Specifications

3.1.5 Four-terminal pair configuration

The four-terminal pair (4TP) configuration solves the effects of mutual coupling between the leads

by employing the following techniques either 1) and/or 2).

1) The outer shield conductors work as the return path for the test signal current (they are not

grounded). The magnetic fields produced by the inner and outer currents cancel each other out

because of the opposite directions and same amount of current flow. Hence there is little

inductive magnetic field, test leads do not contribute to additional errors due to self or mutual

inductance between the individual leads (Fig 3-8. (e))

2) A vector voltmeter measures the differential voltage between the inner and outer conductors. The

differential measurement method can minimize the influence of the mutual inductance. (Fig 3-8. (a))

As a result, the mutual coupling problem is eliminated. The 4TP configuration can improve the impedance

measurement range to below 1 mW. The measurement range achieved by this configuration depends on how

well the 4TP configuration is strictly adhered to up to the connection point of the DUT.

Note: If the shielding conductors of the coaxial test cables are not interconnected properly at the

ends of the cables, the 4TP configuration will not work effectively and, as a result, the

measurement range will be limited, or in some cases, measurements cannot be made.

Figure 3-8. Four-terminal pair (4TP) configuration

(a) Typical schematic diagram

DUT

Null

detector

Return current

Test signal current

(b) Connection image

Do not interconnect the outer

shielding conductors of the cables

at UNKNOWN terminals side

Connect here

(d) Typical impedance

measurement range

DUT

Test current

(through inner cable)

Return current

(through outer shield)

(e) Cancellation of magnetic fluxes

Magnetic flux generated by

return current

Magnetic flux generated by

test current

1 m 10 m 100 m 1 10 100 1 K 10 K 100 K 1 M 10 M (Ω)

3-6