Agilent Technologies 4294A Specifications download pdf (Page 66)

When the test port is extended, calibration should be performed at the end of extension cable, as

discussed in section 3. Thereby, the calibration plane is moved to the end of cable.

To perform measurements met to specified accuracy, the instrument should be calibrated before

measurement is initiated and each time the frequency setting is changed. The calibration defines

the calibration reference plane at which measurement accuracy is optimized.

If a component could be measured directly at the calibration plane, it would be possible to obtain

measured values within the specified accuracy of the instrument. However, the real-world compo-

nents cannot be connected directly to the calibrated test port and, a suitable test fixture is used for

measurements. Calibration is not enough to measure the DUT accurately. Because measurement is

made for the DUT connected at the contact terminals of the test fixture (different from calibration

plane), the residual impedance, stray admittance and electrical length that exist between the cali-

bration plane and the DUT will produce additional measurement errors. As a result, compensation

is required to minimize those test fixture induced errors.

4-6-2. Error source model

Regarding ordinary non-coaxial test fixtures, consider an error source model similarly to that in low

frequency measurements. Figure 4-14 (a) illustrates typical test fixture configuration and a model

of error sources. The test fixture is configured with two electrically different sections: A coaxial

connector section and a non-coaxial terminal section for connecting DUT. The characteristic of the

coaxial section can be modeled using an equivalent transmission line (distributed constant circuit)

and represented by propagation constants. Normally, as the coaxial section is short enough to

neglect the propagation loss, we can assume that only the phase shift (error) expressed as electrical

length exists. The characteristic of the non-coaxial section can be described using the residual

impedance and stray admittance model in two-terminal measurement configuration as shown in

Figure 4-14 (b). We can assume residual impedance, Zs, in series with DUT and stray admittance,

Yo, in parallel with DUT.

Figure 4-14. Typical error source model

4-14

1 2 ... 61 62 63 64 65 66 67 68 69 70 71 ... 125 126

No comments

Agilent Technologies 4294A Specifications Page 66