Question 1

Duty-cycle conversion at constant frequency using a 74121 one-shot Convert a 33 kHz waveform from 30% duty to 60% duty while keeping the frequency at 33 kHz. Choose suitable Rint and Cext.

Accepted Answer

Introduction / Context:Changing duty cycle at a fixed frequency can be achieved by pulse stretching (or re-timing) with a monostable. The 74121 is a non-retriggerable one-shot whose output width is set by its timing network. Here, we need to expand the high time to 60% while keeping the input frequency at 33 kHz.Given Data / Assumptions:Input frequency fin = 33 kHz → period T ≈ 1 / 33,000 ≈ 30.303 µsTarget duty = 60% → desired high time th ≈ 0.60 * 30.303 µs ≈ 18.18 µs74121 approximate timing: t_w ≈ K * Rint * Cext, where K is a family constant on the order of 0.7 (device- and family-dependent)Options provide discrete Rint and Cext choicesConcept / Approach:Select Rint and Cext such that t_w ≈ desired high time. Using a common approximation t_w ≈ 0.7 * Rint * Cext guides us to an R*C product near th / 0.7.Step-by-Step Solution:Compute target RC: RC_target ≈ th / 0.7 ≈ 18.18 µs / 0.7 ≈ 25.97 µsEvaluate options: (a) 2 kΩ * 0.012 nF = 24 ns → far too small(b) 2 kΩ * 0.012 µF = 24 µs → t_w ≈ 0.7 * 24 µs ≈ 16.8 µs (close to 18.18 µs)(c) 4 kΩ * 0.012 nF = 48 ns → far too small(d) 4 kΩ * 0.012 µF = 48 µs → t_w ≈ 0.7 * 48 µs ≈ 33.6 µs (too large)Verification / Alternative check:Accounting for component tolerances and the device's K variation, option (b) yields a practical pulse width near the target. Fine-tuning is typically done by slight value adjustments or trimming if required.Why Other Options Are Wrong:(a) and (c): RC products in the nanoseconds range cannot realize an ~18 µs pulse.(d): Produces a pulse too long (~33.6 µs), overshooting the 60% target.Common Pitfalls:Confusing 74121 timing constant with 555 values; misreading nF vs µF; forgetting frequency–period conversion when converting duty cycles.Final Answer:Rint = 2 kΩ, Cext = 0.012 µF

Question 2

Timing with 74123 retriggerable monostable — find the output pulse width (t_w). Given: R_ext = 4.7 kΩ, C_ext = 0.022 µF (22 nF). Select the closest value of t_w for a standard 74x123 device.

Accepted Answer

Introduction / Context:Monostable multivibrators (one-shots) such as the 74x123 produce a single output pulse whose width is set by an external resistor and capacitor. This question tests whether you know the proper timing constant for the 74123 family and can convert units correctly.Given Data / Assumptions:IC: 74123 (retriggerable monostable)R_ext = 4.7 kΩC_ext = 0.022 µF = 22 nFUse the typical 74123 timing relation t_w ≈ K * R_ext * C_ext, where K is about 0.32 to 0.45 depending on the specific logic family and operating conditions.Concept / Approach:For the 74123, many datasheets give a nominal constant near 0.32 for common trigger/configurations. Therefore compute RC first, then apply K ≈ 0.32 for a close estimate of pulse width.Step-by-Step Solution:RC = R_ext * C_ext = 4.7 kΩ * 22 nF = 4700 Ω * 22e-9 F = 103.4e-6 s = 103.4 µst_w ≈ K * RCUsing K ≈ 0.32: t_w ≈ 0.32 * 103.4 µs ≈ 33.1 µs (≈ 33.3 µs)Verification / Alternative check:If K were as high as 0.45, t_w would be ≈ 46.5 µs. Options provided include 33.3 µs and 46.5 µs; however, the most widely quoted nominal for the 74123 timing is close to 0.32 in many reference problems, so 33.3 µs is the best match to typical expectations.Why Other Options Are Wrong:290 ms and 29 ms: These are orders of magnitude larger than any K * RC with the given components.290 µs: Would require K ≈ 2.8, not applicable.46.5 µs: Uses K ≈ 0.45; while possible in some families, the common nominal answer for this setup is lower.Common Pitfalls:Confusing the 74123 constant with the 555 monostable constant (≈ 1.1).Unit mistakes when converting µF to F or kΩ to Ω.Final Answer:33.3 µs

Question 3

Monostable 555 timer — how many stable states does it inherently have? Choose the number of stable states for a one-shot (monostable) configuration.

Accepted Answer

Introduction / Context:A monostable multivibrator (one-shot) is designed to have a single stable state and one quasi-stable state. When triggered, it leaves its stable state temporarily and then returns, producing a pulse. This question checks conceptual understanding of the 555 in monostable mode.Given Data / Assumptions:Device: 555 timer in monostable configurationTrigger produces one timing pulseAfter timing, the circuit returns to its resting stateConcept / Approach:The defining property of a monostable is exactly one stable state. The other state exists only transiently (quasi-stable) during the output pulse interval set by R and C.Step-by-Step Solution:Identify the operating states: stable (rest), quasi-stable (timed pulse)Count stable states: there is only one stable stateVerification / Alternative check:Compare with astable (no stable state, constant oscillation) and bistable (two stable states, i.e., flip-flop). The monostable sits between them conceptually with exactly one stable state.Why Other Options Are Wrong:0: Describes astable oscillators, not monostables.2 or 3 or 4: Would imply flip-flop behavior (two) or multi-stable systems; not applicable.Common Pitfalls:Confusing monostable with astable or bistable due to naming similarity.Final Answer:1

Question 4

Applications: pulse stretching, time delay, and one-shot pulse generation — which multivibrator is best? Select the multivibrator type commonly used for these three tasks.

Accepted Answer

Introduction / Context:Different multivibrator types serve distinct timing roles. This question checks recognition of which topology naturally performs pulse stretching, time-delay creation, and single-pulse generation after a trigger.Given Data / Assumptions:Goal: extend a pulse, insert a delay, or generate a one-shot pulseAvailable choices: astable, monostable, multistable, bistable, Schmitt triggerConcept / Approach:A monostable multivibrator (one-shot) produces a single output pulse of controlled width in response to a trigger. By choosing R and C, you implement pulse stretching and time delays without continuous oscillation.Step-by-Step Solution:Identify functionality required: single, timed pulse after a triggerMap to topology: monostable multivibrator (one-shot)Verification / Alternative check:Astable oscillators run continuously, which is unnecessary for one-shot tasks. Bistables store state but do not time a pulse width. Schmitt trigger shapes edges but does not create timed widths by itself.Why Other Options Are Wrong:Astable: produces a free-running square wave.Multistable: generic term, not a standard timing choice here.Bistable: latches a state but needs extra timing to form a pulse.Schmitt trigger: adds hysteresis for clean switching, not timing width control.Common Pitfalls:Assuming any RC with a comparator equals a monostable; the one-shot topology matters.Final Answer:monostable

Question 5

Retriggerable vs non-retriggerable one-shot — what is the key difference? Choose the statement that correctly distinguishes how the output pulse behaves with successive triggers.

Accepted Answer

Introduction / Context:One-shot multivibrators are either retriggerable or non-retriggerable. Understanding how additional triggers affect the timing pulse is crucial in pulse-width control and glitch filtering.Given Data / Assumptions:We compare output behavior under multiple triggers arriving before the pulse expires.One device is retriggerable; the other is not.Concept / Approach:In a retriggerable one-shot, any valid trigger that occurs during the active pulse restarts the timing interval, effectively lengthening (stretching) the total pulse width. In a non-retriggerable one-shot, additional triggers received while the pulse is active are ignored until the device returns to its stable state.Step-by-Step Solution:Consider a pulse that lasts T seconds.Apply a second trigger at t = T/2.Retriggerable: timing restarts, so new end time ≈ t + T, total width > T.Non-retriggerable: timing continues unchanged, total width = T.Verification / Alternative check:Datasheets typically note "retriggering extends the output pulse" for devices like 74x123, contrasting with 74x121 (non-retriggerable).Why Other Options Are Wrong:Nonretriggerable can only be triggered once: false; it can be triggered again after it times out.Retriggerable can be triggered many times: true but incomplete; the distinguishing effect is pulse extension.Nonretriggerable stretches the pulse: incorrect by definition.Successive triggers shorten the pulse: opposite of retrigger behavior.Common Pitfalls:Confusing "ignoring triggers while busy" with "cannot be triggered again ever".Final Answer:The output pulse can be stretched with a retriggerable.

Question 6

Retriggerable monostable multivibrator — identify what is NOT an inherent characteristic. Select the option that does not generally define retriggerable monostables across families.

Accepted Answer

Introduction / Context:This item checks whether you can distinguish universal characteristics of retriggerable monostables from package- or family-specific features. The focus is on traits that are always true vs traits that may be true only for particular ICs.Given Data / Assumptions:We consider behavior and construction across common logic families (TTL/CMOS).Terms like "dual" can describe a package option, not a fundamental property.Concept / Approach:Retriggerability describes timing behavior: new triggers extend the active pulse. Whether an IC is "dual" (two monostables per package) or uses a particular reset polarity varies by part number and family. External timing components are also typical and not unique to retriggerable devices.Step-by-Step Reasoning:Identify core behavior: retriggering extends the output pulse.Check packaging: "dual" refers to two one-shots in one IC (e.g., certain 74x123 variants), not an inherent property.Reset polarity and internal components vary by manufacturer and subfamily.Verification / Alternative check:Some retriggerable parts are single (one per package), others are dual. Therefore "dual" is not universally characteristic.Why Other Options Are Wrong:Active-HIGH reset: many parts provide a reset; polarity can vary, but a reset that halts timing is characteristic behavior broadly.No internal timing resistor: timing typically uses external R and C in both retriggerable and non-retriggerable families.Extends the pulse if retriggered: this is the very definition of retriggerable behavior.none of the above: incorrect because at least one option (dual) is not inherent.Common Pitfalls:Assuming a datasheet feature of a specific IC is universal to all retriggerables.Final Answer:It is a dual multivibrator.

Question 7

Monostable multivibrator at rest — what is the typical Q output level before any trigger? Assume standard TTL/CMOS one-shot behavior where the output goes active for a timed pulse and then returns.

Accepted Answer

Introduction / Context:A monostable multivibrator has one stable state. When idle (no trigger), the output sits in that state; a trigger forces a temporary transition for a timed duration. This question checks default logic level knowledge for common one-shot outputs.Given Data / Assumptions:Standard one-shot where the timed pulse is a HIGH pulse at Q.After timing, the device returns to its stable state.Concept / Approach:Most textbook one-shots are depicted with Q LOW at rest and a positive-going pulse on trigger. Although some devices offer inverted outputs or alternative wiring, the conventional Q output idles LOW and pulses HIGH.Step-by-Step Solution:Identify the polarity of the pulse at Q: typically a HIGH pulse.Therefore, before trigger, Q must be LOW to allow a positive-going pulse.Verification / Alternative check:Diagrams for 555 monostable and 74x121/123 families commonly show Q LOW at rest, going HIGH for t = K * R * C, then returning LOW.Why Other Options Are Wrong:+5 V or HIGH: that would imply a LOW-going pulse, not the typical default.SET: descriptive but not a logic level.Hi-Z: outputs are driven, not tri-stated, in basic one-shots.Common Pitfalls:Confusing Q with /Q (inverted output), which would idle HIGH.Final Answer:LOW

Question 8

Astable multivibrator behavior — which statement best describes its output? Choose the accurate description of an astable circuit's output state over time.

Accepted Answer

Introduction / Context:An astable multivibrator has no stable state. It continuously oscillates between two levels, generating a periodic waveform without requiring external triggers. This question verifies recognition of astable behavior compared to mono- and bi-stable types.Given Data / Assumptions:Astable configuration using active components and RC feedbackFree-running oscillator behavior expectedConcept / Approach:By design, astables perpetually charge and discharge timing capacitors, alternately driving the output HIGH and LOW. No trigger input is required to sustain oscillation, unlike monostables and bistables.Step-by-Step Reasoning:Identify presence of any stable resting state: none in an astable.Result: output keeps toggling between logic levels with a period set by R and C.Verification / Alternative check:555 in astable mode or cross-coupled inverters with RC produce a continuous square wave, confirming the description.Why Other Options Are Wrong:LOW until triggered / HIGH until triggered: describes monostable behavior.Floats until triggered: outputs are not left floating in proper designs.Stays latched unless reset: describes bistable flip-flops.Common Pitfalls:Confusing "needs power-up to start" with "needs a trigger"; many astables self-start due to component tolerances.Final Answer:constantly switches between two states

Question 9

Charging behavior of an RC network — which statements are true as a capacitor charges from a DC source? Assume a simple series R–C connected to a constant DC supply and select the best overall answer.

Accepted Answer

Introduction / Context:Classic RC charging exhibits exponential behavior for both capacitor voltage and loop current. The time constant tau = R * C sets the rate. After about 5 * tau, the capacitor is essentially fully charged for practical purposes. This question checks fundamental time-constant intuition.Given Data / Assumptions:Series R–C driven by an ideal DC sourceInitial capacitor voltage is zeroNo leakage and constant RConcept / Approach:Standard equations for charging are: Vc(t) = V_s * (1 − e^(−t/tau)) and I(t) = (V_s/R) * e^(−t/tau). Engineers often use 5 * tau as the practical "full-charge" time where Vc ≈ 99%.Step-by-Step Solution:tau = R * CVc(t) increases exponentially toward V_sI(t) decreases exponentially toward 0t ≈ 5 * tau gives Vc ≈ 0.993 * V_s, close enough to "charged"Verification / Alternative check:Plotting Vc(t) and I(t) vs time confirms the exponential rise and decay and the 5 * tau rule of thumb.Why Other Options Are Wrong:Options a, b, c are all correct statements; thus "all of the above" is the best choice."None of the above" contradicts established RC theory.Common Pitfalls:Assuming charging is linear; it is exponential.Equating 5 * tau with an exact limit; it is a practical guideline.Final Answer:all of the above

Question 10

One-shot (monostable) timing dependencies — which change increases the output pulse width? Assume a standard one-shot whose width t is set by an external R * C network.

Accepted Answer

Introduction / Context:Monostable pulse width is primarily determined by the product R * C and, secondarily, by internal threshold fractions. Understanding which external parameter directly increases the width is essential in practical design and troubleshooting.Given Data / Assumptions:One-shot with t ≈ K * R * C, where K is a device-dependent constantSupply-voltage dependence is weak over the rated rangeConcept / Approach:If t is proportional to R * C, then increasing C (or R) increases t, while decreasing either decreases t. Changes to UTP or edges typically do not dominate compared to R and C selection for basic one-shots.Step-by-Step Solution:t ≈ K * R * CIf C ↑ then R * C ↑ ⇒ t ↑If R ↓ then R * C ↓ ⇒ t ↓Verification / Alternative check:Datasheet curves generally show near-linear scaling of pulse width with timing capacitance across practical ranges.Why Other Options Are Wrong:Supply voltage increases: often minimal effect on t within spec.Timing resistor decreases: reduces t.UTP decreases: threshold variations have smaller, device-specific effects.Faster trigger edge: affects triggering reliability, not width set by R and C.Common Pitfalls:Overestimating supply sensitivity for timing; R and C dominate.Final Answer:timing capacitance increases

Question 11

Terminology: another common name for a bistable multivibrator. Select the widely used digital design term that refers to a bistable.

Accepted Answer

Introduction / Context:A bistable multivibrator has two stable states and stores one bit of information. In digital electronics, it is commonly implemented and discussed under a more familiar name. This question checks your vocabulary alignment with standard digital design terminology.Given Data / Assumptions:Two stable output statesExternal inputs can set, reset, or toggle the stateConcept / Approach:The ubiquitous term for a bistable multivibrator in digital systems is "flip-flop". It can be edge- or level-controlled and is the building block of registers, counters, and state machines.Step-by-Step Reasoning:Identify behavior: two stable states ⇒ bistable storage element.Map to digital term: flip-flop (or latch, depending on control), but the most general and common name in synchronous logic is flip-flop.Verification / Alternative check:Textbooks equate the bistable multivibrator with flip-flops (SR, JK, D, T) used in sequential logic.Why Other Options Are Wrong:On-off switch: a mechanical analogy, not a precise circuit type.Oscillator: describes astable behavior.Latch with analog feedback: imprecise and not a standard term here.Schmitt trigger: a comparator with hysteresis, not a memory element.Common Pitfalls:Confusing latches and flip-flops; both are bistables, but "flip-flop" is the canonical term.Final Answer:a flip-flop

Question 12

RC discharge in basic electronics A 0.3 µF capacitor is initially at 2.7 V. It is discharged through a 10 kΩ resistor for 6.5 ms. What voltage remains across the capacitor after this time?

Accepted Answer

Introduction / Context:Capacitors in RC circuits discharge exponentially. This question tests your ability to apply the standard RC decay formula to find the remaining voltage after a given time constant interval.Given Data / Assumptions:Capacitance C = 0.3 µF = 0.3 * 10^-6 F.Resistance R = 10 kΩ = 10,000 Ω.Initial voltage V0 = 2.7 V.Elapsed time t = 6.5 ms = 6.5 * 10^-3 s.Concept / Approach:The capacitor voltage during discharge follows V(t) = V0 * exp( - t / (R * C) ). The product R * C is the time constant tau, which sets the exponential decay rate.Step-by-Step Solution:1) Compute tau: tau = R * C = 10,000 * 0.3 * 10^-6 = 3 * 10^-3 s = 3 ms.2) Form the exponent: t / tau = 6.5 ms / 3 ms ≈ 2.1667.3) Evaluate the exponential: exp( -2.1667 ) ≈ 0.1145.4) Remaining voltage: V(t) = 2.7 * 0.1145 ≈ 0.30915 V ≈ 0.309 V.Verification / Alternative check:Since 6.5 ms is a little over two time constants (2 tau = 6 ms), a remaining fraction near exp( -2 ) ≈ 0.135 is expected; our more precise 0.1145 is reasonable because 6.5 ms > 6 ms.Why Other Options Are Wrong:0.0 V: Ideal exponential never reaches exactly zero at finite time.2.7 V: That is the initial voltage; no discharge applied.2.295 V: Implies very little decay; contradicts t > 2 tau.Common Pitfalls:Mixing up units (µF, kΩ, ms), using linear instead of exponential decay, or forgetting to compute tau first.Final Answer:0.309 V

Question 13

Multivibrator taxonomy — Evaluate the claim: “The bistable multivibrator is an R–C flip-flop.” Distinguish from monostable and astable types.

Accepted Answer

Introduction / Context: Multivibrators are classified as astable (oscillator), monostable (one-shot), and bistable (flip-flop). Only the astable and monostable rely inherently on R–C timing networks to create a time constant. A bistable has two stable states and no inherent timing component; it remains in a state until triggered to change.Given Data / Assumptions:Bistable multivibrator = flip-flop with two stable equilibria.Astable and monostable use R–C to generate timing (period or pulse width).Technology may be discrete, TTL, or CMOS; definitions do not change.Concept / Approach: The hallmark of an R–C-based multivibrator is a timing interval determined by the product R*C (e.g., T ≈ 0.693 * R * C in common configurations). Bistables do not require such timing; they change state only upon an input trigger and then remain indefinitely. Therefore, calling a bistable an “R–C flip-flop” is inaccurate; it is a cross-coupled latch/flip-flop, not a timing circuit.Step-by-Step Solution:Identify the function: bistable stores a binary state.Note absence of timing capacitor or charging path determining durations.Contrast with monostable/astable which depend on R–C time constants.Conclude statement is incorrect.Verification / Alternative check:Datasheets for SR, JK, D flip-flops show no R–C timing in the core storage element.Why Other Options Are Wrong:Correct: Would mischaracterize the role of R–C networks.True only in discrete transistor designs / True for CMOS not TTL: Technology does not change the defining behavior.Common Pitfalls:Assuming any multivibrator involves R–C timing; only astable/monostable do.Confusing timing add-ons (like reset delays) with core bistable operation.Final Answer:Incorrect

Question 14

Relaxation oscillator with hysteresis — Evaluate: “A single Schmitt trigger inverter plus an R–C network is sufficient to build a simple astable multivibrator.”

Accepted Answer

Introduction / Context: A Schmitt trigger inverter has two distinct thresholds and therefore naturally supports a relaxation oscillator when combined with a resistor and capacitor. This is a textbook way to produce a square wave with minimal parts for clocks, LED blinkers, and simple timing signals.Given Data / Assumptions:Device is a Schmitt-trigger CMOS/TTL inverter.An R feeds the input node from the output; a C from input node to ground forms the timing network.Supply voltage is within the device’s operating range.Concept / Approach: The capacitor charges through R until the inverter input crosses V_T+; the output toggles LOW, causing the capacitor to discharge through R until the input crosses V_T−; the output toggles HIGH. The cycle repeats indefinitely, producing an oscillation whose period depends on R, C, and the hysteresis thresholds.Step-by-Step Solution:Wire inverter output to input through R; connect C from input to ground.On power-up, C charges toward the current output level through R.When V_in reaches V_T+, the inverter flips; C then discharges toward the opposite rail.Oscillation period approximates T ≈ k * R * C, where k depends on V_T+ and V_T−.Verification / Alternative check:Scope captures show a triangular waveform at the input node and a square wave at the output, confirming astable behavior.Why Other Options Are Wrong:Incorrect: Schmitt hysteresis specifically enables single-inverter oscillation.Only works with two cascaded inverters: A single Schmitt device suffices; two non-Schmitt inverters can also oscillate but need different biasing.True only at 12 V: Works across valid supply ranges; frequency varies with thresholds and supply.Common Pitfalls:Using a non-Schmitt inverter; without hysteresis the circuit may not start or may jitter.Choosing R or C values that violate input leakage or output drive limits.Final Answer:Correct

Question 15

Nonretriggerable one-shot behavior — Evaluate: “In the 74121 (nonretriggerable), any triggers arriving before the end of the timing interval are ignored.”

Accepted Answer

Introduction / Context: Monostable multivibrators (one-shots) generate a single pulse of predetermined width in response to a trigger. The 74121 is specifically a nonretriggerable one-shot, meaning that once the timing cycle begins, further triggers do not extend or restart the pulse until the interval completes.Given Data / Assumptions:Device type: 74121 nonretriggerable monostable.The pulse width is determined by external R and C.Triggers may be negative- or positive-going depending on the selected input.Concept / Approach: In a nonretriggerable design, the internal gating disables the trigger path while timing is active. Therefore, new triggers are effectively ignored until the internal timing node resets, after which the device is ready for the next trigger. This prevents pulse stretching and ensures a constant pulse width independent of incoming trigger bursts.Step-by-Step Solution:Apply an initial trigger; output goes active for the programmed time T.During 0 After t ≥ T and internal reset, the device accepts the next valid trigger.Verification / Alternative check:Compare with 74123 (retriggerable), where triggers extend the pulse; 74121 does not extend.Why Other Options Are Wrong:Incorrect: Conflicts with the nonretriggerable definition.True only for positive-edge triggers / Only valid at room temperature: Behavior is architectural, not edge or temperature specific (within ratings).Common Pitfalls:Expecting pulse stretching on 74121; choose 74123 if extension is required.Miswiring the qualifying inputs that define active trigger polarity.Final Answer:Correct

Question 16

Precision timing sources — Evaluate: “When extremely critical timing accuracy and stability are required, a quartz crystal can be used as the reference.”

Accepted Answer

Introduction / Context: Many timing circuits rely on R–C networks or simple oscillators, which can drift with temperature, voltage, and aging. Quartz crystal resonators offer orders-of-magnitude better frequency stability and accuracy, making them a standard choice when critical timing is needed in communication, measurement, and control systems.Given Data / Assumptions:“Extremely critical” means low ppm-level drift and tight tolerance.Crystal oscillators can be simple Pierce oscillators or packaged XO/TCXO/OCXO modules.Environmental conditions are within device specifications.Concept / Approach: Quartz exhibits a strong piezoelectric resonance with high Q, leading to a sharply defined oscillation frequency. Compared to R–C oscillators, crystals reduce frequency tolerance from percent-level to ppm-level. Temperature-compensated (TCXO) and oven-controlled (OCXO) variants further minimize drift, enabling very stable clocks for microcontrollers, radios, and instruments.Step-by-Step Solution:Select a crystal frequency appropriate for the application (e.g., 16 MHz for MCU clocks).Choose XO/TCXO/OCXO depending on stability requirements and power budget.Design the oscillator or use a clock module; respect load capacitance specifications.Distribute the clock carefully to preserve jitter performance.Verification / Alternative check:Bench measurements of long-term drift and temperature sweep show crystals outperform R–C oscillators by large margins.Why Other Options Are Wrong:Incorrect: Ignores well-known advantages of quartz stability.Only true above 10 MHz: Crystals provide excellent stability across a wide frequency range, including kHz bands (e.g., 32.768 kHz).True only if temperature is fixed: TCXO/OCXO mitigate temperature effects significantly.Common Pitfalls:Ignoring crystal load capacitance; improper loading shifts frequency.Assuming crystals are jitter-free; power supply noise and layout matter.Final Answer:Correct

Question 17

Comparator basics — Evaluate: “A comparator outputs a logic HIGH or LOW strictly based on which input voltage is greater.”

Accepted Answer

Introduction / Context: Comparators are threshold devices that decide which of two analog voltages is larger. They form the basis of zero-crossing detectors, window comparators, and many ADC architectures. The essence is binary: one of two logic states is produced according to the sign of V_plus − V_minus.Given Data / Assumptions:Comparator has two inputs, often labeled non-inverting (+) and inverting (−).Outputs saturate near logic rails or defined logic levels via a separate interface.No intentional hysteresis unless added (some parts integrate it).Concept / Approach: The comparator evaluates the instantaneous difference between inputs. If V_plus > V_minus, output drives HIGH; if V_plus Step-by-Step Solution:Define reference voltage on one input; apply measured signal to the other.When measured signal crosses the reference, the output toggles quickly.Use pull-up or push-pull output stage to interface with logic as required.Verification / Alternative check:Oscilloscope plots show a clean digital transition at the threshold crossing.Why Other Options Are Wrong:Incorrect: Confuses comparator behavior with linear amplification; comparators are decision-making devices.Only true for differential amplifiers: A comparator is a specific form of differential decision element; statement applies generally.True only with positive feedback: Hysteresis improves noise immunity but is not required for basic comparison.Common Pitfalls:Feeding signals outside input common-mode range; outputs then misbehave.Expecting linear output; comparators are not op-amps in linear mode.Final Answer:Correct

Question 18

Retriggerable one-shot behavior — Evaluate: “When a 74123 (retriggerable monostable) finishes its timing cycle, it must be started all over again.” Consider what happens if triggers arrive during the pulse.

Accepted Answer

Introduction / Context: The 74123 is a retriggerable monostable multivibrator (one-shot). Its defining feature is that an additional trigger received while the output pulse is active will extend the timing interval. This behavior contrasts with a nonretriggerable device, which ignores triggers until the interval ends.Given Data / Assumptions:Device type: 74123 retriggerable one-shot with R–C timing.New triggers can occur any time, including during the active pulse.Pulse width nominally T = k * R * C, where k depends on device specifics.Concept / Approach: In a retriggerable one-shot, a valid trigger resets the internal timing capacitor near the start of the cycle, effectively adding time from the trigger instant. If triggers are spaced closely, the output can remain asserted for a long period, because each new trigger extends the pulse. Therefore, the claim that it “must be started all over again” after finishing is misleading: it need not finish before being re-triggered; it can be extended mid-cycle.Step-by-Step Solution:Apply a trigger at t = 0; output goes HIGH for duration T.Apply another trigger at t = T/2; the device extends the pulse roughly to t = T + T/2 (depending on internal reset timing).Continue periodic triggers; output stays HIGH so long as retriggers arrive before the end of the current interval.If no trigger occurs and t reaches T, output returns LOW and the device awaits the next trigger.Verification / Alternative check:Timing diagrams in references show staircase-like extensions of the HIGH interval with each retrigger.Why Other Options Are Wrong:Correct: Implies the device cannot be retriggered mid-pulse, contradicting its core feature.Only true for negative-edge triggers: Polarity selection does not change retriggerable behavior.True if R–C values are small: R–C values set T but do not change the retriggerable nature.Common Pitfalls:Confusing 74121 (nonretriggerable) with 74123 (retriggerable).Forgetting to meet input qualification and pulse-width requirements for valid triggers.Final Answer:Incorrect

Question 19

Quartz-crystal timing accuracy claim: “In practical electronic timing circuits, achieving more than eight significant digits of timing accuracy can be easily accomplished by using quartz crystals.” Decide whether this statement is valid, considering…

Accepted Answer

Introduction / Context:Quartz crystal oscillators are the workhorses of timing in digital electronics, used in microcontrollers, communication systems, and test equipment. The statement claims that “more than eight significant digits” of timing accuracy is easily achieved with crystals. This question probes understanding of real-world oscillator accuracy versus stability terms like ppm (parts per million) and ppb (parts per billion).Given Data / Assumptions:“More than eight significant digits” implies relative accuracy better than 1 part in 10^8 (≈ 0.01 ppm).Typical crystal oscillators: XO, TCXO, OCXO; ambient conditions vary.“Easily achieved” means without specialized disciplining (e.g., GPS, atomic reference) or extensive calibration.Concept / Approach:Uncompensated crystal oscillators (XO) are commonly 10–50 ppm. Temperature-compensated (TCXO) units are around 0.5–2 ppm, while oven-controlled (OCXO) may reach 0.01–0.1 ppm over short intervals, but long-term aging and environmental factors worsen absolute accuracy. Achieving Step-by-Step Solution:Relate significant digits to ppm: 8 significant digits ⇒ 1e-8 ≈ 0.01 ppm.Compare to typical XOs: 10–50 ppm ⇒ 1e-5 to 5e-5 (far worse).Compare to TCXO: ~0.5–2 ppm ⇒ 5e-7 to 2e-6 (still worse).Compare to high-grade OCXO: best-case short-term ~1e-9 to 1e-10 Allan deviation, but absolute accuracy without disciplining rarely guarantees Verification / Alternative check:Datasheets list frequency tolerance at 25°C, stability over temperature, load pulling, aging per day/year. Summing these error sources shows practical absolute accuracy typically well above 1e-8 unless the crystal is disciplined to a higher standard (e.g., GPSDO) or periodically calibrated against an atomic reference.Why Other Options Are Wrong:Correct: Overstates crystal capability; not generally true.Only true with atomic disciplining (GPSDO/OCXO): This acknowledges that extra infrastructure is needed; the original claim said “easily” with crystals alone.Only true at room temperature: Even then, manufacturing tolerance and aging prevent 1e-8 “easy” accuracy.Common Pitfalls:Confusing stability (short-term fluctuation) with absolute accuracy; ignoring aging and load capacitance; assuming OCXO guarantees 1e-8 accuracy without calibration.Final Answer:Incorrect

Question 20

74S124 device characterization: “The 74S124 TTL integrated circuit is a voltage-controlled oscillator (VCO) that generates a specific output frequency at Vout.” Evaluate this statement for accuracy.

Accepted Answer

Introduction / Context:Members of the classic 74-series logic family include specialized timing parts. The 74S124 is known historically as a Schottky-TTL voltage-controlled oscillator. Understanding what this IC does helps distinguish it from monostables like the 74121/74123 and general-purpose logic gates.Given Data / Assumptions:Device: 74S124 (Schottky TTL).Function: frequency generation with voltage control plus external timing components.Output node labeled Vout delivering a logic-level oscillation.Concept / Approach:A VCO outputs a periodic signal whose frequency is a function of a control voltage and timing components (R, C). The 74S124 embodies this behavior: varying the control voltage changes the charge/discharge rate internally, thus changing output frequency. This contrasts with retriggerable monostables (pulse generators) that produce single shots, not continuous oscillations unless externally re-triggered.Step-by-Step Solution:Identify the behavior: continuous oscillation present at Vout.Relate oscillation frequency to control voltage and RC network.Confirm that the device is not merely a one-shot; its intended mode is VCO.Conclude the statement accurately describes the 74S124.Verification / Alternative check:Reference timing IC catalogs show the 74S124 as a dual VCO (or VCO block) with logic-level outputs and specified frequency versus control voltage characteristics. Application notes illustrate frequency modulation and clock generation uses.Why Other Options Are Wrong:Incorrect: Conflicts with standard descriptions of 74S124.Only correct for the 74124: 74124 is a different part (monostable family); 74S124 specifically denotes the Schottky VCO.Only correct if used as a monostable: A monostable is edge-triggered pulse generation, not VCO behavior.Common Pitfalls:Confusing close part numbers (74121/74123/74124) with 74S124; assuming all “74xx124” are monostables; overlooking that the “S” indicates Schottky TTL variant.Final Answer:Correct

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