HP 54750A, User Manual, Page: 436 / 478

How the Instrument W orks
System Architecture
P erhaps you have connected an analyzer to a circuit for troubleshooting
only to have the circuit operate correctly after connecting the probes . The
capacitive loading of the probes can attenuate a glitch, remove ringing or
overshoot, or slow an edge just enough that a setup or hold time violation no
longer occurs .
The inductance of the probe ground lead forms an LC circuit with the
probe's capacitance and the output capacitance of the circuit under test,
including any parasitic capacitance of PC board traces , and so on. The ringing
frequency of this circuit is:
F =
1
2
p
LC
If the rise time of the signal is sucient to stimulate this ringing, then it can
appear as part of the captured signal. An approximation of the bandwidth of
the signal is:
S ig nal B andw idth =
0: 35
S ig nal R ise T ime
T o calculate the ringing frequency , assume the probe ground wire has an
inductance of approximately 25 nH per inch. Therefore , a probe with a tip
capacitance of 8 pF and a 4-inch ground wire has a ringing frequency of
approximately 178 MHz (not including the circuit capacitance). In this case , a
signal with a rise time of less than 1.9 ns can stimulate ringing.
13-16