There are a number of features of an oscilloscope that have an influence on the accuracy of an oscilloscope’s measurements. An oscilloscope’s bandwidth is one of the most important measurement characteristics and one of the biggest influences on an oscilloscopes price.
The bandwidth of an oscilloscope is a theoretical frequency limit that the oscilloscope can accurately measure the amplitude of a waveform. In nearly every oscilloscope, the ability of the oscilloscope to measure the amplitude of a signal decreases as the signal frequency approaches the bandwidth limit.
If the oscilloscope does not have enough bandwidth for the signal being measured, the signal amplitude will be highly attenuated and measurement results will be inaccurate. If the bandwidth is excessively high, additional noise will be measured on the signal and fine detail in the signal could be lost. It is important to choose an oscilloscope with the proper bandwidth that can provide a pure and true rendering of the input signal.
Oscilloscopes have a ‘Low Pass Filter’ on the channel inputs to reduce the noise on the waveform, resulting in a cleaner signal display. This filter allows signals to pass through them at full amplitude until the signal frequency approaches the bandwidth limit of the oscilloscope. This frequency response ‘rolls off’ beginning at around one-third of the bandwidth frequency and resembles a downward curve as the frequency increases. When the signal is decreased by 3 dB or 70.7% of its reference amplitude, that is the cutoff point for an oscilloscope’s bandwidth specification. In other words, at the rated bandwidth limit of the oscilloscope, as much as a 30% measurement error for the amplitude can be indicated on the oscilloscope display.
Due to this fact, it is generally recommended that the bandwidth limit of an oscilloscope should be three to five times greater than the frequency of the measured waveform. This range will allow accurate measurements for all sinewaves and the more complex waveforms, such as square waves and data pulses.
The amplitude of the measurement is the characteristic that is affected most by the bandwidth limit. The frequency measurement is not affected and the oscilloscope can measure frequencies accurately even beyond the bandwidth limit.
Bandwidth is also an influence in oscilloscope probes. Probe manufacturers also specify a bandwidth limit so that the amplitude losses do not affect the measured signal. Probes should also be selected with a bandwidth that is three to five times higher than the input signal so that the amplitude error is minimized.
Calibration laboratories check the bandwidth capabilities when calibrating an oscilloscope. To perform the calibration, a ‘leveled sine wave’ signal is input into the oscilloscope channel at a reference frequency of typically 50 kHz. A 50 Ω termination is used in line with the signal to match the impedance at the oscilloscope input. The amplitude of the input signal is then adjusted to a nominal amplitude value as displayed on the oscilloscope.
The frequency is then increased to the bandwidth frequency of the oscilloscope. The displayed amplitude must be equal to or greater than the 3 dB point (70.7%) of the value at the reference frequency. For example, with an amplitude value of 3 V at the reference frequency of 50 kHz, the measured signal at the oscilloscope bandwidth frequency must be above 2.12 V to pass the calibration check. The bandwidth check is performed on every oscilloscope input channel.
If the customer requests that the actual 3 dB point be reported, the frequency of the input signal is adjusted until the amplitude is exactly at the 3dB value of the reference amplitude. The frequency value of the input signal is then recorded and can be reported to the customer.
Should you be calibrating your instruments in-house or outsourced? Read our guide to find out.