Agilent Technologies 54701A, User'S And Service Manual

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Chapter 2: Probing Considerations
Resistive Loading Effects
Figure 12 shows waveforms measured by the 54701A (100 k
Ω
, 0.6 pf) and the
1-M
Ω
, 6-pF probe; both probes are connected to a 1-GHz oscilloscope.
Figure 12
Probe Resonance Effects
Waveform 1 shows the pulse response of a 6-pF probe measuring a 400-ps
step. The ringing on the pulse is caused by the input capacitance of the probe
and by the inductance of the ground return. The period of the ringing
measures 1.72 ns, representing a frequency of 581 MHz. The circuit had a
ground return of 1/2 inch. Using equation 3 to calculate the resonant
frequency (12.5 nH and 6 pF) results in 580 MHz. The measurement and the
calculation yield the same result, showing how probe resonance causes
problems when probing high speed signals.
Waveform 2 shows the pulse response when the same 6-pF probe measures an
800-ps edge. Notice that the overshoot and ringing are still present, but are
significantly reduced. This is because the slower signal edge has less energy
at the resonant frequency of the probe.
Waveform 3 shows the pulse response when the 6-pF probe measures a 1.25-
ns edge. The ringing is nearly subdued and doesn't play a significant role in
the measurement.
Waveform 4 shows the 54701A 0.6-pF probe, with a one-inch ground lead,
measuring the 400-ps edge. Because of its much lower capacitance, and even
with a longer ground lead, its resonant frequency is much higher and it shows
no ringing in the response.
Waveform 1
Waveform 2
Waveform 3
Waveform 4