Agilent Technologies 4294A Specifications download pdf (Page 37)

SECTION 3

Fixturing and cabling

When interconnecting a device under test (DUT) to the measurement terminals of the auto balancing

bridge instrument, there are several connection configurations to choose from. This section will

introduce the basic theory and use of each connection configuration focussing on the auto balancing

bridge instrument. In RF impedance measurements, the usable connection configuration is the two

terminal method only. Since the measurement technique for RF impedance is different from that

for LF, it is described separately after the discussion of the auto balancing bridge instrument.

3-1. Terminal configuration

An auto balancing bridge instrument is generally equipped with four BNC UNKNOWN terminals (Hc,

Hp, Lp and Lc) on its front panel. There are several connection configurations used to interconnect

a DUT to the UNKNOWN terminals. Because each method has advantages and disadvantages, the

most suitable method must be selected based on the DUT’s impedance and required measurement

accuracy.

The two-terminal (2T) configuration is the simplest way but contains many error sources. Lead induc-

tance, lead resistance, and stray capacitance between two leads have been added to the measure-

ment result (Figure 3-1). Because of the existence of these error sources, the typical impedance

measurement range (without doing compensation) is limited to 100 Ω to 10 kΩ.

The three-terminal (3T) configuration employs coaxial cables to reduce the effects of stray capacitance.

The outer conductors (shield) of the coaxial cables are connected to the guard terminal.

Measurement accuracy is improved on the higher impedance measurement range but not on lower

impedance measurement range because lead inductance and resistance still remain. (See Figure 3-

2.) The typical impedance range will be extended above 10 kΩ. If the outer conductor is connected

as shown in Figure 3-2 (d), lower impedance measurement accuracy is a little improved. (Shielded

2T configuration.)

The four-terminal (4T) configuration can reduce the effects of lead impedances because the signal cur-

rent path and the voltage sensing cables are independent (Figure 3-3). Accuracy for the lower

impedance measurement range is improved typically down to 1 Ω. When the DUT’s impedance is

lower than 1 Ω, a large signal current flows through the current path and mutual (M) coupling to the

voltage sensing cable will cause an error.

The five-terminal (5T) configuration is a combination of the 3T and 4T configurations. It is equipped

with four coaxial cables and all of the outer conductors of the four cables are connected to the guard

terminal (Figure 3-4). This configuration has a wide measurement range from 1 Ω to 10 MΩ, but the

mutual coupling problem still remains. If the outer conductor is connected as shown in Figure 3-4

(d), lower impedance measurement accuracy is a little improved. (Shielded 4T configuration.)

3-1

1 2 ... 32 33 34 35 36 37 38 39 40 41 42 ... 125 126

No comments

Agilent Technologies 4294A Specifications Page 37