App-4
IM 701923-01E
Appendix—Probing
Changes in Cable Characteristics
A coaxial cable is used to transfer signals from the probe tip to the oscilloscope.
Cables that can offer the right balance of both flexibility and high frequency
performance are selected after considering the ways in which the cables will
actually be handled. Even so, if a cable is bent sharply, the dielectric can break,
altering the characteristic impedance and subsequently weakening the cable’s
throughput and reflectivity. This in turn will affect the high frequency components
of the observed waveform.
This is one of the causes of poor repeatability in the observation of high
frequency waveforms. The repeatability can be increased by bending the cables
as little as possible in a uniform manner.
Getting the Best Performance Out of Your Probe
When using probes, inductance-based resonance is the biggest problem during
waveform measurement, and it is most important to suppress it.
Make the probe’s input pins and ground lead as short as possible when the goal
is to easily check signals (browsing). When highly reliable waveform observation
is called for, prepare a thru-hole PCB for connecting the probe and directly
connect the probe’s signal input pins onto the PCB trace or to another fitting (No.
1 and No. 2). Connect the probe to a prepared copper wire or plate that is as
thick as possible to reduce inductance in the ground.
If it is not possible to connect a short wire, you can insert a 50 to 100-
Ω
resistor
to dampen the resonance (No. 3). In this case, the measurable frequency
bandwidth is reduced by the resistance, but you can approximate the original
waveform more closely by suppressing the resonance effects of overshooting
and ringing.
Also, try securing the cable to the workbench with tape so that the bends in
the cable do not change (see the next page), because high frequency signal
components are, as stated above, affected by cable bending. This may improve
the repeatability of the observed waveform.
Probing Methods
Registor
Obstacle
Solder
PBA2500
PBA2500
Solder a pin in the
GND thru-hole
Microstrip line
Peel off the
solider resist,
then solider
a pin
Microstrip line
GND
GND thru-hole
No. 1
No. 2
No. 3
