Choosing an oscilloscope: what matters for bandwidth, sample rate and channels?

How much bandwidth and sample rate do I need?

Bandwidth is the most important feature of an oscilloscope. It states up to which frequency a signal is still displayed with adequate amplitude; at the bandwidth limit the display is already attenuated by about 3 dB, roughly 30 percent. As a rule of thumb the bandwidth should be at least five times the highest signal frequency you want to measure.

For digital signals it is not the fundamental frequency that counts but the steep edges, which contain high harmonics. To see square waves cleanly you therefore need considerably more bandwidth than the clock frequency alone suggests. The sample rate in Sa/s (samples per second) describes how often the instrument samples the waveform per second. Depending on interpolation and signal shape it should reach about 2.5 to 5 times the bandwidth.

How many channels, how much memory and which resolution?

The number of channels sets how many signals are captured and compared at the same time. Two channels are enough for many standard measurements, four channels let you relate several signals to each other, such as clock and data or input and output. For digital buses there are mixed-signal instruments with additional logic inputs.

The memory depth (record length) determines how many sample points are stored per acquisition. It decides how long you can record at the full sample rate: when the time base is stretched, a shallow memory lowers the effective sample rate. A deep memory keeps the high rate over longer time windows and reveals fast detail within long recordings.

Vertical resolution is given by the A/D converter in bits. The standard is 8 bit, which corresponds to 256 steps. Instruments with higher resolution, such as 10 or 12 bit, resolve small amplitude differences more finely and suit weak signals or small disturbances riding on a larger level. For many general tasks 8 bit is sufficient.

What matters for probes, storage type and triggering?

The probe connects the device under test and the oscilloscope and affects the result directly. Passive probes with switchable attenuation are standard: in the 10:1 setting they divide the signal to one tenth, raise the input resistance and reduce the capacitive load, which allows higher frequencies and less loading. In the 1:1 setting there is no division, which gives more amplitude for small signals but lowers the bandwidth.

A 10:1 probe must be compensated at the instrument so that square waves appear undistorted; the calibration output serves this purpose. Modern instruments are digital storage oscilloscopes (DSO): they digitize the signal, store it and display it stably, even for single events. Classic analog instruments show the waveform directly on a cathode-ray tube but cannot store it or analyze it numerically.

The trigger produces a stable, repeatably displayed picture by tying the acquisition to a defined event. Besides the edge trigger, pulse, video and bus triggers help to single out specific events. Make sure the bandwidth of instrument and probe match, because the weaker value sets the bandwidth you can actually use.

Frequently asked questions