Sharpen Rising and Falling Edges

Wherever you find a digital control signal, you find sharp rising and falling edges that need characterization. These edges occur on switching power supplies that drive power MOSFETS and they occur in logic gates. Unless your measurement system—oscilloscope and probe—have sufficient bandwidth and sample rate.

POL (point of load) switching regulators have reached 1 nS edge speed and off-the-shelf high-speed logic gates, such as the NC7SZ04, have rise times of roughly 500 pSec. While many engineers measure edges on these components, they are likely getting incorrect results. The right system bandwidth will result in cleaner, more accurate measurements.


Bandwidth and Sample Rate The first considerations for measuring high speed edges are the bandwidth and sampling rate required to permit measurement of the edge without limiting the fidelity of the results due to the test equipment.  The capabilities of oscilloscopes are varied and often masked; some offer Gaussian response while others offer a flatter response (Ref. 1).  Specifications can be misleading. The minimum bandwidth and sample rate required are somewhat, though not strongly, related to this performance and so we can estimate the requirements for a 3% accurate measurement  (10%-90%) on a Gaussian scope from Equation 1.

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