Tektronix, Inc. P6240, Руководство пользователя

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Probing of Circuits
TLS 216 User Manual
3–103
V
source
R
source
Probe
R
in 1 M
W
Probe
2.5 pF
Probe
Lead Inductance L
t
Ground Lead Inductance L
gl
Figure 3-49: A Probe Adds Resistance, Capacitance, and Inductance
The series inductances represented by the probe lead and ground lead can result
in a parasitic resonant circuit that may “ring” within the bandwidth of the logic
scope. Figure 3-50 shows the effect of the same signal through the same probe
with different ground leads.
4
I
Ground Lead
Low-Inductance
Ground Connection
Figure 3-50: Signal Variation Introduced by Probe Ground Lead (1 ns/division)
Ringing and rise time degradation may be hidden if the frequency of the induced
ringing is beyond the bandwidth of the logic scope. If you know the inductance
(L) and capacitance (C) of your probe and ground leads, you can calculate the
approximate resonant frequency (f
0
) at which that parasitic circuit will resonate:
f
0
+
1
2
p
LC
Ǹ
Reducing the ground lead inductance will raise the resonant frequency. Ideally,
the inductance is low enough that the resulting frequency is above the frequency
at which you want to take measurements. To minimize inductance, take your
measurements directly with the probe tip on adjacent pins when possible.