Probes - HBM Genesis GEN5i User Manual

GEN5i
D.4 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, but otherwise they are equivalent to using a cable to
connect the instrument. Figure D.13 shows the circuit diagram for an instrument
input connected to a circuit under test. The circuit under test is modeled as a
voltage source with a series resistor. The 1X probe (or cable) will introduce a
significant 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.
Figure D.13: Input connected using a 1X probe
The impedance of the circuit and the input impedance of the instrument together
produce a lowpass filter. 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 significant and loads down
the voltage seen by the instrument. Figure D.14 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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