Question 1

Oscilloscope measurements: Using a triggered sweep on an oscilloscope primarily improves the accuracy/repeatability of which measurement dimension?

Accepted Answer

Introduction / Context: An oscilloscope's triggered sweep stabilizes repetitive waveforms by initiating the timebase at a consistent point on the signal. This capability is essential for accurate time-related measurements such as period, pulse width, rise/fall times, and delay. Given Data / Assumptions: Repetitive signal with a well-defined trigger condition (edge/level). Oscilloscope supports triggered, auto, and free-run modes. Interest in measurement accuracy and stability. Concept / Approach: Triggered sweep synchronizes the horizontal deflection (time axis) to the signal’s trigger event, making each acquisition begin at the same phase point. This yields a stable trace with minimal jitter, enabling precise timing measurements. While amplitude accuracy depends on vertical calibration and probe compensation, timing benefits most directly from consistent horizontal synchronization. Step-by-Step Solution: Set trigger source to the signal under test.Choose an appropriate trigger mode and level to capture a consistent edge.Observe the stabilized display; use cursors or graticule to measure time intervals.Result: improved timing accuracy and repeatability. Verification / Alternative check: Compare measurements in free-run vs. triggered mode; only the latter yields a steady, phase-locked display suitable for precise timing. Why Other Options Are Wrong: Amplitude accuracy is governed mainly by vertical gain and probe calibration. “Graticule activity” is not a technical metric. Probe compensation is adjusted separately and does not rely on trigger synchronization. Common Pitfalls: Incorrect trigger level or coupling can cause unstable displays; ensure adequate signal amplitude and bandwidth for reliable triggering. Final Answer: timing

Question 2

Troubleshooting digital ICs: After identifying that one internal gate of an integrated circuit is faulty, what is the appropriate corrective action in standard repair practice?

Accepted Answer

Introduction / Context: Digital integrated circuits encapsulate many gates and support structures on a single die. When one gate fails, it typically indicates irreversible internal damage. Field repair focuses on board-level replacement rather than die-level rework. Given Data / Assumptions: Confirmed diagnosis: a specific gate inside an IC is nonfunctional. Through-hole or SMD package on a PCB. No specialized chip-level re-fabrication capability on site. Concept / Approach: ICs are monolithic; you cannot “repair” an individual gate. Best practice is to replace the entire device, ensuring pin-compatible, same or higher speed grade, and same logic family. Before replacement, you may verify power rails and clock/reset just to avoid replacing a good part due to external faults, but the corrective action remains IC replacement. Step-by-Step Solution: Confirm fault with logic probe/scope and substitution if available.Power down and follow ESD precautions.Desolder and remove the defective IC; clean pads/vias.Install a known-good replacement; test operation. Verification / Alternative check: After replacement, verify functionality at the gate’s inputs/outputs; confirm no downstream shorts or upstream drive issues remain. Why Other Options Are Wrong: “Repair the gate” is not feasible in monolithic ICs. Resoldering tracks does not fix an internal die failure. Rechecking power is good diagnostics but not the corrective action once the gate is confirmed bad. Removing only certain pins is impractical and destructive. Common Pitfalls: Overlooking root causes such as overvoltage or ESD; always address the stressor to prevent repeat failures. Final Answer: replace the IC involved

Question 3

PCB power rail diagnosis: A regulated +5 V supply on a board has sagged to about +3.4 V under load. Which fault is the most likely immediate cause?

Accepted Answer

Introduction / Context: Power rails that read significantly below their nominal value often indicate excessive loading. In digital systems, unintended shorts or partial shorts can pull a regulated rail down to an intermediate level where the regulator current limits or the source impedance causes droop. Given Data / Assumptions: Nominal +5 V rail measures +3.4 V in-circuit. Supply previously operated correctly. No evidence of supply being set to +3.3 V intentionally. Concept / Approach: Two broad categories cause low rails: source failure (regulator fault) and excessive load (short/overload). A drop from 5.0 V to about 3.4 V strongly suggests the supply is being dragged down by a low-resistance path, often a solder bridge, failed IC, or connector issue. The “half-split method” is a troubleshooting technique, not a fault. Opens usually cause rails to float or rise, not sag under load. Step-by-Step Solution: Isolate: Power off, measure resistance from rail to ground.If abnormally low, disconnect sections (pull fuses, unplug cards) to localize the short.Use thermal camera or IPA evaporation to find the hot component/spot.Repair the shorted device or bridge; verify rail returns to +5 V. Verification / Alternative check: Bench supply with current limit: bring rail up and observe current draw; a short will pull high current immediately. Why Other Options Are Wrong: An open usually removes load and does not pull rails down. A faulty regulator can cause low voltage, but the most common sudden sag to ~3.4 V is heavy loading. Half-split is a method, not a failure. Probe compensation is unrelated to DC rail measurements. Common Pitfalls: Confusing an intentional +3.3 V rail with a pulled-down +5 V; always check schematics and labels. Final Answer: a circuit short

Question 4

Measuring digital pulse width: At what relative amplitude should pulse-width measurements be taken to obtain consistent and comparable results?

Accepted Answer

Introduction / Context: Pulse width is the time between two amplitude thresholds on a pulse waveform. Standards are necessary so different instruments and engineers obtain matching measurements, independent of overshoot or ringing. Given Data / Assumptions: Digital pulse with finite rise/fall times and possible overshoot. Oscilloscope with cursor measurements or automated timing. Industry-standard thresholding is desired. Concept / Approach: The conventional method measures between the 50% amplitude points of the leading and trailing edges (mid-levels). This choice reduces sensitivity to edge asymmetry and overshoot/undershoot. Rise/fall time measurements commonly use 10–90% or 20–80%, but pulse width uses the 50% criterion for consistency. Step-by-Step Solution: Determine baseline LOW and HIGH amplitudes (e.g., 0 V and VHIGH).Compute the 50% level: (LOW + HIGH) / 2.Place time cursors at the instants the waveform crosses 50% on the leading and trailing edges.Pulse width = Δt between these two crossings. Verification / Alternative check: Compare automated pulse-width readout to manual cursor measurement at 50%; they should agree within instrument tolerance. Why Other Options Are Wrong: Overshoot/undershoot points are nonstandard and vary with loading. 10–90% applies to rise/fall time, not width. Damping/ringing envelopes are irrelevant to width metrics. Peak overshoot is transient and unsuitable for width timing. Common Pitfalls: Failure to average noisy signals or trigger stably can cause jitter in crossings; use proper triggering and bandwidth limits if needed. Final Answer: 50% of the leading and 50% of the trailing amplitude

Question 5

Logic probe and pulser diagnosis: A probe on a gate's output shows a dim (weak) indication, and exercising each input with a logic pulser does not change the output indication. What fault does this most strongly suggest?

Accepted Answer

Introduction / Context: Logic probes typically display HIGH, LOW, or pulsing states, and some indicate a “weak” or indeterminate level with a dim LED. When a logic pulser applied to inputs fails to elicit any deterministic response at the output, a high-impedance or floating node is suspected. Given Data / Assumptions: Probe indicates a dim/weak output level. Inputs are individually driven with a logic pulser. No output state change occurs. Board power and ground are otherwise present. Concept / Approach: An “open” output (internally damaged transistor pair or disconnected pin) leaves the node floating. The probe senses neither a firm HIGH nor a firm LOW, resulting in a dim indication. Because the output stage is not actively driving, changing inputs does not propagate to the output node. A genuine tri-state could also float, but it is usually controlled by an enable pin and would be documented; here, the symptom is consistent with an internal open. Step-by-Step Solution: Confirm probe ground and supply are good (quick sanity check).Pulse each input; observe no change at output → logic not coupling to node.Interpret dim/floating output as high-Z with no active drive.Conclude: output stage is open; replace the IC. Verification / Alternative check: Scope the output; a floating node may show noise pickup. Ohmmeter to ground/VCC with power off may show abnormally high resistance compared to a known-good gate. Why Other Options Are Wrong: Low supply would affect many nodes, not just one output, and the pulser would still move levels. A bad probe ground gives erratic readings everywhere, not reproducible behavior on one gate. Tri-state requires a disabled enable pin; check OE first, but the symptom here points to damage. Probe bandwidth limits produce missed pulses, not a constant dim DC indication. Common Pitfalls: Forgetting to verify output-enable pins on bus drivers; confirm OE is asserted before condemning the IC. Final Answer: The output of the gate appears to be open.

Question 6

Digital pulses and duty cycle: which formula correctly defines the duty cycle of a repetitive pulse train (express your answer in terms of pulse width and period)?

Accepted Answer

Introduction / Context: The duty cycle of a digital or pulse waveform is a core timing specification used in electronics, embedded systems, and communications. It tells us what fraction of each full period a signal remains in its active (usually logic HIGH) state. Understanding the correct formula is essential for power calculations, PWM motor control, LED dimming, and clock timing budgets. Given Data / Assumptions: A repetitive pulse train with a well-defined period T. Pulse width (also called TON or HIGH time) is the time the pulse stays active within each period. Duty cycle is typically expressed as a percentage. Concept / Approach: The duty cycle compares the active time of the signal to its total cycle time. If a signal is HIGH for half the period, its duty cycle is 50%. Expressing this as a formula, we divide the pulse width by the period and then multiply by 100% to convert to a percentage. Step-by-Step Solution: Let pulse width = TON, period = T.Compute the fraction of the period that is HIGH: TON / T.Convert to percent: duty cycle = (TON / T) * 100%.Therefore, Duty cycle = (pulse width / period) * 100%. Verification / Alternative check: If the pulse is HIGH for 2 ms and the period is 10 ms, then duty cycle = (2/10)*100% = 20%, matching intuitive expectations. If pulse width equals period, duty cycle is 100%; if pulse width is zero, it is 0%. Why Other Options Are Wrong: Duty cycle = (rise time / period) * 100%: rise time measures edge speed, not the active fraction. Duty cycle = (fall time / pulse width) * 100%: mixes edge parameter with width; not a definition. Duty cycle = (period / pulse width) * 100%: the reciprocal; would exceed 100% for typical pulses. Common Pitfalls: Confusing frequency with duty cycle; frequency changes period, but duty cycle remains a ratio. Mixing average value with duty cycle; average depends on amplitude as well as duty cycle. Final Answer: Duty cycle = (pulse width / period) * 100%

Question 7

In digital and pulse waveform measurements, the time required for a pulse to transition from 10% to 90% of its final amplitude is defined as:

Accepted Answer

Introduction / Context: Accurate time-domain characterization of digital signals requires standard definitions so different engineers and instruments report consistent results. One of the most common parameters is rise time, which captures how quickly a signal edge moves toward a new logic level. Given Data / Assumptions: The pulse transitions from a lower steady value to a higher steady value. Standard measurement thresholds are 10% and 90% of the final amplitude. We are not measuring width, period, or duty cycle, but specifically the edge speed. Concept / Approach: Rise time focuses on the central part of the edge where the slope is most meaningful and least affected by noise or saturation. The 10% and 90% thresholds avoid slow tails and baseline jitter that can skew a 0%–100% metric. Step-by-Step Solution: Identify the lower level and final upper level of the transition.Compute 10% and 90% of the amplitude step.Measure the elapsed time as the waveform crosses from 10% to 90%.This elapsed time is, by definition, the rise time. Verification / Alternative check: Oscilloscopes typically provide automatic rise time measurements using these thresholds. Datasheets for logic families (e.g., CMOS, TTL) specify typical rise times in nanoseconds based on this standard. Why Other Options Are Wrong: Pulse width: duration the signal stays in the active level; unrelated to edge speed. Propagation delay: time through a device from input transition to output response. Duty cycle: ratio of pulse width to period; again not an edge-speed metric. Common Pitfalls: Using 0%–100% bounds increases sensitivity to noise; industry uses 10%–90% (or 20%–80%). Confusing rise time with bandwidth; while related, they are not the same measurement. Final Answer: rise time

Question 8

For determining a digital signal’s fundamental frequency and viewing its time-domain waveshape directly, which test instrument is best suited?

Accepted Answer

Introduction / Context: When troubleshooting digital circuits, you need both the numeric frequency and the actual time-domain shape (edges, overshoot, ringing). Instruments differ in what domain they observe and what information they provide. Given Data / Assumptions: Goal: determine frequency and visualize the waveform shape. Signal is a repetitive digital pulse or clock. We want immediate time-domain insight (voltage vs. time). Concept / Approach: An oscilloscope displays voltage as a function of time, allowing direct measurements of period, frequency, pulse width, rise/fall times, and anomalies such as ringing. While spectrum analyzers reveal frequency components, they do not show the time-domain shape directly. Step-by-Step Solution: Connect the probe to the node of interest and ground reference.Set time base to capture several cycles; adjust volts/div to fit amplitude.Measure period T from the screen (or use automated measurement), then compute f = 1 / T.Inspect the edge quality, overshoot, undershoot, and duty cycle from the time trace. Verification / Alternative check: Many oscilloscopes provide built-in frequency counters and automated timing measurements, confirming manual readings. A spectrum analyzer complements by revealing harmonics, but the oscilloscope remains primary for waveshape. Why Other Options Are Wrong: Multimeter: Reads average or RMS values; lacks time-domain detail. Spectrum analyzer: Frequency-domain only; does not directly show time-domain shape. Frequency generator: Produces signals; it is not a measuring instrument. Common Pitfalls: Using too much probe capacitance can distort high-speed edges; use 10x probes and proper bandwidth. Failure to set proper trigger can make the waveform appear unstable. Final Answer: an oscilloscope

Question 9

In practical digital-circuit troubleshooting, a handheld multimeter is best used to measure which quantities on a static or slowly changing node?

Accepted Answer

Introduction / Context: A multimeter is a fundamental tool for electronics work, but it is limited by measurement speed and display type. Knowing what it measures well in digital circuits prevents misinterpretation and speeds diagnosis. Given Data / Assumptions: Standard DMM with DC/AC voltage, resistance, and current ranges. Digital signals may be static levels (logic HIGH/LOW) or fast pulses. We seek what a DMM accurately measures in this context. Concept / Approach: Multimeters excel at scalar measurements: DC voltage, average AC voltage (within bandwidth limits), and resistance of unpowered networks. They cannot observe rapid timing parameters such as pulse width or duty cycle and are not ideal for fast current pulses without specialized shunts and bandwidth. Step-by-Step Solution: For a logic rail, use DC volts to confirm nominal level (e.g., 3.3 V or 5 V).For pull-ups/pull-downs, power off and measure resistance to confirm value and continuity.Avoid using a DMM to measure pulse widths; use a scope or logic analyzer. Verification / Alternative check: Manufacturers specify DMM update rates (often a few readings per second) and limited AC bandwidth, confirming unsuitability for timing measurements but reliability for DC voltage and resistance checks. Why Other Options Are Wrong: Pulse width / pulse trains: Require time-domain instruments (oscilloscope, logic analyzer). Current: While DMMs can measure current, inserting in series can disturb circuits; it is not the typical quick check in digital IC debugging. Common Pitfalls: Attempting to infer duty cycle from an averaged DMM reading; results are ambiguous. Measuring resistance on a live circuit leads to incorrect readings and potential damage. Final Answer: voltage or resistance

Question 10

PCB fault-finding: which instrument best helps trace current paths to locate a shorted track or shorted component on a printed circuit board?

Accepted Answer

Introduction / Context: Short circuits on printed circuit boards can be difficult to localize by visual inspection alone. Specialized tools can help follow actual current flow to the precise fault location, reducing rework time. Given Data / Assumptions: There is an unintended short between nodes or rails. We need to follow the path of highest current draw. Board is powered in a controlled, current-limited way during tracing. Concept / Approach: A current tracer (paired with an injection source or current probe) senses magnetic fields produced by current and lets the technician follow the flow along traces. The signal peaks near the short location, enabling pinpoint diagnosis even in multilayer designs. Step-by-Step Solution: Power the board with a safe, current-limited supply.Use the current tracer probe along the suspect net, observing the indicator strength.Follow increasing signal intensity until the maximum is found—this is the shorted region. Verification / Alternative check: Confirm by isolating components, thermal imaging (hot-spot), or resistance/voltage drop measurements across small copper lengths to corroborate the tracer’s indication. Why Other Options Are Wrong: Multimeter: Useful for resistance checks but poor at spatial localization of shorts on complex layouts. Logic pulser: Injects logic pulses for functional testing, not for tracing DC shorts. Oscilloscope: Excellent for waveforms but cannot “sniff” current paths over copper planes. Common Pitfalls: Not limiting current during tracing may overheat components. Misinterpreting ground-return currents; ensure proper reference and path selection. Final Answer: current tracer

Question 11

To compare input and output signals simultaneously during troubleshooting (e.g., verify gain, phase, and distortion), which test instrument is most appropriate?

Accepted Answer

Introduction / Context: Many diagnostics require observing two or more nodes at once—such as an amplifier’s input vs. output to assess gain and phase, or two related digital lines to examine timing relationships. The chosen instrument must display multiple traces concurrently with a shared time base. Given Data / Assumptions: Need to visualize two or more signals at the same time. Accurate timing correlation is required (same horizontal sweep). Qualitative and quantitative comparisons (amplitude, delay) are desired. Concept / Approach: A multitrace (dual- or multi-channel) oscilloscope displays multiple signals stacked vertically but time-aligned horizontally. This allows direct, visual comparison of amplitude, phase shift, propagation delay, and distortion. Step-by-Step Solution: Connect probes to input and output nodes; select identical attenuation.Set time base and trigger on the input for stable display.Adjust vertical scales; measure gain (Vout/Vin) and phase/time shift.Use math functions if available (e.g., A–B) for further analysis. Verification / Alternative check: Frequency response can be corroborated using a network analyzer, but for quick, in-circuit comparisons, a multitrace oscilloscope remains the fastest approach. Why Other Options Are Wrong: Single-channel oscilloscope: Cannot show both signals simultaneously with shared timing. Logic probe: Indicates logic levels but not analog shape or timing. Spectrum analyzer: Frequency-domain visualization; not time-domain comparison across nodes. Common Pitfalls: Mismatched probe compensation skews waveform comparisons. Incorrect ground referencing can introduce ground loops or ringing. Final Answer: a multitrace oscilloscope

Question 12

Signal integrity terminology: a series of sinusoidal oscillations whose amplitude gradually decreases with time after a step or edge is properly called:

Accepted Answer

Introduction / Context: Real-world interconnects and circuits have inductance, capacitance, and resistance. When a fast edge excites these parasitics, the response can be a damped oscillation. Correctly naming and recognizing this behavior is essential for debugging and mitigation. Given Data / Assumptions: Observation: oscillatory waveform following an edge, diminishing in amplitude. Cause: underdamped RLC behavior or transmission-line reflections. Goal: identify the correct term. Concept / Approach: “Ringing” describes damped oscillations about a nominal steady-state value after a transition. If the first excursion exceeds the target level it is “overshoot,” and if it dips below, it is “undershoot”; these can occur within a ringing envelope. Step-by-Step Solution: Identify oscillations about the final value following a step.Check that amplitude decays with time (damped response).Label the phenomenon: ringing. Verification / Alternative check: Simulating a step response of an RLC series circuit or a mismatched transmission line shows classic ringing that decays at a rate determined by damping (R) and energy storage (L and C). Why Other Options Are Wrong: Slew: Refers to the rate of change (slew rate), not oscillation. Overshooting: Single excursion above the final value; not necessarily oscillatory. Undershooting: Single excursion below the final value; again, not inherently oscillatory. Common Pitfalls: Confusing occasional overshoot with persistent ringing; ringing implies successive alternating excursions. Ignoring terminations and layout that can reduce ringing. Final Answer: ringing

Question 13

Best tools for locating shorts in digital circuits  Select the instrument pair that technicians commonly use to inject a stimulus and trace abnormal current flow to locate shorted nodes on a digital board.

Accepted Answer

Introduction / Context: Finding shorts on a dense digital PCB can be challenging. Rather than guessing, technicians use targeted tools to excite the circuit and observe where current actually flows. The right combination makes it quicker to track the fault to a specific trace or component. Given Data / Assumptions: We need to detect unintended low-resistance paths that draw current. A safe stimulus must be applied without damaging logic devices. A sensitive probe should indicate where the current is traveling along copper traces. Concept / Approach: A logic pulser momentarily drives a pin or net with controlled pulses at logic levels. A current tracer (a handheld probe with a magnetic or resistive sensor) detects the resulting pulse current through traces, allowing you to follow it physically across the board. When a short exists, the current concentrates near the fault, producing a stronger indication and guiding you to the exact location. Step-by-Step Solution: Isolate the suspect net and remove power to sensitive loads if needed.Attach the logic pulser to the node to inject low-duty-cycle pulses.Sweep the current tracer along the trace path, watching for signal strength increase.Identify the point where current abruptly diverts or peaks—this is near the short. Verification / Alternative check: Technicians corroborate by lifting components or cutting jumpers near the indicated spot; continuity tests (DMM) confirm the short after the board cools and the stimulus is removed. Why Other Options Are Wrong: Logic pulser + logic probe: the probe reads logic levels, not current flow along traces.Logic probe/clip: useful for state observation, but not for tracing shorts.Current tracer + logic clip: the clip is a powered logic indicator, not a stimulus source. Common Pitfalls: Overdriving the node with an improper pulser setting; misinterpreting inductive pickup from adjacent power rails as the short path; forgetting to disconnect heavy loads that mask the short’s signature. Final Answer: current tracer, logic pulser

Question 14

Digital systems control: In synchronous digital circuits, what signal type is most commonly used to coordinate state changes and sequence operations across flip-flops and registers?

Accepted Answer

Introduction / Context: In synchronous digital electronics, thousands or even millions of gates and storage elements must change state in a predictable, coordinated way. The mechanism that aligns these transitions is fundamental to reliable operation. This question asks which signal class most commonly provides that coordination in practical digital systems such as microprocessors, FPGAs, and synchronous state machines. Given Data / Assumptions: The circuit in question is a synchronous digital system using edge-triggered or level-sensitive storage. Flip-flops/registers require a timing reference to capture inputs and update outputs. Noise and glitches may exist but are undesired. Design goals include repeatability, determinism, and timing closure. Concept / Approach: Digital systems rely on a periodic timing reference that defines discrete instants for sampling inputs and updating states. A global or locally distributed clock provides these timing instants. By contrast, random pulses are nondeterministic; “selected frequencies” is vague and does not imply a systemwide reference; and “sophisticated gating” describes combinational logic shaping data, not the timing reference itself. Step-by-Step Solution: Identify the need: storage elements must transition simultaneously or within a controlled skew window.Recognize the standard solution: a periodic clock edge triggers all relevant flip-flops.Exclude alternatives: data gating configures functionality but does not replace temporal coordination.Conclude that clock signals are the industry-standard control mechanism. Verification / Alternative check: Examine any synchronous timing diagram: inputs are prepared during a clock period and captured at a defined edge (rising or falling). Static timing analysis and clock distribution networks (H-trees, meshes) are built specifically around this concept, confirming the central role of a clock. Why Other Options Are Wrong: selected frequencies: Frequency choice exists, but without a coordinated global clock it does not ensure synchronous capture.sophisticated gating: Shapes logic but is not a timing reference.random pulses: Undeterministic; would cause metastability and unreliable behavior. Common Pitfalls: Confusing asynchronous handshakes or event-driven designs with mainstream synchronous control; assuming a gated clock equals “control”—gating is a power/timing technique, not a fundamental replacement for the master clock. Final Answer: clock signals

Question 15

Troubleshooting digital circuits: What common cause best explains erratic or seemingly unexplained activity on logic lines in an otherwise functioning system?

Accepted Answer

Introduction / Context: Digital systems sometimes exhibit unexpected toggles or invalid states that appear sporadically. Understanding the most typical root cause helps technicians prioritize their measurements and fixes, whether using a logic analyzer, oscilloscope, or a simple logic probe. Given Data / Assumptions: The system is clocked and intended to be synchronous. Intermittent, short-duration anomalies are observed. No deliberate asynchronous interface has been designed to tolerate hazards. Power rails and grounds are assumed adequate until proven otherwise. Concept / Approach: A glitch is a brief unintended pulse or hazard on a digital line, often originating from unequal path delays in combinational logic, improper synchronization of asynchronous inputs, or insufficient debouncing. Glitches can momentarily drive logic to incorrect levels, triggering downstream latches or counters unexpectedly. Recognizing a glitch as the likely culprit prompts targeted checks: hazard analysis, input synchronization, and layout or termination review. Step-by-Step Solution: Observe symptom: spurious transitions or occasional miscounts.Relate to causes: path-delay hazards, crosstalk, ringing, or metastability at clock boundaries.Instrument the line: use an oscilloscope with sufficient bandwidth to capture sub-nanosecond pulses if needed.Fix: add proper synchronization (two-flop synchronizers), re-time logic, add debouncing, or improve signal integrity. Verification / Alternative check: Trigger the oscilloscope on narrow pulse width or use logic analyzer glitch-capture features. If the anomaly coincides with logic reconvergence or asynchronous inputs, the diagnosis is reinforced. Why Other Options Are Wrong: a random pulse: Describes the symptom, not the typical engineering term/cause; “glitch” is the recognized name.clock frequency changes: Usually controlled; frequency drift alone rarely causes sporadic single-pulse errors.a change in slew rate: Can exacerbate SI problems but is not, by itself, the standard root cause descriptor. Common Pitfalls: Blaming the clock first; overlooking asynchronous inputs and debouncing; ignoring termination or ground bounce in fast edge-rate environments. Final Answer: a glitch

Question 16

Test equipment modes: On which instrument would you typically find a mode selector labeled “train” and “single” for generating multiple or single pulses?

Accepted Answer

Introduction / Context: Digital troubleshooting often employs a small set of handheld tools. Knowing which tool provides which function speeds fault isolation. The terms “train” and “single” refer to generating either a burst (train) of pulses or a one-shot (single) stimulus applied to a node in a digital circuit. Given Data / Assumptions: The goal is to inject known logic transitions into a circuit. “Single” means one controlled pulse; “train” means repetitive pulses. The instrument connects to a digital node and delivers TTL/CMOS-compatible levels. The context is board-level troubleshooting, not measurement-only instruments. Concept / Approach: A logic pulser is specifically designed to drive logic-level pulses into a circuit to check response paths, clock in data, or step counters. Its modes commonly include single-pulse output and pulse-train output. A logic probe, by contrast, is primarily a sensing device indicating HIGH/LOW/pulse presence, not a stimulus source. Multimeters measure voltage/current/resistance and do not generate logic pulse trains. Current tracers locate current flow along PCB traces; they also are not pulse generators. Step-by-Step Solution: Identify need: generate controlled digital pulses.Match function to tool: only a logic pulser provides selectable “single” and “train” outputs.Exclude others: probe senses; multimeter measures; current tracer detects current in traces.Conclude the correct instrument is a logic pulser. Verification / Alternative check: Check typical handheld logic pulser datasheets/user guides: features include pulse width selection, repetition rate, and “single”/“burst” (train) modes for stimulating logic circuits. Why Other Options Are Wrong: multimeter: no pulse generation; only measurement.current tracer: senses magnetic fields from current, not a stimulus generator.logic probe: detects logic states and pulses but does not usually generate them. Common Pitfalls: Confusing a logic pulser with a logic probe because both are handheld and used together; assuming a DMM with a frequency function can generate pulse trains—it cannot at logic levels suitable for digital IC stimulation. Final Answer: logic pulser

Question 17

Finding an open PCB track: Which handheld troubleshooting tool most effectively helps trace where current stops along a printed conductor without cutting traces?

Accepted Answer

Introduction / Context: Open-circuit faults on printed circuit boards (PCBs) can be difficult to locate visually, especially in fine-pitch or multilayer designs. Field technicians rely on tools that localize where current flow ceases along a conductor to pinpoint the break without desoldering or cutting copper. Given Data / Assumptions: The board can be powered safely to allow current flow for tracing. Goal: identify the exact location where current no longer flows along a net. Access may be limited due to solder mask or internal layers. Non-invasive methods are preferred to avoid further damage. Concept / Approach: A current tracer senses the magnetic field generated by current in a conductor and provides a visual or audible indication of current presence. By moving the probe along the trace, you find the point where the indication stops—right at or just before the open. A multimeter can detect opens at two points but not easily localize mid-trace faults without probing many vias. Logic probes indicate logic levels, not current flow. Oscilloscopes can show signal activity but are less convenient for continuous current path localization. Step-by-Step Solution: Bias the net with a safe current (through the circuit or a current-injection jig).Follow the trace with the current tracer from the source toward the load.Note where the indication drops out—this marks the open location.Confirm with a visual inspection or continuity test at that spot. Verification / Alternative check: After identifying the suspected break, use a DMM in continuity mode across the localized region to confirm the open, and then repair via microjumper, reflow, or trace patch as appropriate. Why Other Options Are Wrong: multimeter: good for end-to-end checks but slow for mid-trace localization.logic probe: senses logic HIGH/LOW; not a current-flow localizer.oscilloscope: powerful for signal integrity but not optimized for tracing DC/low-frequency current paths. Common Pitfalls: Using too little current to be detected; tracing on powered boards without observing safety and ESD precautions; misinterpreting results on parallel paths where current may bypass a section. Final Answer: current tracer

Question 18

Powering a logic probe: Before a handheld logic probe can indicate HIGH/LOW/pulse states, its red and black leads must be connected to what?

Accepted Answer

Introduction / Context: A logic probe is a simple but essential tool that reports logic levels and activity on digital nodes. Like most active test instruments, it requires power to operate its internal comparators, indicators, and pulse-stretch circuitry. This question clarifies the necessary initial connection before probing any signal line. Given Data / Assumptions: Standard logic probe with red (V+) and black (ground) leads. Target circuit provides suitable supply rails (e.g., +5 V and ground for TTL, or +3.3 V for modern CMOS). The probe thresholds are referenced to the connected supply. The probe is not a passive device like a simple LED tester. Concept / Approach: The logic probe’s detection thresholds and pulse indicators must be referenced to a known supply to correctly interpret what constitutes a HIGH or LOW. Connecting the red lead to the system’s V+ and the black lead to ground powers the probe and aligns its thresholds to the system’s logic family. Without a power source, the probe cannot operate or may misreport levels due to floating references. Step-by-Step Solution: Connect red lead to the target system’s positive supply.Connect black lead to the system ground.Verify the probe’s power indicator is active.Probe signals; interpret HIGH/LOW/pulse indicators per the manual. Verification / Alternative check: Most probes include a power LED that lights when properly powered; some provide a self-test or reference pad to confirm proper operation before use. Why Other Options Are Wrong: logic pulser: separate tool for generating pulses; does not power the probe.logic clip: accessory for monitoring many pins; not a power source.HIGH or TRUE signal: probing a signal without powering the probe yields unreliable results. Common Pitfalls: Powering the probe from a different voltage than the circuit under test (threshold mismatch); forgetting ground reference; probing high-voltage or non-logic lines and damaging the probe. Final Answer: power source

Question 19

Improving logic probe usage: Which accessory enhances troubleshooting by simultaneously showing logic states on many IC pins while you selectively probe signals?

Accepted Answer

Introduction / Context: While a logic probe helps check a single node at a time, troubleshooting often benefits from a broader view of multiple related nodes (e.g., all pins on a counter or latch). An accessory that attaches to an IC and displays the logic state of many pins in parallel can dramatically speed diagnosis. Given Data / Assumptions: Standard DIP/SOIC logic families (TTL/CMOS) are being tested. Goal: correlate pin states without moving the probe for each pin. Accessory should be non-invasive and easy to clip on/off. Budget/bench-top context favors handheld tools. Concept / Approach: A logic clip is designed to clamp over an IC package and present each pin’s logic state via LEDs or indicators. While a logic probe remains useful for detailed node-by-node inspection, the clip offers an overview—revealing stuck pins, floating inputs, or unexpected toggling patterns. Frequency counters and oscilloscopes are powerful but serve different purposes; current tracers locate opens/shorts based on current flow, not logic state. Step-by-Step Solution: Attach the logic clip to the suspected IC (observing orientation and pin count).Power it according to the circuit’s logic supply to set thresholds correctly.Observe multiple pin indicators simultaneously to spot anomalies.Use the logic probe for deeper inspection of specific nodes flagged by the clip. Verification / Alternative check: Diagnose a stuck bus line: the logic clip quickly shows which lines are permanently HIGH/LOW; then the probe or scope can investigate causes like shorted outputs or missing enables. Why Other Options Are Wrong: frequency counter: measures frequency of a single signal; does not show many pins at once.oscilloscope: excellent for waveforms but not a multi-pin, at-a-glance indicator.current tracer: targets current paths, not logic level visualization. Common Pitfalls: Misaligning the clip across pins; using an incompatible clip for the package; forgetting ground reference; drawing conclusions without correlating with the circuit’s truth table. Final Answer: logic clip

Question 20

Troubleshooting flow: In a structured repair process, internal IC failures are most likely to be recognized during which phase?

Accepted Answer

Introduction / Context: Professional troubleshooting typically follows a staged workflow: verification of symptoms, diagnostic testing, isolation of the faulty block, component-level repair or replacement, and final test. Recognizing when internal IC faults become evident helps technicians choose the right tools and tests at the right time. Given Data / Assumptions: A standard, methodical troubleshooting process is used. Diagnostic activities include applying known stimuli, measuring expected responses, and comparing to specifications. Isolation further narrows the fault to a board area or subcircuit after diagnostic clues are gathered. Final test verifies complete functional recovery after repairs. Concept / Approach: Internal IC failures—like stuck outputs, excessive input leakage, or internal short/opens—are revealed when targeted diagnostic tests expose inconsistencies between expected and observed logic/function. The diagnostic phase is when you first perform these structured tests: loopback checks, stimulus-response tests (using logic pulsers), signature analysis, or boundary scans. While isolation deepens the localization, the “aha” that an IC is misbehaving typically originates during diagnostics. Step-by-Step Solution: Start with symptom verification to ensure the complaint reproduces.Perform diagnostic tests: check power rails, clocks, resets, and key signals against expected behavior.Note anomalies indicative of internal IC faults (e.g., output fighting, thermal hot spots, stuck-at levels).Proceed to isolation to pinpoint the specific IC or local net, then repair and retest. Verification / Alternative check: Use cross-substitution (swap a known-good IC), or consult datasheet truth tables during diagnostics; if the device fails the table consistently, the fault attribution occurs in that phase, reinforcing the answer. Why Other Options Are Wrong: isolation: narrows location but the initial recognition typically arises from diagnostics.repair: component replacement occurs after identification.final test: confirms the fix; not the discovery stage. Common Pitfalls: Skipping early diagnostic basics like power rail checks; assuming the first suspect IC is bad without corroborating tests; misattributing system-level timing issues to a single chip. Final Answer: diagnostic

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