Radio General Exam - G7 Question Set - Practical Circuits Show Answers: