The oscilloscope
The instrument that shows the shape of a signal, not just its value. A multimeter says “5 V”; the oscilloscope tells you whether those 5 V are clean DC, a sine wave, a square wave with overshoot, or DC with 200 mV of hum riding on top.
01What the screen shows
An oscilloscope draws a graph of voltage versus time. The vertical axis is voltage and the horizontal axis is time; the screen is divided into a grid of divisions (typically 8 vertical × 10 horizontal), and two knobs set what each division is worth:
- VOLTS/DIV — how many volts each vertical division represents.
- TIME/DIV (or SEC/DIV) — how much time each horizontal division represents.
Every oscilloscope measurement comes down to counting divisions and multiplying.
02The cathode-ray tube
The analog oscilloscope is based on a cathode-ray tube (CRT). Although new instruments today are digital with an LCD screen, understanding the CRT explains the origin of all the controls that are still around.
- Electron gun. A cathode heated by a filament emits electrons. The control grid (G) regulates how many get through: this is the INTENSITY control. The focus anodes concentrate the beam into a point: this is the FOCUS control.
- Y plates (vertical). They receive the amplified signal. They deflect the beam up or down in proportion to the voltage.
- X plates (horizontal). They receive the timebase, which sweeps the beam from left to right at constant speed.
- Screen. Coated with phosphor, it glows where the beam hits and keeps that light for a moment (persistence), which lets you see a continuous curve instead of a single dot.
03The timebase
If a ramp that rises linearly and drops back abruptly (a sawtooth) is applied to the X plates, the beam travels across the screen from left to right at constant speed and returns instantly. This sweep turns the horizontal axis into a time axis.
If another signal is applied to the X plates instead of the timebase, the screen plots one signal against the other. This is useful for viewing the characteristic curve of a diode (voltage versus current) or for Lissajous figures, which are used to compare two frequencies: if the ratio is exact, the figure stands still.
04Synchronization: the trigger
This is the most important concept of the oscilloscope and the one that causes the most problems. If each sweep started at an arbitrary moment of the signal, each trace would be drawn shifted relative to the previous one and the screen would be a smear. The trigger (trigger circuit) makes every sweep start at the same point of the wave, so the picture stays still.
🔬 Instrument simulator A two-channel analog oscilloscope and a function generator, with the front-panel knobs working. You can move the trigger level until you lose it and watch the image drift away, which is exactly what this section explains. ›| Control | What it does | How to use it |
|---|---|---|
| LEVEL | Voltage at which the sweep is triggered. | It must lie within the range of the signal. If it goes beyond the peak, it never triggers. |
| SLOPE | Whether it triggers on a rising (↑) or falling (↓) crossing. | Changes the point where the trace starts. Useful for viewing the edge you care about. |
| SOURCE | Which channel it takes its reference from: CH1, CH2, EXT or LINE. | When comparing two signals, always trigger on the same one: otherwise the measured phase shift is false. |
| MODE · AUTO | If there is no trigger, it sweeps anyway. | Default mode: you always see something, even if it is just a line. This is the one to start with. |
| MODE · NORMAL | Sweeps only when there is a valid trigger. | Blank screen if it does not trigger. Better for slow or sporadic signals. |
| MODE · SINGLE | Triggers only once and freezes. | For capturing a one-time event: a motor starting, a switching spike. |
| HOLDOFF | Dead time after each trigger. | Stabilizes complex signals that cross the level several times within a single cycle. |
- Trigger mode on AUTO, source set to the channel being used.
- Channel coupling on DC and vertical position at the center.
- VOLTS/DIV at a large value (5 V/div) and TIME/DIV at a middle value (1 ms/div).
- Check that the channel is turned on and the probe is properly connected, with the ground clip on the correct point.
- Only then adjust the trigger level and fine-tune the scales.
05Coupling and probes
Input coupling
Passes everything: DC and AC. It is the default mode and the one that shows the signal as it really is.
Inserts a capacitor: it blocks DC and leaves only the variation. Useful for viewing the ripple of a 12 V power supply at a sensitivity of 50 mV/div.
Disconnects the input and ties it to ground. It is used to mark where zero is on the screen before measuring.
Probes
| Probe | Attenuation | Input Z | When to use it |
|---|---|---|---|
| ×1 | None | 1 MΩ · 100 pF | Small, low-frequency signals. It loads the circuit more. |
| ×10 | Divides by 10 | 10 MΩ · 15 pF | The normal working position. It loads the circuit much less and reaches a higher bandwidth. |
- Tell the oscilloscope. If the probe is on ×10 and the instrument thinks it is ×1, all voltage readings come out 10 times too low. On digital scopes this is set in a menu; on analog ones you have to multiply in your head.
- Compensate it. With the tip on the oscilloscope's own square-wave calibration output, adjust the probe's trimmer screw until the square wave has square corners. If they look rounded or show overshoot, the probe is out of compensation and distorts every fast signal.
The probe ground is connected to the oscilloscope ground and, through the power cord, to the third conductor of the installation (the earth wire). Consequences:
- Connecting the ground clip to a point that is not ground shorts that point to earth. In a circuit powered directly from the mains without an isolation transformer, that can destroy the equipment or the oscilloscope, or be dangerous for the operator.
- With two channels, the two ground clips are tied together. You cannot measure two points with different grounds at the same time.
- To measure floating signals, use an isolation transformer or a differential probe. Never cut off the earth pin of the oscilloscope's plug.
06What is measured and how
| Quantity | How it is obtained |
|---|---|
| Peak-to-peak voltage | Divisions between the maximum and the minimum × VOLTS/DIV. |
| Peak voltage | Half of the previous one, if the signal is symmetrical. |
| RMS value (sine wave) | VRMS = Vp / √2 = 0.707 · Vp. Only for a pure sine wave. |
| DC component | Set the channel to GND to mark zero, switch to DC and measure how far the trace shifted. |
| Period and frequency | T = divisions of one cycle × TIME/DIV. Then f = 1/T. |
| Rise time | Time between 10% and 90% of the edge amplitude. |
| Duty cycle | (time high / period) × 100%. |
| Phase shift | With two channels: measure Δt between the zero crossings and apply the formula below. |
With VOLTS/DIV = 2 V (probe on ×10, correctly configured) and TIME/DIV = 20 µs, the signal spans 3 divisions peak to peak and one cycle measures 5 divisions.
- Vpp = 3 × 2 V = 6 V → Vp = 3 V
- VRMS = 3 / 1.414 = 2.12 V (if it is a sine wave)
- T = 5 × 20 µs = 100 µs → f = 1/100 µs = 10 kHz
If the probe were on ×10 and the instrument set to ×1, the reading would have been 0.6 V: an error of one order of magnitude, and the most frequent mistake when starting out.
Instrument bandwidth
07Analog and digital
A digital oscilloscope (DSO) does not deflect a beam: it samples the signal with an A/D converter, stores the samples in memory and draws them on an LCD screen. That changes everything for the better, except for one detail you need to know about.
- Freezes the image and lets you save or export it.
- Pretrigger: shows what happened before the trigger. Impossible in analog.
- Automatic measurements (Vpp, frequency, RMS, duty cycle).
- Cursors, math between channels, FFT.
- Captures one-time events with SINGLE.
- Aliasing. If the sampling rate is insufficient, the screen shows a signal of a false frequency, a lower one, that looks perfectly real.
- With few samples per cycle, the shape looks angular even if the signal is a sine wave.
- Very fast, isolated events (glitches) can fall between two samples and not show up.
To reconstruct a signal you must sample it at more than twice its maximum frequency (Nyquist). In practice oscilloscopes use 5 to 10 samples per cycle. If the screen shows a suspiciously slow signal, speed up the timebase: if the displayed frequency changes, it was aliasing. This theorem reappears in Telecommunications II and in the A/D conversion topic of Year 5.
08Measurements in amplifier circuits
This is the direct application of the instrument in this subject, on the circuits of the small-signal amplifiers topic.
| What you look for | How | What it indicates |
|---|---|---|
| Gain | CH1 at the input, CH2 at the output. Av = Vout(pp) / Vin(pp). | Compare it with the calculated value. If it differs a lot, check CE and the load. |
| Phase inversion | Both channels at once. | In a common-emitter stage the output is at 180°. If it is not, the stage is not what you think it is. |
| Clipping and Q-point | Raise the input until the output flattens. | In a common-emitter stage, if it clips at the top first, the Q-point has shifted toward cutoff; if it clips at the bottom, toward saturation. |
| Power supply ripple | Channel on AC coupling, 50 mV/div, across the filter capacitor. | You see the sawtooth at 100 Hz and measure its peak-to-peak amplitude. |
| Distortion | Compare the shape of the output with a sine wave. | Asymmetries, flattening or the step of crossover distortion in power stages. |
| Frequency response | Fixed input amplitude, sweep the frequency. | You find the cutoff frequencies (where the output drops to 0.707 of its maximum, that is, −3 dB). |
| Parasitic oscillations | Fast timebase (1 µs/div) with no input signal. | If a signal of hundreds of kHz appears, the amplifier is oscillating: decoupling is missing. |
09In the lab
Connect the probe to the calibration terminal (1 kHz, 2 Vpp on most instruments). Adjust intensity, focus, position and scales until you see two or three complete cycles. Check that the reading matches what the label on the terminal says. Adjust the probe's compensation trimmer until you get square corners; sketch in your folder the three possible shapes (undercompensated, correct, overcompensated).
Function generator at 1 kHz, 4 Vpp. Use the oscilloscope to measure the amplitude, the period and the frequency. Calculate the RMS value and compare it with a multimeter on the AC range. Repeat with a square wave and a triangle wave at the same amplitude: a multimeter without True RMS will give values different from the correct one, and that is where you see why.
On the rectified and filtered power supply from the semiconductors topic: with DC coupling you see an almost straight line at the DC level; switching to AC and raising the sensitivity reveals the sawtooth ripple at 100 Hz. Measure its peak-to-peak amplitude and compare it with the value calculated from Vr = I/(f·C).
Series circuit with R = 1 kΩ, C = 100 nF, driven with 1 Vpp at 1 kHz. CH1 at the input, CH2 across the capacitor (with both grounds at the same point). Measure Δt between the zero crossings and calculate φ. The theoretical result is φ = −arctan(2πfRC) = −32°. Repeat at 10 kHz and check that the phase shift approaches 90°.
10Common mistakes
| Symptom | Usual cause |
|---|---|
| All voltages read 10 times too low (or 10 times too high) | The probe is on ×10 and the instrument is set to ×1, or the other way around. |
| The image “runs” or looks blurry | Trigger set incorrectly: the level is outside the range of the signal, or the trigger source is the wrong channel. |
| Blank screen | Trigger mode on NORMAL with no valid trigger. Switch to AUTO. |
| The square wave shows rounded corners or “ears” | Probe out of compensation. Adjust it using the calibration terminal. |
| A slow sine wave appears that should not be there | Aliasing on a digital oscilloscope. Change the timebase and see whether the displayed frequency changes. |
| Connecting the ground clip trips the residual-current device (RCD) or burns components | The clip was connected to a point that is not ground. Mains-connected equipment requires an isolation transformer. |
| The signal is full of 50 Hz noise | Ground lead too long, or ground clip far from the measurement point. |
| A horizontal line is displayed even though there is a signal | Coupling on GND, or the channel is turned off, or the timebase is too slow for that frequency. |
11Self-assessment
With VOLTS/DIV = 0.5 V and TIME/DIV = 2 ms, a signal is 6 divisions tall and one cycle measures 4 divisions. What are Vpp, T and f?
Vpp = 6 × 0.5 = 3 V. T = 4 × 2 ms = 8 ms. f = 1/0.008 = 125 Hz.
What is AC coupling for, and when should you NOT use it?
It is for viewing a small variation riding on a large DC level (the ripple of a power supply, for example), because it blocks the DC and lets you raise the sensitivity. It is not suitable when the absolute level of the signal matters, nor with very slow signals or signals with a low duty cycle, because the coupling capacitor distorts them.
What exactly does the trigger circuit do?
It makes each sweep always start at the same point of the signal: when it crosses a given voltage level with a given slope. That way successive traces overlap exactly and the image looks still.
Why is the ×10 probe preferable even though it attenuates the signal?
Because it presents 10 MΩ and about 15 pF instead of 1 MΩ and 100 pF: it loads the circuit much less, especially at high frequency, where the capacitance of the ×1 probe can completely alter the circuit's operation. The attenuation is compensated by the instrument's scale.
Two 1 kHz signals are 250 µs apart. What is the phase shift?
T = 1 ms. φ = (250 µs / 1000 µs) × 360° = 90°.
What is aliasing and how is it detected?
It is a false signal, of a lower frequency than the real one, that appears on a digital oscilloscope when the sampling rate is insufficient. It is detected by changing the timebase: if the displayed frequency changes, it was aliasing; a real signal keeps its frequency.
The output of an amplifier flattens only at the top. What does that indicate?
That the Q-point has shifted and the signal hits one end of the load line sooner. You need to recalculate the biasing to center the Q-point and recover the maximum symmetrical swing.
Why can't the ground clips of the two channels be connected to different points?
Because internally they are tied together and to the earth of the plug. Connecting them to points at different potentials shorts those two points through the oscilloscope, with a risk of damage to the circuit, the instrument and the person.
What bandwidth does an oscilloscope need to display a 1 MHz square wave properly?
At least 5 MHz, and preferably 10 MHz or more: a square wave contains odd harmonics of the fundamental, and if the instrument cuts them off, the edges look rounded. With the formula tr = 0.35/BW, a 20 MHz instrument shows edges of down to 17.5 ns.