Agilent Technologies 4294A Specifications download pdf (Page 67)

4-6-3. Compensation method

As the error source model is different for the coaxial and non-coaxial sections of the test fixture,

compensation method is also different for each of them.

Electrical length compensation eliminates measurement errors induced by the phase shift in the coaxial

section. Agilent RF impedance analyzers and RF LCR meters facilitate the electrical length compen-

sation by allowing you to choose the model number of desired test fixture from among the displayed

list, instead of entering the specified electrical length of that test fixture to the instrument. (It is

also possible to input the specified electrical length value.)

Open/short compensation is effective for residuals in the non-coaxial section. It is based on the same

compensation theory as described for low frequency measurements. (Refer to paragraph 4-2-2 for

details.) The Yo and Zs can be known by measuring with the contact terminals opened and shorted,

respectively.

As the test fixture is configured with the coaxial and non-coaxial sections, both compensations are

required to minimize combined errors. Load compensation is not required for normal measure-

ments using Agilent supplied test fixtures.

When a test port extension or a user-fabricated test fixture is used, error sources will not match the

model assumed for the open/short compensation and affect measurement results. In such cases

that measurement errors cannot be sufficiently removed, consider attempting the open/short/load

compensation. Actually, the open/short/load compensation is substituted by the open/short/load

calibration using working-standard devices because these two functions are equivalent to each

other. Note that when the open/short/load calibration is executed at measurement terminals, the

test port calibration data is invalidated (because the calibration plane is moved.) Consequently,

measurement accuracy depends on the calibrated accuracy of the short and load working standard

devices (open calibration requires no device) as well as proper contact when these standard devices

are inserted into the test fixture. It is important to take special consideration for the precision of

the standard values, contact resistance and positioning of the standard device on the test fixture.

4-6-4. Precautions for open and short measurements in RF region

To discuss calibration and compensation issues, we need to consider how residual parameters have

large effects on measurement results at high frequencies.

Assume that, for example, a residual inductance of 0.1 nH and a stray capacitance of 0.1 pF exist

around the measurement terminals of the test fixture. Notice how the effects of these small residu-

als differ depending on frequency. Relationships of the residual parameter values to the typical

impedance measurement range are graphically shown in Figure 4-15. In low frequency region, the

residual parameter values are much smaller than the values of normally measured devices. It is

because the capacitors and inductors, which are designed for use in low frequency electronic equip-

ment, possess large values compared to small residuals. In high frequency region, however, such

devices as which are employed for higher frequency circuits and equipment have lower values. In

the frequency range typically above 100 MHz, the majority of the DUTs are low value devices (in the

low nanohenries and the low picofarads) and their values come closely to the values of the residu-

als.

4-15

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Agilent Technologies 4294A Specifications Page 67