Homebrew loop gain test transformer

You can purchase test equipment that will allow you to examine the loop gain and loop phase properties of an operating feedback loop and, that purchased equipment will deliver useful results over a very wide range of frequencies. However, the feedback loops in the power supplies I’ve encountered only needed to be characterized over fairly limited frequency ranges. That being the case, going “homebrew” with some low-cost components worked out quite well.

Loop gain and phase measurements were a lecture topic I first encountered at the Dallas, Texas PowerCon back in 1981. A presentation was made, at that time by Dean Venable, where I first became aware of a particular technique for the measurement of gain and phase of a feedback loop such as the one shown in Figure 1.

Figure 1 A feedback loop that requires testing.

We choose a point in the feedback path where will focus our attention. Where the “X” is seen above, we insert a transformer and introduce a network analyzer (Figure 2).

Figure 2 A feedback loop ready for testing.

With malice a forethought, we choose our “X” point where the source impedance of A1 is zero and the input impedance of A2 is infinite. Since the inserted test transformer will not disturb the feedback loop’s normal operation, our gain and phase measurements will be representative of those two parameters under the conditions in which the feedback loop will actually see service.

Using at the time, an Agilent 4395A network analyzer, an instrument now superseded but still a nice piece of electronics anyway, a “homebrew” test transformer was devised as shown in Figure 3.

Figure 3 A homebrew test transformer and it’s test usage.

A typical power supply test result obtained from this test method looks like Figure 4.

Figure 4 A gain and phase test result.

What the actual bandwidth of the homebrew transformer might be, I have no clue. Still, it’s broadband enough to let me get a pretty good look at the loop gain and phase characteristics of any power supply I’ve had to deal with yet.

John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).

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