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June 2015 • The PCB Design Magazine 35 AVOIDING OVERLOAD IN GAIN-PHASE MEASuREMENTS continues Inserting the test signal at such a loca- tion guarantees the lowest possible alteration of loop characteristics by the measurement. Since the injection point sits at the DC out- put voltage, we need an isolation transform- er as shown in Figure 1. We can measure the loop gain as the complex ratio of the voltages at the two sides of the injection transformer with respect to ground. There are dedicated instruments for this purpose, called frequency response analyzers. There is one remaining challenge though, which leads us to the title of this article. As you can see on the chart, the loop gain can vary by orders of magnitudes as frequency changes; usually it is in the 40–70 dB range at very low frequencies. Unless the converter is our discrete component design, we may not know exactly what circuitry we have along the control loop, so it may be hard to guess the proper level of the test signal. If we are not careful and use too large a test signal, we can easily overload the control loop. This in turn will create invalid results. An illustration of such a case is shown in Figure 3. If, on the other hand, we preemptively try to select a very low injected test signal level, our output data may be buried under noise. How to maintain the proper injected level to be above the noise floor, yet avoid control- loop saturation, will be explained in a future column. PCBDESIGN References 1. Dynamic Characterization of DC-DC Converters, DesignCon 2012, Santa Clara, CA, January 30–February 2, 2012. Dr. Istvan Novak is a distin- guished engineer at oracle, working on signal and power integrity designs of mid-range servers and new technology developments. With 25 patents to his name, novak is co-author of "Frequen- cy-Domain Characterization of Power Distri- bution networks." To read past columns, or to contact novak, click here. Figure 3: An example of gain-phase plot showing the result of error-amplifier overloading. quiet power

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