Understanding and measuring an oscilloscope’s frequency response

Frequency response is defined as:

A measure of the effectiveness with which a circuit, device, or system transmits the different frequencies applied to it. It is a phasor whose magnitude is the ratio of the magnitude of the output signal to that of a sine-wave input, and whose phase is that of the output with respect to the input.

Frequency response curves are used as a measure of an oscilloscope’s performance or quality.An oscilloscope’s frequency response is important and should be understood when purchasing one because the flatter the frequency response, the more accurate the oscilloscope will represent the signal it is measuring.

This means greater measurement repeatability and accuracy. As a result, oscilloscopes strive to achieve a “flat” response, with vendors often using digital signal processing (DSP) to improve the flatness of their hardware response. Typically, oscilloscope vendors do not specify their frequency response characteristics, but the response can be measured or you can ask the oscilloscope vendor for this data (they will frequently satisfy such requests).

The frequency response of an oscilloscope is usually characterized by the magnitude of the system's response (measured in decibels) and the phase (measured in radians) versus frequency. There are several different methods used for measuring the frequency response of a system2. For the purpose of this paper, we will focus on the method oscilloscope vendors use to specify their measurement – simply sweep a constant-amplitude pure tone through the entire frequency range of the oscilloscope and measure the output level and phase shift relative to the input.

In addition to the multiple methods used to measure the response, there are two ways to plot the frequency response (the Bode plot or the Nyquist plot). This paper will focus on the Bode plot.A flat frequency response (ideal for oscilloscopes) means the plot stays near 0 (either dB or rads) until the high frequency of the oscilloscope.Any variation from zero means less accuracy.

This article looks in detail at the tests and measurement of a scope's bandwidth. It is presented as a pdf (no registration required); to read it, click here .

About the author
Brig Asay manages product planning and strategic marketing for Agilent’s high-performance oscilloscope business. He joined Agilent Technologies in 2005 as a Technical Support Engineer. During his five years with Agilent, he has held positions as Marketing Operations Manager, where he oversaw the marketing budget and managed the technical support and learning products teams; and Technical Support Engineer, which he helped solve numerous customer problems. Previously to Agilent, Brig worked at Micron Technologies, Inc. as a Test Engineer.Brig graduated with an MBA from Northwest Nazarene University and BS Electrical Engineering from the University of Wyoming.He is a published technical author.

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