Simple Signal Injector Aids Control-Loop Analysis

Sep 1, 2008 12:00 PM
By Brian Keeney, Senior Electrical Engineer, Oxford Instruments X-Ray Technology Group, Scotts Valley, Calif.

Network analysis is a powerful and well-established method of characterizing and optimizing a control system.^[1] Unfortunately, making a successful measurement can be difficult and frustrating without the proper instrumentation. Having a good network analyzer is not enough. There must be a means for injecting a test signal into a closed loop over the frequency range of interest.

A common method of signal injection is to insert a 100-W resistor in the control loop, typically between the error amp and the plant, which is everything between the control output and the feedback input. For example, a buck converter plant consists of the sawtooth generator and comparator, the power transistor, the catch diode and the LC filter. The injection point must be between a low-impedance output and a high-impedance input. A transformer is then used to generate an ac test signal across the resistor. The reference signal is then measured at the plant input and the response is measured at the error-amplifier output.^[2]

Fig. 1 shows a typical control loop and two common locations for signal injection and measurement. It is very difficult to design a transformer that will provide a flat signal both at very low frequencies (< 1 Hz) and at higher frequencies (200 kHz). An engineer can forego this design and purchase a transformer, but the commercial versions still have the same frequency limitations, typically only operating over one or two decades. Electronic injection circuits exist but are expensive, around $1000. At least one open-source design exists according to published literature^[3], but it suffers from chassis grounding issues.

Another Method

Fig. 2 shows a simple circuit that performs the forementioned tasks very well. An instrumentation amplifier (inamp) isolates the sine-wave test signal from the network analyzer ground. The reference input is driven by the low-impedance output of an error amp or buffer. The sine wave then rides on the reference node, or control point of the circuit. It is important to realize that the reference input to an inamp is not a high-impedance node. Therefore, make sure that this node is truly driven by a low-impedance source.

The output of the inamp is connected to the high-impedance input of the plant. A designer does not actually need the 100-W resistor with this design, but its presence prevents accidental open-loop conditions if the device under test (DUT) is powered up before the test fixture. Analog Device's AD620^[4] is able to be run on ±18-V rails, allowing for injection at nearly any conceivable point in most loops. Lastly, note the 1-MW resistor from the inverting input to ground. The inamp needs nanoamps of input bias current, which isn't much, but it is ill-advised to float the inputs completely. The resistor allows a very small amount of bias current without changing the return current paths appreciably.

Design Verification

The injection circuit was tested using a Venable 3120 network analyzer. First, the open-circuit performance of the fixture was checked with a test voltage of 500 mV and the reference input biased at 1 V (a graph of open-circuit performance is shown as Fig. A.) The region of practical use is from 0 kHz to 200 kHz. The phase shift is unimportant because it affects neither the network measurement nor the DUT. However, gain is important because the injected signal will start to decrease in the region of gain roll-off, reducing signal-to-noise and resulting in an inferior measurement. A higher-performance inamp would extend the range of the circuit.

It is very easy to believe erroneous results from a network analyzer. There are innumerable ways to compromise the measurement, from incorrect test signal amplitude to forgetting to turn on the injection circuit. It is important to have an oscilloscope hooked up to both the signal injection and measurement points. The signals should be relatively clean — distortion and noise compromise a reading. Once the setup has passed an open-loop test like the one previously described, it is prudent to test a standard, such as a simple RC circuit. As a means of verifying signal integrity, a buffered 10-kW/16-nF low-pass filter was tested in the fixture with a dc bias of 1 V.

Results of this test (see Fig. B.) showed that the performance was very good, matching the expected 3-dB point of 1 kHz. There are many other logical test circuits, but making a standard that closely matches the application is best. The results from a 10-Hz high-pass filter won't yield much information about how the test circuit will behave at 100 kHz.

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