Specifications

In practice, the measurement error is different for the cable termination types of the instrument.
High frequency 4TP instruments, such as the Agilent 4285A and the 4294A, which internally termi-
nate cables with their characteristic impedance, differ from low frequency 4TP instruments without
cable termination.
4.5.2 Cable extension without termination
Extending test cable from the 4TP instrument without cable termination will produce an impedance
measurement error, which is typically given by the following equation:
Error = k ×ΔL × f
2
(%)
Where, k: A coefficient specific to the instrument
ΔL: Cable length difference (m) from standard length (cable length setting)
f: Measurement frequency (MHz)
The k value is a decimal number mostly within the range of –1 to +1 and is different depending on
instruments. As the above equation shows, the error rapidly increases in proportion to the square of
measurement frequency. Using the open/short compensation will not reduce this error. Only the
open/short/load compensation can minimize this error.
4.5.3 Cable extension with termination
Extending the test cables from the instrument with cable termination will not produce a large error
for the magnitude of measured impedance (because the effect of reflections is decreased.) However,
it causes a phase error in proportion to the extension length and measurement frequency. (In prac-
tice, an error for the magnitude of impedance also occurs because the actual cable termination is
not ideal.) Performing the open/short/load compensation at the end of the cable can eliminate this
error.
4.5.4 Error induced by shielded 2T or shielded 4T cable extension
When the 4TP test cables and the shielded 2T (or shielded 4T) extension cables are connected in
series as shown in Figures 3-13 and 3-14, the cable length limitation and measurement error (dis-
cussed in Sections 4.4.2 and 4.4.3) apply to the 4TP test cables only. The cable extension portion in
the shielded 2T or shielded 4T configuration does not cause the bridge unbalance, but produces
additional impedance measurement error. There are some error sources specific to the shielded 2T
or shielded 4T configuration (as described in Sections 3.1.2 and 3.1.4) in the cable extension por-
tion. In this case, different compensation methods are applied to the 4TP test cables and the cable
extension portion, respectively.
Agilent-supplied test cables should be used in order to apply the cable length correction to the 4TP
test cables. The cable length correction moves the calibration plane to the tip of the 4TP test cables
from the UNKNOWN terminals. To minimize errors, perform the cable length correction for the
Agilent test cables and then the open/short/load compensation at the tip of extension cables. When
the cable extension is sufficiently short and is used in the low frequency region, the open/short
compensation can be used in place of the open/short/load compensation. Note that the cable length
correction must be done to avoid the bridge unbalance caused by the phase shift of the measure-
ment signal in the 4TP test cables.
4-13