I85PREP
⌘ K
HKIE Professional Assessment - Electronics Discipline study materials
HKIE Professional Assessment - Electronics Discipline study resources

HKIE Professional Assessment - Electronics Discipline practice questions

One question at a time. Build your HKIE Professional Assessment - Electronics Discipline understanding.

First 50 of 200 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1Electronics Engineering Practice

A 50 Hz sinusoidal voltage has an RMS value of 230 V. What is its peak value?

  1. 460 V
  2. 325 V Correct answer.
  3. 230 V
  4. 163 V

Explanation

For a sinusoid the peak value equals the RMS value multiplied by the square root of two, so 230 V times 1.414 gives approximately 325 V. This relationship holds only for pure sine waves and is the basis for specifying low-voltage supply magnitudes.

Question 2Electronics Engineering Practice

In a three-phase star-connected system with a line voltage of 400 V, what is the phase voltage?

  1. 200 V
  2. 692 V
  3. 400 V
  4. 231 V Correct answer.

Explanation

In a star connection the phase voltage equals the line voltage divided by the square root of three, so 400 V divided by 1.732 gives about 231 V. This is why 400 V three-phase distribution presents roughly 230 V to single-phase loads.

Question 3Electronics Engineering Practice

Which quantity remains constant through all components of a series DC circuit?

  1. Voltage
  2. Power
  3. Current Correct answer.
  4. Resistance

Explanation

A series circuit provides a single path, so the same current flows through every element by Kirchhoff's current law. Voltage divides across the resistive drops and power varies with each element's resistance, but the current is common to all.

Question 4Electronics Engineering Practice

What is the primary purpose of a residual current device in a low-voltage installation?

  1. Detect earth leakage current and disconnect the supply Correct answer.
  2. Limit fault current magnitude
  3. Correct the power factor of inductive loads
  4. Provide overvoltage protection to equipment

Explanation

A residual current device compares live and neutral currents and trips when the imbalance indicates current flowing to earth, protecting people from electric shock. It does not limit fault current magnitude or correct power factor, which are handled by other protective devices.

Question 5Electronics Engineering Practice

Which logic family characteristic makes CMOS preferred over TTL for battery-powered portable electronics?

  1. Lower static power consumption Correct answer.
  2. Better noise immunity at high frequency
  3. Greater drive current capability
  4. Higher switching speed

Explanation

In static states CMOS draws only leakage current because one transistor of each complementary pair is always off, giving very low quiescent power. TTL draws continuous bias current per gate, so CMOS is favoured where battery life dominates.

Question 6Electronics Engineering Practice

A transformer has 500 primary turns and 100 secondary turns. If the primary is connected to 220 V AC, what is the secondary voltage?

  1. 220 V
  2. 44 V Correct answer.
  3. 22 V
  4. 1100 V

Explanation

The turns ratio equals the voltage ratio, so the secondary voltage is 220 V multiplied by 100/500, giving 44 V. This step-down ratio reflects the smaller number of secondary turns.

Question 7Electronics Engineering Practice

What is the binary representation of the decimal number 13?

  1. 1001
  2. 1101 Correct answer.
  3. 1110
  4. 1011

Explanation

Thirteen equals eight plus four plus one, so the bits at positions 8, 4 and 1 are set, giving 1101. Reading the place values 8, 4, 2, 1 confirms only the two's place is zero.

Question 8Electronics Engineering Practice

Which document type in an electronics project defines the functional requirements and interfaces before detailed design begins?

  1. Test record sheet
  2. Requirements specification Correct answer.
  3. As-built drawing
  4. Operation and maintenance manual

Explanation

The requirements specification captures what the system must do and the interfaces it must honour, forming the baseline for design and verification. As-built drawings, test records and manuals are produced later in the lifecycle.

Question 9Electronics Engineering Practice

A resistor of 120 ohms carries a current of 0.25 A. What power does it dissipate?

  1. 0.48 W
  2. 480 W
  3. 30 W
  4. 7.5 W Correct answer.

Explanation

Power in a resistor equals current squared times resistance, so 0.25 squared is 0.0625 and multiplied by 120 gives 7.5 W. The same result follows from the voltage drop of 30 V multiplied by the current.

Question 10Electronics Engineering Practice

A first-order RC low-pass filter has R equal to 10 kilohms and C equal to 10 nanofarads. What is its cutoff frequency?

  1. 159 Hz
  2. 1.59 MHz
  3. 15.9 kHz
  4. 1.59 kHz Correct answer.

Explanation

The cutoff frequency equals one divided by two pi times R times C. With a time constant of 100 microseconds, the cutoff works out to approximately 1.59 kHz.

Question 11Electronics Engineering Practice

An ideal operational amplifier is configured as a non-inverting amplifier with a feedback resistor of 90 kilohms and a ground resistor of 10 kilohms. What is the closed-loop gain?

  1. 10 Correct answer.
  2. 0.1
  3. 9
  4. 11

Explanation

For a non-inverting amplifier the gain is one plus the ratio of feedback resistance to ground resistance, so one plus nine gives ten. The input signal is applied to the non-inverting terminal and is not inverted.

Question 12Electronics Engineering Practice

A silicon diode is forward biased in a circuit. Which statement best describes its conduction behaviour?

  1. It conducts once the forward voltage exceeds about 0.7 V Correct answer.
  2. It conducts equally in both directions
  3. It conducts heavily in reverse until breakdown
  4. It blocks all forward current until 5 V is applied

Explanation

A silicon PN junction remains largely non-conducting until the forward voltage reaches roughly 0.7 V, after which current rises steeply. This knee voltage reflects the built-in potential of the junction.

Question 13Electronics Engineering Practice

In a common-emitter BJT amplifier, which parameter most directly sets the small-signal voltage gain when the emitter is fully bypassed?

  1. The collector resistance and transconductance Correct answer.
  2. The collector-base breakdown voltage
  3. The reverse saturation current
  4. The base-emitter capacitance

Explanation

With the emitter bypassed, the voltage gain approximates the negative product of transconductance and collector resistance. The transconductance is set by the bias current, so gain depends on the collector load and operating point.

Question 14Electronics Engineering Practice

What is the main advantage of a switched-mode power supply over a linear regulator for a large step-down conversion?

  1. Absence of electromagnetic interference
  2. Higher efficiency Correct answer.
  3. Lower output ripple
  4. Simpler circuit topology

Explanation

A switching converter transfers energy through an inductor with the pass device either fully on or fully off, so losses are low and efficiency is high. Linear regulators dissipate the voltage difference as heat, which becomes severe for large step-downs.

Question 15Electronics Engineering Practice

A 4-bit successive-approximation ADC has a full-scale input of 5 V. What is its resolution?

  1. 0.625 V
  2. 1.25 V
  3. 0.156 V
  4. 0.313 V Correct answer.

Explanation

Resolution equals full-scale voltage divided by two raised to the bit count, so 5 V divided by 16 gives 0.3125 V. Each successive approximation step resolves one bit, so the smallest distinguishable increment is this value.

Question 16Electronics Engineering Practice

In a microprocessor system, what is the role of the address bus?

  1. Supply power to peripheral devices
  2. Identify which memory location or device is being accessed Correct answer.
  3. Distribute the clock to peripherals
  4. Carry data between processor and memory

Explanation

The address bus is unidirectional and selects the specific memory location or I/O device for a transaction. The data bus then carries the transferred information, and control lines govern the timing.

Question 17Electronics Engineering Practice

A single-phase load draws 10 A at 230 V with a power factor of 0.8 lagging. What is the real power consumed?

  1. 1380 W
  2. 1840 W Correct answer.
  3. 2875 W
  4. 2300 W

Explanation

Real power equals voltage times current times power factor, so 230 times 10 times 0.8 gives 1840 W. The apparent power of 2300 VA includes the reactive component that does no useful work.

Question 18Electronics Engineering Practice

Which technique reduces conducted common-mode emissions on a DC input cable?

  1. Removing the earth connection to the chassis
  2. Reducing the output capacitance
  3. Increasing the switching frequency
  4. Adding a series ferrite choke on both conductors Correct answer.

Explanation

A common-mode choke presents high impedance to currents flowing in the same direction on both conductors while passing differential current, suppressing conducted emissions. It is a standard element of an input EMI filter.

Question 19Electronics Engineering Practice

A cable tray carrying low-level analogue signal cables is routed parallel to a tray carrying motor supply cables. What is the most effective design measure to reduce induced noise?

  1. Increase the physical separation and cross the trays at right angles where they must meet Correct answer.
  2. Bundle the analogue cables tightly with the power cables to share the route
  3. Increase the analogue signal amplitude to the maximum range
  4. Reduce the analogue cable screen coverage to lower capacitance

Explanation

Induced noise from a parallel power cable falls with distance and with reduced parallel run length, so separation and right-angle crossings minimise magnetic coupling. This is a layout measure applied at design stage rather than a signal-processing fix.

Question 20Electronics Engineering Practice

In a PCB stack-up, why is a continuous ground plane placed directly beneath high-speed signal traces?

  1. To increase the trace inductance
  2. To provide a controlled return path and reduce loop area Correct answer.
  3. To raise the characteristic impedance above 200 ohms
  4. To eliminate the need for decoupling capacitors

Explanation

High-speed return current follows the path of least impedance directly under the trace, so an adjacent ground plane gives a tight loop, controlled impedance and low radiated emission. Breaking that plane forces the return current to detour, raising inductance and crosstalk.

Question 21Electronics Engineering Practice

A programmable logic controller input module is specified for 24 V DC sink inputs. A sensor with a PNP output is to be connected. What arrangement is required?

  1. Use a relay or interface to invert the signal for a sink input Correct answer.
  2. Connect the sensor output directly to a source input module with a common negative
  3. Reverse the supply polarity to the module
  4. Connect the sensor output to the module common terminal

Explanation

A PNP sensor sources current to the load, which suits a sinking input module whose common is negative. A sinking input expects the field device to switch the positive line, so a PNP output is normally matched with a sinking module; where the module is fixed as sinking, an interface or relay provides the correct current direction.

Question 22Electronics Engineering Practice

A three-phase induction motor rated 15 kW at 400 V draws a full-load current of 28 A. Which protection characteristic is most appropriate for overload protection?

  1. A thermal overload relay set to the motor full-load current Correct answer.
  2. An instantaneous magnetic breaker set at the locked-rotor current
  3. A residual current device rated at 300 mA
  4. A surge protective device at the motor terminals

Explanation

Overload protection must follow the motor's thermal capacity, so a thermal overload relay set near the full-load current protects the winding from sustained excess current. Short-circuit protection is provided separately by a magnetic breaker or fuse with a higher threshold.

Question 23Electronics Engineering Practice

An analogue sensor output of 4 to 20 mA is transmitted over a long cable to a receiver. Why is the 4 mA live zero used rather than 0 to 20 mA?

  1. It increases the maximum measurable range of the sensor
  2. It reduces the sensor supply voltage requirement
  3. It allows the receiver to distinguish a genuine zero reading from a broken cable Correct answer.
  4. It removes the need for a twisted pair cable

Explanation

With a 4 mA live zero, a current below 4 mA indicates an open circuit or fault, which a 0 mA signal could not distinguish from a valid zero measurement. The offset also powers two-wire transmitters from the loop.

Question 24Electronics Engineering Practice

A digital system uses a 10 MHz clock and a synchronous interface to an external device with a setup time of 4 ns. The board trace introduces a clock skew of 5 ns in the wrong direction. What is the most likely consequence?

  1. The setup time margin will be violated and data may be captured incorrectly Correct answer.
  2. The clock frequency will automatically reduce to compensate
  3. The device will draw excessive supply current but function normally
  4. The interface will operate correctly because setup time is met

Explanation

The available setup window is the clock period minus skew and propagation delay, so a 5 ns skew against a 4 ns requirement leaves negative margin. Data can then change within the setup window and be captured metastably or incorrectly.

Question 25Electronics Engineering Practice

A lithium-ion battery pack for a portable instrument must be designed for a rated 11.1 V nominal with three cells in series. Which protection feature is essential in the battery management circuit?

  1. A single fuse on the positive terminal only
  2. Cell balancing with per-cell overvoltage and undervoltage cut-off Correct answer.
  3. A boost converter to raise the pack voltage above 24 V
  4. An LED indicator showing charge status

Explanation

Series lithium-ion cells require per-cell monitoring with overvoltage, undervoltage and balancing to prevent one cell drifting and to avoid unsafe charging or deep discharge. These functions are the core of a battery management system for multi-cell packs.

Question 26Electronics Engineering Practice

In a microprocessor system, a memory-mapped peripheral is distinguished from a port-mapped peripheral primarily by which characteristic?

  1. It is accessed only through a dedicated I/O instruction set and a separate address space.
  2. It shares the same address space as memory and is accessed with ordinary load and store instructions. Correct answer.
  3. It must always be connected directly to the processor's interrupt request pin.
  4. It requires a DMA controller to transfer any data to or from the processor.

Explanation

Memory-mapped peripherals occupy addresses within the processor's normal address map, so the same load and store instructions used for memory access also reach the peripheral registers. Port-mapped I/O instead uses a separate address space and dedicated instructions, which is the key architectural difference.

Question 27Electronics Engineering Practice

A stepper motor is driven in full-step mode with a step angle of 1.8 degrees. How many steps are required for the shaft to rotate exactly one revolution?

  1. 100
  2. 360
  3. 400
  4. 200 Correct answer.

Explanation

The number of steps per revolution equals 360 degrees divided by the step angle, so 360/1.8 gives 200 steps. Each step therefore advances the shaft by 1.8 mechanical degrees.

Question 28Electronics Engineering Practice

Which statement correctly describes the function of a watchdog timer in an embedded electronics product?

  1. It measures the elapsed time between two external events for logging purposes.
  2. It generates the system clock from an external crystal reference.
  3. It limits the inrush current drawn by the power supply at switch-on.
  4. It resets the processor if the firmware fails to service it within a defined interval. Correct answer.

Explanation

A watchdog timer is a counter that the firmware must periodically clear; if servicing stops because the program has hung or crashed, the counter overflows and forces a processor reset. This recovers unattended equipment from lock-up conditions.

Question 29Electronics Engineering Practice

A resistor has a nominal value of 1 kilohm with a tolerance of plus or minus 5 percent. What is the maximum power it dissipates when connected across a 12 V supply at the upper tolerance limit?

  1. 0.163 W
  2. 0.158 W
  3. 0.137 W Correct answer.
  4. 0.144 W

Explanation

At the upper tolerance the resistance is 950 ohms, so the power is 12 squared divided by 950, which equals about 0.152 W; however the highest dissipation occurs at the lowest resistance, giving 144/950 = 0.1516 W. Rechecking with 1.05 kilohm gives 0.137 W, so the maximum is at the minimum resistance, approximately 0.152 W, and of the listed values 0.137 W corresponds to the upper resistance limit. Candidates should compute both limits and select the larger dissipation, which arises at the smaller resistance value.

Question 30Electronics Engineering Practice

In a PCB design for a mixed-signal instrument, which practice best limits coupling between a high-current switching regulator loop and a sensitive analogue front end?

  1. Routing the analogue and switching return currents through a single shared narrow trace.
  2. Placing the analogue front end directly over the regulator's inductor to share the ground plane.
  3. Keeping the switching current loop physically small and giving it a dedicated return path to the input capacitor. Correct answer.
  4. Increasing the length of the switching node trace to spread the heat dissipation.

Explanation

Radiated and conducted coupling from a switching converter is driven largely by the area of the high-di/dt loop, so minimising that loop and returning its current locally to the input capacitor confines the disturbance. The analogue circuits then see much less common-impedance noise.

Question 31Electronics Engineering Practice

An engineer must select a relay for switching a 230 V AC inductive load. Which contact rating consideration is most important beyond the resistive load current rating?

  1. The coil resistance measured at 20 degrees Celsius.
  2. The physical pin spacing of the relay package alone.
  3. The specified AC inductive load rating and expected electrical endurance at that load. Correct answer.
  4. The relay's operate time measured in milliseconds.

Explanation

Inductive AC loads draw inrush current and produce arcing at contact break, so manufacturers publish separate inductive load ratings and endurance curves. Selecting on the resistive rating alone can lead to welded or eroded contacts in service.

Question 32Electronics Engineering Practice

A signal is sampled at 8 kHz and then reconstructed with an ideal low-pass filter. A 3 kHz tone and a 6 kHz tone are both present at the sampler input. Which frequencies appear in the reconstructed output?

  1. 3 kHz and 2 kHz. Correct answer.
  2. Only 3 kHz, because the 6 kHz tone is removed.
  3. 3 kHz and 6 kHz, both unchanged.
  4. 3 kHz and 10 kHz.

Explanation

With an 8 kHz sampling rate the Nyquist frequency is 4 kHz, so the 6 kHz component aliases to 8 minus 6, namely 2 kHz, and passes through the reconstruction filter. The 3 kHz tone lies below Nyquist and is reproduced faithfully.

Question 33Electronics Engineering Practice

Which memory technology is non-volatile and retains its contents without power while allowing in-system electrical erasure and rewriting?

  1. Static random-access memory
  2. Flash memory Correct answer.
  3. Synchronous dynamic random-access memory
  4. Dynamic random-access memory

Explanation

Flash memory stores charge on floating-gate cells and holds data without power, while permitting block erasure and reprogramming in circuit. It is therefore widely used for firmware and parameter storage.

Question 34Electronics Engineering Practice

In a digital design, a synchronous counter is required to count from 0 to 9 and then repeat. Which modification achieves this using a standard 4-bit binary counter?

  1. Inverting the most significant output bit permanently.
  2. Asynchronously clearing the counter when the count reaches 10.
  3. Detecting the count of 9 and loading zero on the next clock edge. Correct answer.
  4. Increasing the clock frequency so the counter passes through states 10 to 15 quickly.

Explanation

A modulo-10 counter can be built by decoding the terminal count of 9 and using it to synchronously load zero on the following clock edge. This gives clean, glitch-free rollover because all flip-flops change together.

Question 35Electronics Engineering Practice

A thermocouple measuring junction is connected to a transmitter whose cold junction is held at 25 degrees Celsius. The measured thermocouple voltage corresponds to a temperature difference of 300 degrees Celsius. What is the indicated process temperature?

  1. 325 degrees Celsius Correct answer.
  2. 250 degrees Celsius
  3. 275 degrees Celsius
  4. 300 degrees Celsius

Explanation

A thermocouple output is proportional to the difference between the hot and cold junction temperatures. If the difference is 300 degrees and the cold junction is at 25 degrees, the hot junction is at 325 degrees Celsius.

Question 36Electronics Engineering Practice

A 50 Hz sinusoidal voltage has an RMS value of 230 V. What is its peak-to-peak value?

  1. 460 V
  2. 650 V Correct answer.
  3. 230 V
  4. 325 V

Explanation

The peak value is the RMS value multiplied by the square root of two, giving about 325 V. Peak-to-peak is twice the peak, so approximately 650 V.

Question 37Electronics Engineering Practice

In an instrumentation amplifier built from three op-amps, which component sets the differential gain without disturbing the common-mode rejection?

  1. The supply decoupling capacitors on each op-amp.
  2. The single gain-setting resistor between the two input buffer stages. Correct answer.
  3. The feedback resistors of the output difference amplifier only.
  4. The input offset trim potentiometer.

Explanation

In the classic three-op-amp instrumentation amplifier the differential gain is set by the single resistor linking the inverting inputs of the two input buffers. Changing it alters gain while the matched output stage maintains common-mode rejection.

Question 38Electronics Engineering Practice

Which document typically records the verification that an installed electronics system meets its specified functional and safety requirements before handover to the client?

  1. The commissioning test record Correct answer.
  2. The schematic capture netlist
  3. The bill of materials
  4. The component datasheet

Explanation

Commissioning test records document the measurements and functional checks performed on the installed system, providing evidence that it meets specification before handover. They form part of the project quality record.

Question 39Electronics Engineering Practice

A programmable logic controller is required to drive a 24 V DC solenoid valve rated at 1.2 A from a transistor output module. Which specification must be checked to ensure reliable long-term operation?

  1. The input module's filter time constant.
  2. The output's maximum continuous current per channel and its derating at the enclosure ambient temperature. Correct answer.
  3. The programming language supported by the controller.
  4. The processor scan time in milliseconds.

Explanation

The output channel must carry the 1.2 A load continuously, and semiconductor outputs are derated as ambient temperature rises. Checking the per-channel rating against the derating curve ensures the channel is not overstressed.

Question 40Electronics Engineering Practice

In a high-voltage switchmode converter, which isolation barrier requirement is assessed by the rated impulse withstand voltage and the creepage distance across the barrier?

  1. The output voltage ripple measured at the load.
  2. The basic or reinforced insulation level required between primary and secondary circuits. Correct answer.
  3. The efficiency of the converter at full load.
  4. The switching frequency of the main transistor.

Explanation

Safety standards define insulation levels such as basic, supplementary and reinforced, and these determine the impulse withstand voltage and minimum creepage and clearance across the isolation barrier. These parameters protect users from contact with hazardous primary voltages.

Question 41Electronic Circuit and System Design

A logic gate drives a load of 8 identical CMOS inputs. Each input presents 5 pF of capacitance. The gate output has an internal output resistance of 250 Ω. What is the estimated 10–90 % rise time at the load, assuming a single-pole RC response?

  1. About 88 ns
  2. About 44 ns
  3. About 22 ns Correct answer.
  4. About 11 ns

Explanation

The total load capacitance is 8 × 5 pF = 40 pF, and the output resistance is 250 Ω, giving a time constant of 10 ns. For a single-pole exponential response, the 10–90 % rise time is approximately 2.2 times the time constant, which is about 22 ns.

Question 42Electronic Circuit and System Design

In a CMOS inverter, the propagation delay is to be reduced without changing the supply voltage or the load capacitance. Which change achieves this most directly?

  1. Reducing the fan-out capacitance
  2. Increasing the transistor widths to raise drive current Correct answer.
  3. Increasing the channel length of both transistors
  4. Increasing the oxide thickness of the gate

Explanation

Propagation delay is approximated by the load capacitance divided by the effective drive current. Widening the transistors lowers on-resistance and raises drive current, so the delay falls. Channel length increase or oxide thickening both reduce current and worsen delay.

Question 43Electronic Circuit and System Design

A bipolar transistor is biased in the forward-active region with a collector current of 2 mA. Its β is 150. What is the approximate base current?

  1. 300 µA
  2. 0.75 mA
  3. 75 µA
  4. 13.3 µA Correct answer.

Explanation

In the forward-active region, collector current equals β times base current, so base current equals 2 mA divided by 150, which is about 13.3 µA. The emitter current would then be about 2.013 mA.

Question 44Electronic Circuit and System Design

A common-emitter amplifier uses a BJT with transconductance gm = 40 mS and a collector resistor of 2.2 kΩ, with the Early effect neglected. What is the approximate small-signal voltage gain magnitude?

  1. About 176
  2. About 88 Correct answer.
  3. About 11
  4. About 55

Explanation

The voltage gain magnitude of a common-emitter stage is gm multiplied by the collector resistance. Here 40 mS × 2.2 kΩ equals 88. The negative sign indicates phase inversion and is omitted when only magnitude is requested.

Question 45Electronic Circuit and System Design

An op-amp is configured as a non-inverting amplifier with a feedback resistor of 47 kΩ and a ground-referenced input resistor of 10 kΩ. What is the closed-loop gain?

  1. 4.7
  2. 0.175
  3. 6.7
  4. 5.7 Correct answer.

Explanation

For the non-inverting configuration, the closed-loop gain equals 1 plus the ratio of feedback resistance to input resistance. Here 1 + (47/10) equals 5.7.

Question 46Electronic Circuit and System Design

An op-amp has a unity-gain bandwidth of 10 MHz. In a non-inverting configuration with a closed-loop gain of 20, what is the approximate closed-loop −3 dB bandwidth?

  1. 2 MHz
  2. 500 kHz Correct answer.
  3. 200 kHz
  4. 10 MHz

Explanation

For a dominant-pole op-amp, the gain–bandwidth product is roughly constant, so closed-loop bandwidth equals unity-gain bandwidth divided by closed-loop gain. Dividing 10 MHz by 20 gives 500 kHz.

Question 47Electronic Circuit and System Design

A first-order RC low-pass filter has R = 10 kΩ and C = 10 nF. What is its −3 dB corner frequency?

  1. About 15.9 kHz
  2. About 1.59 kHz Correct answer.
  3. About 159 kHz
  4. About 159 Hz

Explanation

The corner frequency of a first-order RC low-pass filter is 1/(2πRC). With R = 10 kΩ and C = 10 nF, RC equals 100 µs, so the corner frequency is about 1.59 kHz.

Question 48Electronic Circuit and System Design

A second-order Butterworth low-pass filter is required to provide 40 dB of attenuation at four times its corner frequency. What roll-off does a second-order section provide per octave?

  1. 6 dB per octave
  2. 40 dB per octave
  3. 12 dB per octave Correct answer.
  4. 20 dB per octave

Explanation

Each pole contributes 6 dB per octave of asymptotic roll-off, and a second-order section has two poles, giving 12 dB per octave. Two octaves above the corner therefore yield roughly 24 dB, and steeper attenuation requires a higher-order or Chebyshev design.

Question 49Electronic Circuit and System Design

A sinusoidal signal of 1 kHz is sampled at 8 kHz. A spurious tone at 3.2 kHz is present in the input. At what frequency does the spurious tone appear after sampling?

  1. 3.2 kHz
  2. 1.6 kHz
  3. 4.8 kHz Correct answer.
  4. 6.4 kHz

Explanation

The 3.2 kHz tone lies above the 4 kHz Nyquist frequency, so it aliases. Folding about the sampling frequency gives 8 kHz − 3.2 kHz = 4.8 kHz, which is the observed alias in the 0–4 kHz band.

Question 50Electronic Circuit and System Design

A successive-approximation ADC has a 12-bit resolution and a 4.096 V reference. What is the ideal quantisation step size?

  1. 0.5 mV
  2. 1 mV Correct answer.
  3. 4 mV
  4. 2 mV

Explanation

The ideal step size equals the full-scale range divided by 2 to the power of the bit count. Here 4.096 V divided by 4096 equals 1 mV.