The most versatile tool in the test & measurement world is the oscilloscope. Similar to a multi-purpose pocket tool, not only can a scope be used to view time-domain waveforms (voltage vs time), which is the primary cutting blade of an oscilloscope, but many of today’s scopes have additional blades that can perform measurements that were formerly relegated to specialized test gear, including spectrum analysis (FFT), DVM, counter, logic analysis, serial protocol analysis, and arbitrary waveform generation.
The latest blade that Keysight has added to this multi-purpose tool is frequency response analysis. With frequency response analysis, a voltage sine wave source is swept from a lower frequency to an upper frequency while Vin and Vout are measured and the ratio is plotted (Gain in dB = 20 x Log(Vout/Vin). This is the primary function of Vector Network Analyzers (VNA), which are sometimes called Frequency Response Analyzers (FRA).
If you can remember back to your college engineering days, you may recall the dreaded Bode plots of gain and phase versus frequency. For my EE class assignments I used a slide rule (I’m ancient), a pencil with a really big eraser, and traditional green engineering graph paper to predict theoretical frequency response results. And then to verify predicted results in the lab, we used a 2-channel analog oscilloscope along with a sinewave generator to perform multiple measurements at discreet frequency settings. We then plotted the results manually on paper for comparison.
Most of today’s EE students use off-the-shelf PC apps such as MATLAB or LabVIEW to generate their theoretical Bode plots. But since most universities can’t afford to purchase and fully equip EE teaching labs with specialized test equipment, the method of verification is often the same method that I used 40 years ago.
Frequency response measurements (Bode plots) are not just something that you are required to do in college. Many electronic designs, including filters and amplifiers, must meet frequency response specifications. One common example where frequency response testing should be performed is when testing the stability of switch mode power supplies.
All power supplies have a feedback amplifier network. If the output load of a power supply suddenly increases (sudden increase in current), output voltage will momentarily drop until the feedback amplifier responds to pull it back up. If the feedback amplifier responds too quickly, the net result could be excessive overshoot/undershoot with significant output ringing, or even worse, oscillation. To insure stability, the feedback network of power supplies should be tested. But even in industry today, VNAs and FRAs are often hard to come by and also are not that easy to use. However, almost every test bench has an oscilloscope. And if it’s a Keysight InfiniiVision 3000T or 4000 X-Series oscilloscope, problem solved.
Figure 1 shows an example of the setup menu used to perform a power supply Control Loop Response measurement (Bode gain & phase) using a Keysight InfiniiVision 3000T X-Series oscilloscope with the power measurements option (DSOX3PWR).
Figure 1. Control Loop Response (Bode) setup menu.
In this example the oscilloscope uses its built-in waveform generator to sweep an input test signal (sine wave) from 100 Hz to 20 MHz using a fixed amplitude of 200 mVpp at each test frequency. Note that amplitude profiling is also possible. When “Apply” is pressed, the oscilloscope begins the one-time sweep and produces the gain and phase shown in Figure 2.
Figure 2. Gain and phase plot of the feedback network of a switch mode power supply.
The blue trace represents the gain plot with its scaling factors shown on the left vertical axis, while the orange trace represents the phase plot with its scaling factors shown on the right vertical axis. At the completion of the sweep, the gain and phase plots are automatically scaled for optimum display resolution, but these plots can also be scaled manually. Also shown in this plot are automatic measurements of the phase margin at the 0 dB cross-over frequency (PM = 41.52ᵒ at 62.21kHz) and gain margin at the 0ᵒ cross-over frequency (GM = 9.89dB at 130.8kHz). These are important measurements that give you an indication of the stability of your power supply’s feedback amplifier. Note that you can also manually slide the measurement markers along the plots to measure gain and phase at any frequency.
To learn more about power supply Control Loop Response testing using an oscilloscope, download Keysight’s application note on this topic.