Agilent Technologies 4294A Specifications download pdf (Page 114)

A-2

The equation for the test fixture’s additional error is shown below:

Ze = ± { A + (Zs/Zx + Yo•Zx) × 100} (%)

De = Ze/100 (D ≤ 0.1)

Ze : Additional error for impedance (%)

De : Additional error for dissipation factor

A : Test fixture’s proportional error (%)

Zs/Zx × 100 : Short offset error (%)

Yo•Zx × 100 : Open offset error (%)

Zs : Test fixture’s short repeatability (Ω)

Yo : Test fixture’s open repeatability (S)

Zx : Measured impedance value of DUT (Ω)

Proportional error, open and short repeatability are mentioned in the test fixture’s operational man-

ual and in the accessory guide. By inputting the measurement impedance and frequency (propor-

tional error, open and short repeatability are usually a function of frequency) into the above equa-

tion, the fixture’s additional error can be calculated.

Proportional error:

The term, proportional error (A), was derived from the error factor, which causes the absolute

impedance error to be proportional to the impedance being measured. If only the first term is taken

out of the above equation and multiplied by Zx, then ∆Z = A•Zx (Ω). This means that the absolute

value of the impedance error will always be A times the measured impedance. The largeness of pro-

portional error is dependent upon how complicated the test fixture’s construction is. Conceptually,

it is dependent upon the stability of each element of the fixture’s equivalent circuit model. From pre-

vious experience, proportional error is proportional to the frequency squared.

Short offset error:

The term, Zs/Zx × 100, is called short offset error. If Zx is multiplied to this term, then ∆Z = Zs(Ω). It

can be concluded that this term affects the absolute impedance error, by adding an offset. Short

repeatability (Zs) is determined from the variations in multiple measurements of the test fixture in

short condition. After performing short compensation, the measured values of the short condition

will distribute around 0 in the complex impedance plane. The maximum value of the impedance vec-

tor is defined as short repeatability. This is shown in the figure below. The larger short repeatability

is the more difficult it is to measure small impedance values. For example, if the test fixture’s short

repeatability is 100 mΩ, then the additional error of an impedance measurement under 100 mΩ will

be more than 100 %. In essence, short repeatability is made up of a resistance and an inductance

part, which become larger as the frequency becomes higher.

Definition of short repeatability

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Agilent Technologies 4294A Specifications Page 114