When purchasing an oscilloscope, a buyer has to consider various oscilloscope specifications in order to meet his needs for a particular requirement.
One significant oscilloscope specification is related to the waveform’s speed and determined by the oscilloscope’s bandwidth. It was discovered that the ability of the oscilloscope to display the waveforms accurately declines with the increase of frequency. The manner in which it is specified can be viewed in IEEE 1057, which describes electrical bandwidth as the state when the amplitude of the sine wave input is lowered by three dB with regards to its level at a minimal reference frequency.
The bandwidth specification of an oscilloscope will usually be quoted in this format — Bandwidth = -3dB at 1500 MHz. If the specification of the oscilloscope for the -3dB point is not adequately high, it will be noted that the edges of square waves and pulses are also slower attributed to the reduced high frequency components.
For adequate oscilloscope specification, one must ensure that the oscilloscope bandwidth is higher than its operating frequency. A Five Times Rule is generally employed as a rule of thumb. Thus, a bandwidth should be five times the signal’s highest frequency component. With this rule, errors caused by frequency limitations will be lower than ± 2%.
Another important oscilloscope specification is vertical DC gain accuracy. Since oscilloscopes are not designed to be utilized in lieu of digital multimeters, its voltage elements will not likely be as accurate. Oscilloscope users are therefore advised to be aware of the accuracy of the measurement made when measuring the signal’s amplitude.
Because most oscilloscopes today employ all digital techniques converting the X axis or incoming vertical voltage to a digital format, oscilloscope users should be aware of the scale’s resolution. Dynamic range and resolution determine the largest measurement made without the need of clipping the waveform and the measurement’s “granularity”, respectively. Checking the oscilloscope’s resolution will ensure that the device will deliver correct resolution and dynamic range.
Rise time specification is also an important specification particularly for digital circuits where edges on pulses and square waves are of paramount importance. An oscilloscope should have fast rise time for it to accurately capture rapid transitions, otherwise the results could be misleading and significant information may not be displayed.
Rise time is comparable to an operational amplifier’s slew rate, where the voltage change’s rate is the limiting factor. And like bandwidth, rise time should also be sufficiently high to enable the oscilloscope to capture the required detail.
The influx of digital oscilloscopes gave birth to another important oscilloscope specification — sample rate, which is specified as samples per second (S/s). An oscilloscope with fast sampling rate also offers greater resolution of the waveform’s detail. Faster sampling rate also reduces the chance of losing any critical information.