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UNDERSTANDING INPUTS AND USAGE OF PROBES
UNDERSTANDING INPUTS AND USAGE OF PROBES15
15.3 Probe bandwidth calibration
A probe makes a physical and electrical connection between a test point or signal
source and the instrument. Depending on the measurement needs, this connection can
be made with something as simple as a length of wire or with something as sophisti-
cated as an active differential probe.
For the purpose of this document, we only describe attenuating probes within two
categories: 1X Probes and 10X Probes.
15.3.1 1X Probes
1X probes, also known as 1:1 (one-to-one) probes, simply connect the input of the
instrument to the circuit being measured. They are designed for minimum loss and
easy connection. Fig. 15.11 shows the circuit diagram for an instrument input con-
nected to a circuit under test. The circuit under test is modeled as a voltage source
withaseriesresistor.The1Xprobe(orcable)introducesasignicantamountof
capacitance that appears in parallel with the input of the instrument. A 1X probe may
have around 40 to 60 pF of capacitance.
5
6
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9
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Fig. 15.11 Input connection using a 1X probe
The impedance of the circuit and the input impedance of the instrument produce a
lowpasslter.Forverylowfrequencies,thecapacitoractsasanopencircuitandhas
little or no effect on the measurement. For high frequencies, the capacitor's impedance
becomessignicantandreducesthevoltagedetectedbytheinstrument.Fig.15.12
shows this effect in the frequency domain. If the input is a sine wave, the amplitude
tends to decrease with increasing frequency and the phase is shifted
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