Operating The Probe - Agilent Technologies 1156A User Manual

Operating the Probe

Introduction
The Agilent 1156A active probe comes with accessories that place a
resistor as close as possible to the circuit being probed. The purpose of
this resistor is to dampen the resonant circuit created when the probe
tip makes physical contact with the circuit. The resonant circuit is made
from the parasitics of the physical connection between the point being
probed and the internal attenuator of the probe.
All probes suffer from the effects of this resonant circuit at the input of
the probe. However, with lower bandwidth probes the resonant
frequency is much higher than the bandwidth of the probe making this
effect less noticeable.
The bandwidth of the 1156A active probe is higher than the resonant
frequency of the physical connection to the probed circuit, even when
using the smallest possible physical connection. Placing a resistor
between the circuit being probed and the parasitics of the physical
connection isolates the circuit being probed from this resonance. This
allows the response of the probe (V
bandwidth of the probe.
It is important to understand that the resistor placed at the point being probed
is part of the probe. The optimum value of this resistor depends on the geometry
of the physical connection to the circuit being probed. The optimum value of
resistance is used in each of the suggested configurations shown in table 2-1.
Table 2-1
Suggested Configurations and Characteristics
Configuration
110 Ω Resistive Signal Pin (green) and
Solderable Ground Socket
130 Ω Resistive Signal Pin (orange) and
Ground Blade
5 cm Resistive Signal Leads and
Solderable SMT or Through-hole
Ground Pin
Socket-end 10 cm Resistive Signal Lead
and Solderable SMT or Through-hole
Ground Pin
Each suggested configuration is optimized for flattest response and highest input
impedance. For response, this means that the waveform at the output of the
probe will be as close as possible to the waveform at the input of the probe
without introducing overshoot, ringing, or other unwanted perturbations. For
input impedance, this means that the input impedance is as high as possible
throughout the entire bandwidth of the probe, thus never resonating low due to
18
/V
out
Bandwidth Rise time
> 1.5 GHz < 235 ps
1.7 GHz 205 ps
1.4 GHz 250 ps
850 MHz 410 ps
) to be flat across the entire
in
Input C
0.9 pF
0.8 pF
1.2 pF
1.8 pF
Minimum |Z |
in
125 Ω
165 Ω
230 Ω
275 Ω