Directed Energy, Inc.
            Knowledge Base Directed Energy Support General

            Choosing an oscilloscope to view pulses

            We're occasionally asked about choosing an oscilloscope for viewing the output of a current monitor or a voltage monitor.

            On the surface, it seems that the bandwidth should be easy to select. For example, using the standard rule of thumb, viewing a 1 MHz sine wave requires an oscilloscope that's 5 times faster. An entry-level 100 MHz scope would be more than sufficient.

            But we're not dealing with sine waves, we're dealing with pulses. The sharpness of the edges is not necessarily linked to the pulse repetition rate. If we truly want to see the pulse's rising and falling edges with good fidelity, including overshoot, ringing, glitches, etc., we will need a compatible oscilloscope.

            For example, to accurately see pulses from a laser diode driver with an 8 µs rise time specification (which translates to a frequency of 125 kHz), we'll need a scope with a bandwidth of at least 625 kHz.

            For a voltage pulser with a 60 ns rise time (16 MHz), the bandwidth becomes 80 MHz.

            For a fast laser diode driver with a rise time of 10 ns (100 MHz), the bandwidth becomes is 500 MHz.

            And laser diode drivers with single-digit rise times push the bandwidth requirement over 1 GHz.

            While we can certainly use a slower scope for our measurements, we'll lose detail.

            Unquestionably, a 1 GHz or even a 500 MHz oscilloscope costs much more than a 100 MHz oscilloscope. Part of that cost is due to the fast, low-capacitance probes that are included. When using a cable with a VMON- or IMON-compatible connector instead of a probe, we should be sure the cable has a suitable bandwidth specification.

            Updated: 19 Feb 2019 06:57 AM
            Help us to make this article better
            0 0