Interview questions & answers
Q1. What is the bandwidth of an oscilloscope and how do you choose the right bandwidth for your measurement?
Oscilloscope bandwidth is the frequency at which the measured amplitude drops to -3 dB (70.7%) of the true amplitude — a 100 MHz oscilloscope starts attenuating signals above about 20–30 MHz. The rule of thumb is to use a scope with bandwidth at least 5× the highest frequency component of interest — for a 10 MHz digital clock, a 50 MHz scope gives reasonable amplitude accuracy but a 100 MHz scope is better. For digital signals, it is the rise time that determines bandwidth need: BW = 0.35/tr, so a signal with 10 ns rise time needs at least 35 MHz of scope bandwidth to avoid being visibly rounded.
Follow-up: What is the bandwidth of a scope probe and how does it interact with oscilloscope bandwidth?
Q2. What is the difference between analog and digital oscilloscopes?
An analog oscilloscope deflects an electron beam in real time using the signal being measured — it cannot capture single-shot events and the display brightness shows signal frequency of occurrence. A digital oscilloscope (DSO) samples the input using an ADC, stores the data, and reconstructs the waveform — it can capture single-shot transients, do post-processing (FFT, math, protocol decode), and store screenshots. A Tektronix TBS1202 DSO with 200 MHz bandwidth and 2 GS/s sample rate can capture a 50 ns glitch in a serial bus that an analog scope would completely miss, which is why analog scopes are rarely used in new designs.
Follow-up: What is the Nyquist criterion and how does it determine the minimum sample rate for a digital oscilloscope?
Q3. What is the difference between a 1:1 and 10:1 oscilloscope probe, and when do you use each?
A 1:1 probe passes the signal directly to the scope input with no attenuation — it has the full scope input impedance (typically 1 MΩ || 20 pF) loading the circuit. A 10:1 probe uses a 9 MΩ series resistor and compensation capacitor to present 10 MΩ input impedance and only 12–15 pF to the circuit, but reduces the signal by 10×. For measuring a 5 V logic signal on a PCB with high-impedance gates, a 10:1 probe is almost always used because the lower capacitance (12 pF vs 100 pF for a 1:1 at cable capacitance) avoids distorting high-frequency edges — 1:1 probes are only used when signal amplitude is very low (millivolts) and loading is acceptable.
Follow-up: How do you compensate a 10:1 probe and why is it important?
Q4. What is triggering in an oscilloscope and what are the common trigger modes?
Triggering synchronises the oscilloscope sweep start to a specific feature of the input signal, stabilising the display — without proper triggering, the waveform scrolls or appears blurred. The common trigger modes are: Edge (triggers on rising or falling edge crossing a set level — the default for most measurements), Pulse Width (triggers only if a pulse exceeds or falls within a width range — useful for glitch hunting), and Video (synchronises to composite video sync pulses). When debugging an intermittent I²C bus error on an STM32 microcontroller, setting an oscilloscope to trigger on SCL going low while SDA is low (I²C start condition) and filtering for pulses shorter than the expected clock period catches the corrupted transactions that appear perhaps once per minute.
Follow-up: What is hold-off time in oscilloscope triggering and when would you adjust it?
Q5. What does the sample rate (samples per second) of a digital oscilloscope mean, and what is the effect of undersampling?
Sample rate is the number of discrete voltage measurements the ADC makes per second — a 1 GS/s oscilloscope takes one sample every 1 ns. Undersampling (sampling below 2× the signal bandwidth — Nyquist criterion) causes aliasing: a 900 MHz signal sampled at 1 GS/s appears as a spurious 100 MHz alias frequency on the display. A Rigol DS1054Z at 1 GS/s can faithfully capture signals up to about 200–250 MHz in practice (accounting for anti-aliasing filter roll-off) despite its 50 MHz bandwidth rating limiting amplitude accuracy — the distinction between sample rate and bandwidth is a common exam topic.
Follow-up: What is equivalent time sampling (ETS) and how does it achieve apparent sample rates far beyond the ADC hardware speed?
Q6. What is the input impedance of a typical oscilloscope and why does it matter?
Standard oscilloscope input impedance is 1 MΩ in parallel with 20–30 pF — this is high enough for most circuit measurements but the 20–30 pF capacitance can seriously load high-frequency circuits. At 100 MHz, 20 pF has a reactance of only 80 Ω, which would load an RF circuit or high-speed digital buffer significantly, changing the signal being measured. For RF and microwave work, 50 Ω input-terminated scopes (or use of a SMA 50 Ω cable and scope input termination) are used to match transmission line impedance and avoid reflections that create false ringing on the waveform.
Follow-up: What problem arises when you use a 50 Ω terminated oscilloscope input to measure a DC-coupled 5 V signal?
Q7. What is the FFT function on an oscilloscope and what can it reveal that the time-domain display cannot?
The FFT (Fast Fourier Transform) function computes the frequency spectrum of the captured time-domain waveform, showing amplitude versus frequency — it reveals harmonic content, interference frequencies, and noise sources that are invisible in the time domain. A 415 V industrial motor drive measured with an oscilloscope FFT reveals switching harmonics at 5, 7, 11, and 13 times the fundamental, caused by the 6-pulse rectifier bridge, as sharp spikes at 250, 350, 550, and 650 Hz. The time-domain waveform shows distortion, but only the FFT identifies which harmonic orders are dominant and whether they exceed IEEE 519 limits.
Follow-up: What is the effect of windowing function choice (Hanning vs Rectangular) on FFT measurement accuracy?
Q8. How do you measure phase difference between two signals using an oscilloscope?
Phase difference is measured either by comparing zero-crossing points on the time axis (time difference method: φ = (Δt/T) × 360°) or by displaying both channels in X-Y mode and measuring the Lissajous pattern (phase = arcsin(Y-intercept/Y-max)). For a 1 kHz signal where the time difference between zero crossings is 100 µs, phase = (100 µs / 1 ms) × 360° = 36°. The X-Y Lissajous method is faster for fixed frequency but requires careful calibration of both channels at the same volts/div and same probe type to avoid introducing a false amplitude-related phase error.
Follow-up: How does the scope channel-to-channel skew affect phase measurements at high frequencies?
Q9. What is the difference between a mixed-signal oscilloscope (MSO) and a standard DSO?
An MSO (Mixed Signal Oscilloscope) combines analog oscilloscope channels with 8 or 16 digital logic channels on the same instrument, allowing simultaneous capture of analog waveforms and digital bus activity time-correlated to the same trigger event. A Keysight MSOX3054T combines 4 analog channels at 500 MHz with 16 digital channels, letting a firmware engineer trigger on a SPI chip-select edge and simultaneously see both the analog clock and data waveforms and the decoded byte values on the digital channels. For debugging an SPI peripheral, this eliminates the need to switch between an oscilloscope and a logic analyser.
Follow-up: What is the difference between an MSO and a logic analyser in terms of channel count and timing resolution?
Q10. What is a differential probe and why is it needed for high-side measurements?
A differential probe measures the voltage difference between two arbitrary nodes, neither of which needs to be at ground — it uses a high CMRR instrumentation amplifier at the probe tip to reject common-mode voltage up to hundreds of volts. Measuring the voltage across the high-side IGBT in a 3-phase inverter bridge requires a differential probe because the source terminal of the high-side device swings between 0 V and 600 V at switching frequency — connecting a standard probe here would short the inverter bus to ground through the scope chassis. A Tektronix TMDP0200 differential probe has ±1500 V differential range and 200 MHz bandwidth for such measurements.
Follow-up: What is CMRR of a differential probe and how does it affect measurement accuracy?
Q11. How is time base (sweep speed) calibrated on an oscilloscope and what errors arise from incorrect time base?
Time base accuracy on modern DSOs is typically ±50–100 ppm referenced to an internal crystal oscillator — a 100 ppm error on a 1 ms/div time base gives only 100 ns error per division, negligible for most measurements. Significant time base error arises in older analog scopes where the sweep circuit ages — a 2% time base error measuring a 50 Hz signal''s period (20 ms) gives a frequency reading of 49 or 51 Hz. For precision frequency measurement, the frequency counter function (triggered zero-crossing counter) is more accurate than reading off the time base, as it gates over thousands of cycles and averages.
Follow-up: How do you use an oscilloscope''s cursor measurement functions to improve time and amplitude measurement accuracy beyond the graticule?
Common misconceptions
Misconception: Higher sample rate always means better measurement, so a 2 GS/s scope is always better than a 500 MS/s scope.
Correct: Sample rate determines time resolution but the oscilloscope bandwidth limits frequency response — a 50 MHz bandwidth scope with 2 GS/s oversamples but still cannot accurately measure amplitudes above 50 MHz regardless of sample rate.
Misconception: A 10:1 probe reduces the oscilloscope's effective bandwidth by a factor of 10.
Correct: A 10:1 probe reduces signal amplitude by 10× but does not reduce bandwidth by the same factor — good 10:1 probes are designed to match or exceed the scope bandwidth; the bandwidth reduction is typically much less than 10×.
Misconception: Oscilloscope input coupling should always be set to DC for accurate measurements.
Correct: AC coupling removes the DC component and shows only the AC variation — it is preferred when measuring small AC ripple on a large DC supply, where DC coupling would use most of the vertical range for the DC component and compress the AC detail.
Misconception: Persistence mode on a digital oscilloscope works the same as the natural persistence of an analog CRT scope.
Correct: Digital persistence is a software-rendered display feature that retains old waveform data on screen with colour grading by hit frequency; analog CRT persistence is a physical property of the phosphor decay time — digital persistence can be set to infinite while analog persistence is limited by phosphor chemistry.