Notes / Test & Debug Instruments
Oscilloscope: reading a circuit from its waveforms
Trigger, time base, probe compensation, ground clip. A multimeter shows an average; an oscilloscope shows change over time.
Three axes of the screen
- Vertical (V/div): volts per division
- Horizontal (s/div): time per division
- Trigger: the reference that holds the display still. Set as, for example, a rising edge at 1.5 V
Without the right trigger the waveform runs across the screen and cannot be read. A signal that does not repeat (the first byte of a transfer, the instant of reset) is captured once with a single trigger.
Probes
- Use a ×10 probe, and when you first connect it, do probe compensation on the scope's calibration terminal (make the corners of the square wave flat).
- A long ground clip shows false ringing on fast signals. Keep it short with a spring ground tip.
- The scope's ground is in most cases connected to mains earth. Clip it to a point that is not ground (a high-side node, AC mains) and you make a short circuit. Use a differential probe for those.
What you look at most
| Situation | What to check |
|---|---|
| PWM | Frequency, duty, rise time |
| Power | Ripple, voltage dips when the load changes |
| Communication | Bit width, levels, ringing |
| Reset | The order and timing in which the supplies come up |
A bandwidth of at least 5 times the highest frequency of the signal to be measured is good.