Choose bandwidth at roughly five times the fastest signal you need to measure, and treat sample rate as a separate specification. A 100MHz scope does not measure a 100MHz signal accurately — it measures it badly. The five-times rule exists because a square wave contains harmonics well above its stated frequency, and losing those harmonics distorts the shape you are trying to read.
Bandwidth: the number that matters most
Bandwidth is the frequency at which the scope's own response has fallen by 3dB — about 30% of amplitude. A signal measured at the bandwidth limit will appear smaller and rounder than it really is.
For a square wave, the fifth harmonic carries most of the edge definition. That is where the five-times rule comes from: to see a 20MHz square wave properly you want at least 100MHz of bandwidth.
Working guide:
- Audio, sensors, power supplies, Arduino work — 50MHz is comfortable.
- Microcontroller buses, SPI, I2C, general electronics repair — 100MHz.
- Faster digital work, switching supplies with fast edges — 200MHz and above.
Sample rate is not bandwidth
These get conflated constantly. Bandwidth is an analogue property of the front end. Sample rate is how many points per second the digitiser captures.
You want a sample rate of at least five times the bandwidth for a single channel — so a 100MHz scope wants 500MS/s or better. Watch for a catch: many scopes quote a maximum sample rate that is only available on one channel. Turn on the second channel and it halves. Check the per-channel figure rather than the headline.
Channels
Two channels is the practical minimum, because most useful measurements are comparisons — input against output, clock against data, before against after a component.
Four channels is genuinely useful for bus work and multi-rail power supplies, and unnecessary for most repair benches.
Handheld or benchtop
Handheld scopes have transformed field work. They run on battery, which also means they are not referenced to mains earth — so you can probe a floating circuit without the earth loop that makes benchtop measurement risky.
Benchtop units give larger screens, deeper memory and more analysis features. If the instrument lives on a bench, that is the better value. If it travels to installations, handheld wins.
What a first scope really needs
Beyond the headline numbers, three things affect daily use far more than most buyers expect:
- Memory depth — determines how long a capture you can hold at full sample rate. Shallow memory forces you to trade time window against resolution.
- Trigger options — edge triggering is the minimum; pulse-width and serial triggering save enormous time when hunting intermittent faults.
- Probe quality — a 100MHz scope with 20MHz probes is a 20MHz scope. Check what is in the box.
Browse Digital Oscilloscopes and Oscilloscope Multimeters.
Frequently asked questions
Is a 2-in-1 scope-meter good enough?
For field diagnostics and general repair, yes — and carrying one instrument instead of two is a real advantage. For detailed bench analysis, a dedicated scope is better.
Do I need four channels?
Only for bus work or multi-rail supplies. Two covers most repair and hobby use.
Can I measure mains with an oscilloscope?
Not directly with a standard earthed probe — it creates a short through the earth path. Use a differential probe or a battery-powered handheld scope.
What bandwidth for automotive work?
50–100MHz is ample. Automotive signals are slow; the value is in triggering and capture, not raw bandwidth.