Oscilloscope (HAMEG HMO1524)

An oscilloscope draws a graph of voltage against time. Where the multimeter gives you one number — the average voltage at a point — the scope shows you the shape of what’s actually happening there: a square wave’s edges, a clock’s frequency, noise riding on a supply rail, a pulse that’s the right height but half the width it should be. Reach for it when the voltages all measure correct but the circuit still misbehaves. The bench scope is a Rohde & Schwarz HAMEG HMO1524, with four channels at 150 MHz, so you can watch four points in a circuit at once and compare their timing.

WARNING

The ground clips of all four probes are connected to each other and to the building’s earth, through the scope’s power cord. Clip two of them to different points in your circuit and you’ve wired those points together — which will short whatever sits between them. Every ground clip goes to your circuit’s ground, and nowhere else.

Before you start

• No training or checkout is required, but a scope rewards a bit of patience. If you’ve never used one, the fastest way in is to probe something you already understand, like a blinking LED’s control pin, and watch the trace change as you adjust the controls.
• Check the probe attenuation. Scope probes usually have a switch on the body marked ×1 and ×10. In ×10 the probe divides the signal by ten, and the scope has to be told, or every voltage it shows you will be wrong by a factor of ten. Set the switch and set the channel’s probe setting to match — this is the single most common scope mistake.
• Low-voltage bench work only — see the hazard callouts on the Electric Workbenches page.
• For anything beyond the basics here, Rohde & Schwarz publish a full user manual for the HMO1000 series that covers every menu on the scope. Ask a staff member if you need a copy.

The controls

The front panel is grouped by function, and once you know the groups you can find anything.

• VERTICAL controls how tall the trace is. VOLTS/DIV sets how many volts one grid square represents, POSITION slides the trace up and down, and the numbered channel buttons turn each channel on and off and open its menu — including its probe attenuation setting.
• HORIZONTAL controls time. TIME/DIV sets how much time one grid square represents — turn it one way to see the overall shape, the other way to zoom in on a single edge. POSITION slides the view left and right.
• TRIGGER is what makes a repeating signal stand still instead of scrolling across the screen. The LEVEL knob sets the voltage the scope waits to see before it draws; set it somewhere in the middle of your signal’s swing. AUTO draws continuously whether or not it finds that level, which is what you want while you’re hunting; NORM draws only when it triggers, which is what you want for a signal that happens occasionally.
• AUTOSET, in the GENERAL group, makes the scope guess sensible settings for whatever it can see. It is the right first button to press, and a good way to get back to something readable after you’ve lost the trace.
• CURSOR/MENU and ANALYZE hold the measurement tools — cursors you position by hand, and automatic measurements of things like frequency, period, and peak-to-peak voltage.

Operating

1. Switch the scope on and let it finish booting.

2. Set the probe’s ×1 / ×10 switch, then press that channel’s button and set the same attenuation in its menu. Do this before you measure anything, not after you’ve written a number down.

3. Clip the probe’s ground lead to your circuit’s ground, and touch the probe tip to the point you want to see.

4. Press AUTOSET. In most cases a usable trace appears immediately.

5. Adjust from there. Use VOLTS/DIV so the trace fills a good part of the screen vertically without running off the top or bottom, and TIME/DIV so you can see a few cycles of whatever you’re looking at.

6. If the trace won’t hold still, the scope isn’t triggering. Turn the TRIGGER LEVEL knob until it sits within the signal’s range, and check the trigger is set to the channel you’re actually looking at.

7. Measure. The automatic measurements under ANALYZE cover frequency, period, and amplitude; cursors let you measure anything else by placing two lines and reading the difference.

8. To keep a result, save a screenshot to a USB stick in the port on the front.

Finishing up

• Disconnect the probes from your circuit before you power anything down.
• Unclip the ground leads and check the probe tips aren’t bent or missing their tip covers.
• Switch the scope off and coil each probe loosely. Scope probes are surprisingly expensive and a kinked one is a dead one, so don’t wind them tightly or knot them.
• Tell a staff member if a probe is damaged or a tip accessory has gone missing.

Common problems

Every voltage reads ten times too big or too small. The probe’s ×1 / ×10 switch and the channel’s probe setting don’t match. Fix both and take the measurement again.

The trace scrolls across the screen and won’t stand still. The scope isn’t triggering. Set the TRIGGER LEVEL inside the signal’s voltage range, make sure the trigger source is the channel you’re watching, and use AUTO rather than NORM while you’re searching.

There’s a flat line where a signal should be. Check the channel is switched on, that the probe’s ground clip is actually on ground, and that the tip is on the node you meant. Then check VOLTS/DIV — a small signal at a large volts-per-division setting looks exactly like a flat line. Press AUTOSET to rule all of this out at once.

The signal is buried in fuzz. Some of that is real noise in your circuit and worth knowing about. Some of it is the probe’s long ground lead acting as an antenna — keep the ground clip as close to the measurement point as you can. And check you’re not measuring on a scale where the scope’s own noise floor is most of what you see.