Figure 2-2. Switching Waveform
Oscilloscopes also offer filtering of the probes.
shows that both C1 and C2 have a
BWL
icon. This
indicates that bandwidth limiting is turned on. Many scopes offer filtering for 20 MHz, 200 MHz, and no filtering
at all. These filers act as low-pass filters at these set frequencies. Noise from around the lab can be introduced
into the scope, so sometimes it is useful to filter that noise inside the oscilloscope itself (if checking if a MOSFET
is switching, checking a start-up or shutdown, and so forth). However, when checking a maximum voltage on
a MOSFET, or investigating a voltage spike, remove bandwidth limiting. Noise or real, it is critical to find the
maximum voltage, so remove bandwidth limiting. If you are unsure, capture both. It is a simple setting change.
Another important measurement technique is called
tip and barrel
. This is best shown in a picture, please
see below for reference. For oscilloscopes, tip and barrel commonly have a ground clip that attaches to the
probe with a short wire. This short wire can pick up noise and add parasitic inductance, which can alter the
waveform. So, when measuring critical voltage signals, whether that be a maximum voltage on a FET, output
ripple, switching voltages on a gate, the current sense voltage for the feedback loop, use the tip and barrel
method. The tip and barrel method is to remove the ground clip, and replace it with a smaller ground connection
wire. The ground connection wire will be a small attachment of wire that is curly on one side and then a small
straight portion with one corner. The curly portion is what holds the attachment to the probe, and the wire with
the corner is the new ground connection. Many scope probes come with a cover that has a hook, which can be
used to hook onto test points or wires. If this is removed, it exposes the probe tip and a small exposed ring of
metal. This is the ground connection and the curly side connects to the ground of the probe. This method creates
the smallest ground loop between the probe signal and ground, and therefore reduces noise.
2.4 Board Preparation
When prepping a board for debugging, install a wire for RTN, primary ground, so that many oscilloscope probe
ground clips can connect to it easily. Do this for both primary ground and secondary ground. Potentially all
four probes of an oscilloscope could be connected to primary ground, so make the wire long enough for all
four to connect. However, a wire is essentially an antenna, so do not make it excessively long. It should not
exceed the length of 5 milimeters, as a general rule. Other signals to put wires on are VDD, VC, and Vout.
A wire can connect to these signals because these voltages should be steady and are relatively high voltage
(as in connecting a wire to them for the scope probe should not introduce a significant amount of noise to the
measurement versus directly connecting the probe to the board). Trying to hold more than two oscilloscope
probes and operate the power supplies and take good measurements can be difficult, so it is good to have wire
connections with wires, when able. Taking these measures provide a sufficient start for debugging.
Preparation and Measurement Techniques
4
PoE Powered Devices Debug Guidelines
SLVAF74 – JULY 2021
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