Practical Circuits Flashcards
Power supplies; schematic symbols
What is the function of a power supply bleeder resistor?
A. It acts as a fuse for excess voltage
B. It discharges the filter capacitors when power is removed
C. It removes shock hazards from the induction coils
D. It eliminates ground loop current
B. It discharges the filter capacitors when power is removed
Hint: a Bleeder causes a Discharge blood
The bleeder resister of a power supply performs the safety feature of discharging the filter capacitors. The resistor is arranged in parallel with the power supply. The use of the resistor is very important in this application, as it lowers the risk of electric shock from stored electrical energy in the filter capacitors by allowing the voltage level to decay quickly when the power supply is shut off.
Which of the following components are used in a power supply filter network?
A. Diodes
B. Transformers and transducers
C. Capacitors and inductors
D. All these choices are correct
C. Capacitors and inductors
Both capacitors and inductors are FILTERS and are used in a power-supply filter network. The combination of these two components is often referred to as an LC circuit (L is the Inductor, C is the Capacitor). Together they act to “tune” the circuit to smooth out voltage and current pulses from the power supply to provide a constant DC output voltage.
Which type of rectifier circuit uses two diodes and a center-tapped transformer?
A. Full-wave
B. Full-wave bridge
C. Half-wave
D. Synchronous
A. Full-wave
In a full wave [NOT bridge] rectifier, the return currents go to the center tap on the transformer, while each diode conducts for half of the AC cycle, capturing the full wave and converting it to DC.
https://en.wikipedia.org/wiki/Rectifier#Full-wave_rectification
In contrast, a full wave BRIDGE contains a Diode Bridge consisting of 4 diodes.
https://en.wikipedia.org/wiki/Diode_bridge
What is characteristic of a half-wave rectifier in a power supply?
A. Only one diode is required
B. The ripple frequency is twice that of a full-wave rectifier
C. More current can be drawn from the half-wave rectifier
D. The output voltage is two times the peak input voltage
A. Only one diode is required
In this circuit we only wish to capture half of the incoming AC waveform, thus giving the name “Half-wave rectifier”. Only one diode is used since it will conduct during the positive portion of the cycle, and that is all that is required.
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. 180 degrees
A half-wave rectifier will convert 1/2 of the full AC cycle of 360 degrees, so it will convert 180 degrees of the wave to DC.
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. 360 degrees
A FUll-wave = 360°
As its name indicates, a full-wave rectifier will convert the full wave or 360 degrees of the AC waveform to DC.
A half-wave rectifier converts only half of the waveform, and is 180 degrees of the input sine wave.
Hint: Think “going full-circle” - all 360 degrees.
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 series of DC pulses at twice the frequency of the AC input
Hint: a full wave rectifier is TWICE the fun.
The output waveform of an unfiltered full-wave rectifier connected to a resistive load will be a series of DC pulses at twice the frequency of the AC input. The rectifier will change each half cycle of the AC current waveform into pulses of DC current. This results in one pulse for the positive 1/2 cycle and one pulse for the negative 1/2 cycle for a total of 2 pulses DC output for each wavelength of AC converted.
Which of the following is characteristic of a switchmode 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. Inherently more stable
C. High-frequency operation allows the use of smaller components
Hint: a Switchmode uses SMALLER components, NOT FEWER.
An advantage of a switch mode power supply over a linear power supply, is that the high frequency operation allows the use of smaller components. This type of power supply is highly efficient and switches very quickly. The high operating frequencies (typically 50 kHz - 1 MHz instead of much lower 60 Hz AC) means that compact and lightweight transformers can be used. This makes a great difference in the size and weight of the equipment.
What is the purpose of neutralizing an amplifier?
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. To eliminate self-oscillations
This neutralization is done by returning a portion of the amplifier output back into the input in such a way that the signal is 180 degrees out of phase. This cancels the positive feedback loop which if not corrected can cause self-oscillations which may be harmful.
“Neutralization” is an old concept largely replaced by Linville & Rollet stability factor analysis.
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
This class of amplifier has high efficiency, but because they only conduct less than half of the input signal, they have high distortion. This makes them unsuitable for most phone transmissions, but effective for CW operation.
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. Output is high only when both inputs are high
Hint: if they go high, we BOTH go HIGH
The AND gate uses a binary system of Boolean or digital logic to express the “AND” or conjunction of two states. The AND gate follows the truth table shown below.
So notice on the table that Output is only high “1” when input A is high “1” AND input B is high “1” as well. If either or both of the inputs is low “0”, then the output must also be low “0”.
In a Class A amplifier, what percentage of the time does the amplifying device conduct?
A. 100%
B. More than 50% but less than 100%
C. 50%
D. Less than 50%
A. 100%
In a class-A amplifier, 100% of the input signal is used (conduction angle Θ = 360°). The active element remains conducting all of the time.
https://en.wikipedia.org/wiki/Power_amplifier_classes
Silly hint: class-A for Always
How many states does a 3-bit binary counter have?
A. 3
B. 6
C. 8
D. 16
C. 8
Hint: a 3-bit = 2³ (3 bits of 2)
A 3-bit binary counter has 8 states.
The number of states increases exponentially with each bit increase. The equation for finding the number of states in a binary counter is simply 2^N. 2 being the numeric value of the number of states in a single bit (only 2 states possible, 1 or 0) and N is a power equal to the number of bits. So it follows:
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 clocked array of circuits that passes data in steps along the array
Hint: The Employee’s Shift ended when they CLOCKED out
(A). A Shift register is a clocked array of circuits that passes data in steps along the array. The shift register is a series of “Flip Flops” using a common clock input. These “Flip flops” produce a binary array of 0 or 1 values, with the output of one flip flop becoming the input signal for the next flip flop in the sequence.
HINT: for your “shift” at work, you “clock” in and out!
Which of the following are basic components of a sine wave oscillator?
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. A filter and an amplifier operating in a feedback loop
Key word: Feedback
Oscillator is a ground loop so what all is wrong comes around (feedback, not forward)
The basic components of virtually all sine wave oscillators are filter and an amplifier operating in a feedback loop**. The sine wave oscillator uses an amplifier with a positive feedback loop in which the output from the amplifier is filtered for desired frequencies and then fed back into the input of the amplifier. The cycle repeats, taking the signal from a state of noise to a clear sine wave at the desired frequency.
For more info see Wikipedia: Electronic oscillator
Silly Hint: “Sine” in Latin means “without”. So without any waves in the beach, makes the water recede, or ‘feedback”. You may have also seen it as a legal term “sine qua non”. Without it, there’s No Deal!”
SILLY HINT2: you “filter” the signal and “amplify” it on the display.
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. Divide the RF output power by the DC input power
Silly memory trick: The [R]oman [F]leet” can only be defeated if you [D]ivide & [C]onquer” them (RF / DC)
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. The inductance and capacitance in the tank circuit
Hint: Linear (inductor) C (capacitor) = LC The inductance and capacitance in the tank circuit determine the frequency of an LC oscillator. An oscillator uses an amplifier in which the signal output is filtered for the desired frequency and then sent in a feedback loop back into the input of the amplifier. The LC oscillator uses an LC circuit consisting of an inductor (L) and capacitor (C) in parallel for filtering the RF signal in the feedback loop.
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. An amplifier in which the output preserves the input waveform
Silly Hint: Think of a linear descendant PRESERVES the family line.
A linear amplifier is an amplifier in which the output preserves the input waveform. The linear amplifier simply increases the power (amplitude) of the signal. The form of the wave is not changed - what goes in is what comes out!
For which of the following modes is a Class C power stage appropriate for amplifying a modulated signal?
A. SSB
B. FM
C. AM
D. All these choices are correct
B. FM
Hint: FM CLASS(i)C rock
FM operation is an appropriate mode for using a Class C power state for amplifying the modulated signal. Class C amplifiers conduct only a fraction of the input signal (usually less than 50%), so distortion is high, but efficiency is also high. Because of this high distortion, the class C amplifiers are not good for phone signals such as AM or SSB, as these require more complete signal. FM however, works well with the Class C amplifier.
(Note that in the 2015-2019 General pool the correct answer to this question was “CW”, but it was changed in the 2019 pool release)
For more info see: Electronic Amplifiers - 4.4 Class C amplifier
Silly Hint: The only Grade lower then a C is an F.
What circuit is used to select one of the sidebands from a balanced modulator?
A. Carrier oscillator
B. Filter
C. IF amplifier
D. RF amplifier
B. Filter
Hint: A FILTER - PROCESSES
Modulation is the process that combines the audio signal with the radio frequency. In AM (amplitude modulation) this encodes all of the information about the audio signal in the sidebands, at which point the carrier frequency serves no further purpose.
A balanced modulator first modulates and then also suppresses the now superfluous carrier signal.
The output of the balanced modulator gets passed to the filter, which removes one of the sidebands. That single sideband is then forwarded on to the mixer which will produce a signal with the desired frequency.
What output is produced by a balanced modulator?
A. Frequency modulated RF
B. Audio with equalized frequency response
C. Audio extracted from the modulation signal
D. Double-sideband modulated RF
D. Double-sideband modulated RF
A balanced modulator mixes the RF with the carrier, but suppresses the carrier frequency in the output, passing the upper and lower sidebands.
SILLY HINT: Three of the answers have modulate mentioned, but only one has the “balance” of “double-sided”
What is one reason to use an impedance matching transformer at a transmitter output?
A. To minimize transmitter power output
B. To present the desired impedance to the transmitter and feed line
C. To reduce power supply ripple
D. To minimize radiation resistance
B. To present the desired impedance to the transmitter and feed line
One reason to use an impedance matching transformer is to maximize the transfer of power. This type of transformer alters the current and voltages, which changes the impedances between the power source and load. Matching the impedances allows for maximum power transfer, so this is one component which may be used for the function.
SILLY HINT: The correct choice has PRESENT which is closer in meaning to match, where the incorrect choices all have verbs that mean to make something smaller (minimize and reduce).
SILLY HINT 2 : The correct choice is the only one that includes “impedance”
How is a product detector used?
A. Used in test gear to detect spurious mixing products
B. Used in transmitter to perform frequency multiplication
C. Used in an FM receiver to filter out unwanted sidebands
D. Used in a single sideband receiver to extract the modulated signal
D. Used in a single sideband receiver to extract the modulated signal
When receiving a single sideband signal, the product detector combines the received signal with a local oscillator signal. By multiplying or mixing these signals, the product detector creates a new signal that contains the original voice or information.
This demodulated signal can then be further processed and converted into audio or data that we can perceive or interpret. In essence, the product detector plays a vital role in separating the modulated signal from the carrier wave, allowing us to retrieve and use the transmitted communication.
SILLY HINT: The “product” we are detecting is the desired signal (audio) - and it has to be extracted to be useful.
Which of the following is characteristic of a direct digital synthesizer (DDS)?
A. Extremely narrow tuning range
B. Relatively high-power output
C. Pure sine wave output
D. Variable output frequency with the stability of a crystal oscillator
D. Variable output frequency with the stability of a crystal oscillator
Variable frequency with the stability of a crystal oscillator is an advantage of a transceiver controlled by a direct digital synthesizer (DDS). The DDS can be used over a wide range of frequencies and can be precisely controlled to create finely stepped sine wave forms while producing very little phase noise.
Which of the following is an advantage of a digital signal processing (DSP) filter compared to an analog filter?
A. A wide range of filter bandwidths and shapes can be created
B. Fewer digital components are required
C. Mixing products are greatly reduced
D. The DSP filter is much more effective at VHF frequencies
A. A wide range of filter bandwidths and shapes can be created
An advantage of a receiver digital signal processor (DSP) IF (intermediate frequency) filter compared to an analog filter, is that a wide range of filter bandwidths and shapes can be created. The software and processing abilities of the DSP unit make it a very flexible instrument for either filtering or other modifications, signal modulation or other adjustments to the received signal.
What term specifies a filter’s attenuation inside its passband?
A. Insertion loss
B. Return loss
C. Q
D. Ultimate rejection
A. Insertion loss
Hint: To be “inside” is to “insert”
A filter’s pass band is the band that allows the signal to pass through it. However, even in the pass band the filter does attenuate the signal some. This base level of attenuation from having the filter inserted into the signal path is called the “Insertion loss”.
Which parameter affects receiver sensitivity?
A. Input amplifier gain
B. Demodulator stage bandwidth
C. Input amplifier noise figure
D. All these choices are correct
D. All these choices are correct
SILLY HINT: To “receive” something (receiver) means for something to “come in”. There are two answers that start with “Input” therefore there is more than one answer that starts with “input,” so only correct answer must be “D”-All these choices correct.
What is the phase difference between the I and Q RF signals that software-defined radio (SDR) equipment uses for modulation and demodulation?
A. Zero
B. 90 degrees
C. 180 degrees
D. 45 degrees
B. 90 degrees
I & Q signals are used as components of a quadrature signal. By definition, they are separated by 90 degrees (a right angle). You just need to memorize this. It may help to remember they are QUADrature, think of dividing a signal into 4 parts: 360 degrees divided by 4 is 90 degrees.
Silly memory hint: The “average” IQ of a human is generally standardized to be around 100 on most tests, which is closest to 90 of the available options. 180 would be way too high, 45 too low, and 0 is impossible.
What is an advantage of using I-Q modulation with software-defined radios (SDRs)?
A. The need for high resolution analog-to-digital converters is eliminated
B. All types of modulation can be created with appropriate processing
C. Minimum detectible signal level is reduced
D. Automatic conversion of the signal from digital to analog
B. All types of modulation can be created with appropriate processing
I and Q signals are the In-phase and Quadrature components of a signal. They provide enough data for software to be able to decode many different types of modulation.
It is debatable whether “all types” of modulation can be created, but certainly a great many different types can be. That is the power of a Software Defined Radio!
Memory hints:
A person with high IQ can create ALL kinds of things
Software requires processing
Which of these functions is performed by software in a software-defined radio (SDR)?
A. Filtering
B. Detection
C. Modulation
D. All these choices are correct
D. All these choices are correct
The transmitter of the software-defined radio (SDR) is just a “transmitter”, so it can’t do anything since it just sends and receives signal to and from the air and doesn’t try to encode or decode them but just process them directly to the computer.
What is the frequency above which a low-pass filter’s output power is less than half the input power?
A. Notch frequency
B. Neper frequency
C. Cutoff frequency
D. Rolloff frequency
C. Cutoff frequency
A low pass filter passes the low frequencies and rejects the high frequencies. “Cutoff” indicates the point at which frequencies are rejected.
Note that “roll off” describes the shape of the filter’s curve between the pass band and stop band. It isn’t a specific frequency, it is more likely to be described as a rate of change like dB/decade.
Hint: A quarterback also throws a “Lowpass” to “cut-off” any sacks
What term specifies a filter’s maximum ability to reject signals outside its passband?
A. Notch depth
B. Rolloff
C. Insertion loss
D. Ultimate rejection
D. Ultimate rejection
Notch depth describes the ability to accept a signal in the center of a band pass filter.
Rolloff describes the steepness of the edge of a filter between the pass band and stop band.
Insertion loss describes the loss across the band by the insertion of the filter into the circuit.
Ultimate rejection describes the ability to reject signals outside the pass band.
Mnemonics:
A maximum reject is an ultimate reject.
The question and answer both have reject.
Ultimate means “as far as possible”, the filter rejects signals far from its “passband”.
The bandwidth of a band-pass filter is measured between what two frequencies?
A. Upper and lower half-power
B. Cutoff and rolloff
C. Pole and zero
D. Image and harmonic
A. Upper and lower half-power
Hint: a bandwidth has UPPER and LOWER limits
“Upper and lower half-power” is correct. These might also be referred to as the upper and lower cutoff or -3dB frequencies.
Don’t be tricked by: “Cutoff and Rolloff”. Rolloff describes the shape of the edge of the filter’s transfer function, not a specific frequency.
Half-Power Point, aka Half-Power Frequencies
The half-power point is the point at which the output power has dropped to half of its peak value; that is, at a level of approximately -3 dB.
In filters, optical filters, and electronic amplifiers, the half-power point is also known as half-power bandwidth and is a commonly used definition for the cutoff frequency.
