Probing Techniques and Tradeoffs (Part V): Probe Loading

Figure 1: A probe's impedance varies
with frequency

Earlier in this series of posts, we alluded to the topic of probe loading, which is an outcome of the fact that to make a measurement, an oscilloscope probe must "steal" some energy from the circuit or device under test. Thus, the probe's tip must have a finite impedance across the frequency range of interest.

Testing Techniques For Switch-Mode Power Supplies

Figure 1: A simplified schematic of
a switch-mode power supply circuit

On its journey from wall socket to the device being powered, power typically passes through a switch-mode power supply, where the AC signal is rectified into DC before it reaches the device. After that, the DC signal (often 5 V) is passed on to DC-DC converters on the device's PC board for feeding various voltages to branches of the device's power-delivery network. Let's look at some of the measurement techniques and considerations relative to testing switch-mode power supplies.

The Effects of Passive Probe Ground Leads

Figure 1: Teledyne LeCroy's PP108,
a representative passive probe

When you open the box containing your shiny new oscilloscope, one of the items you'll likely find inside is a set of basic 10:1 passive probes (Figure 1). Those probes have a ground lead that you'll want to use when you make measurements. Your probe has a bandwidth specification that's probably somewhere between a few hundred megahertz to 1 GHz; that spec was obtained at the factory with a specialized test jig having a specific ground inductance and source impedance. Now, the way in which you connect your ground lead can have a big impact on the real-world bandwidth and response of the probe.