The Shunt-Thru configuration inherently suffers a ground-loop error at low frequencies. The
current flowing thru the cable shield of the connection to Channel 2 ground introduces a
measurement error that can become significant at frequencies below 10 kHz to 100 kHz
when measuring very low impedance values. To suppress respectively reduce the ground-
loop error at low frequencies, use a common-mode choke or common-mode transformer or
an active isolation device. Note that the Shunt-Thru transformation from S21 to impedance
assumes 50 Ω as source impedance and input impedance.
To improve signal to noise ratio for very low impedance measurements, you can use a 50 Ω
amplifier at the Output of Bode 100 as shown in the image below. Use calibration to remove
the amplifier gain and phase shift.
Calibration
The Shunt-Thru measurement is basically a Gain measurement that is transformed to an impedance.
Therefore two calibrations are possible. Either a Thru-calibration or an Open, Short, Load calibration.
Thru calibration calibrates the underlying S21 measurement and removes the gain/phase shift of the
cable connection, common mode choke or amplifier.
Open, Short, Load calibration shifts the reference plane directly to the calibration plane.
For more details on calibrating a Shunt-Thru measurement, please refer to 10.3.4 Calibrating a Shunt-
Thru or Series-Thru measurement on page 100.
Measurement types and applications
OMICRON Lab
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