An oscilloscope draws voltage against time. It shows the actual shape of a signal: whether a keying waveform has clean edges, whether an audio signal is clipping, whether a power supply has ripple. Where a multimeter gives one number, a scope shows the whole waveform.
Reading the screen
Height times volts-per-division gives volts; width times time-per-division gives time. The trigger sets where the sweep starts.
The grid is 10 divisions across and typically 8 tall. You choose how many volts and how much time each division stands for, then count divisions: height × volts per division is volts, and width of one cycle × time per division is the period, which gives frequency as 1 ÷ period.
The trigger is what makes the picture hold still. The scope starts each sweep when the signal crosses the trigger level, so every sweep begins at the same point on the wave and they overlap. Set the level outside the signal and it never triggers, and the display drifts or smears. Also check the coupling: DC shows the whole signal including any DC level, AC blocks the DC so a small ripple on top of a large DC voltage can be seen.
Probes
A probe isn't just a wire. A standard ×10 probe contains a resistor that forms a divider with the scope's 1 MΩ input, cutting the signal by ten but loading the circuit with about 10 MΩ and only around ten picofarads or so. That is far gentler than a bare lead, which would load a circuit with the cable's capacitance. The price is a smaller signal, and the scope must be told: set its probe-attenuation setting to ×10 or every reading is ten times too small.
Adjust the probe's trimmer until the flat parts of the square wave are as flat as possible.
The probe's capacitance must be tuned to match the scope input. Touch it to the scope's built-in square-wave output and adjust the trimmer until the tops are flat. Keep the ground clip short, as a long ground lead rings and distorts fast edges.
Limits and uses
Bandwidth. A scope rated at 100 MHz reads 30% low at 100 MHz. A common rule is to choose bandwidth at least five times the highest frequency you care about, which keeps the error around 2%.
A scope shows time, not frequency. To see harmonics or distortion products, use a spectrum analyzer.
In the shack, a scope checks a CW keying envelope or SSB two-tone output:
The scope's vertical input is a sample of the transmitter's RF output; the horizontal axis is time.
Across a 50 Ω dummy load, a steady 100 W carrier gives 70.7 V RMS, which is 100 V peak and 200 V peak-to-peak. Use a high-voltage probe, an attenuator or a capacitive pick-off, never a 10× probe on a kilowatt. See Dummy loads.