Practise on a real-feeling oscilloscope

Set TIME/DIV so two or three cycles fill the screen, and trigger so the trace stands still. Then:

• Find where the wave crosses the centre line going up.
• Count the divisions to the next rising crossing. Each small tick is 0.2 div.
• Period T = divisions × TIME/DIV. Frequency f = 1 ÷ T.

For better accuracy, measure across several cycles and divide. Counting 3 cycles over 12.6 divisions is more precise than reading one.

Press CURSOR once for time cursors, twice for voltage cursors and a third time to turn them off. Drag the dashed lines on the screen.

• Time cursors show ΔT and 1/ΔT. Put them on two matching points one cycle apart and 1/ΔT is the frequency.
• Voltage cursors show ΔV using CH1's scale. Put them on the top and bottom of the wave to read peak-to-peak.

Cursors are how you measure things the automatic measurements can't, like the rise time of an edge or the height of a ripple.

With two signals of the same frequency on screen, measure the time between matching points (for example, both rising zero crossings).

If CH2's crossing comes after CH1's, CH2 lags. A 90° lag at 1 kHz is a Δt of 250 µs.

Two channels are also how you compare the input and output of a circuit: you can see gain, delay and distortion at a glance.

DC coupling shows the true voltage, including any steady DC level. AC coupling puts a capacitor in the input, which blocks the DC and shows only the part that changes.

A 15 V supply with 0.3 V of ripple is a flat line at 5 V/div. Switch to AC coupling and turn VOLTS/DIV down to 100 mV/div, and the ripple fills the screen.

Remember to switch back to DC when you want to know the actual voltage level.

A ×10 probe divides the signal by 10 and loads the circuit much less (typically 10 MΩ in parallel with around 10 to 15 pF). It also has far more bandwidth than ×1. Use ×10 for almost everything.

Each ×10 probe must be compensated to the scope input. Clip it to the scope's 1 kHz calibration square wave and adjust the trimmer:

• Rounded leading corners: under-compensated.
• Spikes at every edge: over-compensated.
• Flat, square corners: correct.

Keep the ground lead short. A long ground lead adds ringing to fast edges.

On a normal mains-powered scope, every probe's ground clip is connected to mains earth through the power cable. Clip it to a point that isn't at earth potential and you create a short circuit through the probe, which can destroy it, the scope or the circuit.

• Never clip the ground lead to mains live or neutral, or to the output of a bridge rectifier running straight off mains.
• Never defeat the scope's earth to "float" it. That makes the whole metal case live.
• For mains, drive outputs and other non-earthed circuits, use a differential probe or a battery-powered, isolated scope with the right CAT rating.
• Both channels share the same ground. Two ground clips on two different points is also a short.

If you're unsure whether a point is safe to ground, it isn't. Check the circuit first.

The smoothing capacitor charges at each peak and then discharges into the load until the next peak arrives. The dip between peaks is the ripple.

For a half-wave rectifier f is the mains frequency (50 Hz). A bridge refills the capacitor twice per cycle, so f is 100 Hz and the ripple halves.

Ripple at 50 Hz on a bridge supply is a classic sign of an open diode.

A capacitor charging through a resistor reaches 63% of the step in one time constant, τ = R × C, and is essentially finished (over 99%) after 5τ.

A 555 astable uses the same curve. Its capacitor swings between ⅓ and ⅔ of the supply: