Agilent Technologies 4294A Specifications download pdf (Page 51)

3-8. Test port extension in RF region

In RF measurements, connect the DUT closely to the test port to minimize additional measurement

errors. When there is an unavoidable need of extending the test port, such as in-circuit testing of

devices and on-wafer device measurement using a prober, make the length of test port extension as

short as possible. If the instrument has a detachable test head, it is better for accuracy to place the

test head near the DUT in order to minimize the test port extension length, and interconnect the test

head and the instrument using coaxial cables. (Observe the limit of maximum interconnection cable

length specified for instrument.) Using a long test port extension will involve large residual imped-

ance and admittance of the extension cable in the measurement results and significantly deteriorate

the accuracy even if calibration and compensation are completed.

Figure 3-15 shows an equivalent circuit model of the port extension. The inductance, Lo, resistance,

Ro, capacitance, Co, and conductance, Go, represent the equivalent circuit parameter values of the

extension cable. When the DUT’s impedance, Zx, is nearly 50 Ω, the test signal is mostly fed to the

DUT as the cable causes only a phase shift and (relatively small) propagation loss like a transmis-

sion line terminated with its characteristic impedance. However, most likely DUTs have different

value from 50 Ω. If the impedance of the DUT is greater than that of Co, the test signal current

mainly bypasses through Co, flowing only a little through the DUT. Conversely, if the impedance of

the DUT is lower than that of Lo and Ro, the test signal voltage decreases by voltage drop across the

cable and is applied only a little to the DUT. As a result, the cable residuals lead to measurement

inaccuracy and instability, particularly, in high impedance and low impedance measurements. As

illustrated in Figure 3-15, the Lo, Ro, Co and Go not only get involved in the measurement results

(before compensation), but also affect measurement sensitivity. Note that the measurable imped-

ance range becomes narrow due to port extension even though the calibration and compensation

have been performed appropriately.

In addition, electrical length of the extension cable will vary with environmental temperature and,

thereby causing phase measurement instability. Using longer extension makes measurement results

more susceptible to the influence of environmental temperature changes. Bending the cable will

also cause variance in measured phase angle, deteriorating measurement repeatability.

Accordingly, in any applications the port extension should be minimized. Discussion for practical

method of extending test port follows.

The RF I-V and network analysis instruments commonly employ an N-type or 7 mm type coaxial

connector as the UNKNOWN terminal. Naturally, test port extension is made using a low loss, elec-

trically stable coaxial transmission line (cable) with 50 Ω characteristic impedance. When choosing

the cable, care should be taken for both temperature coefficients of propagation constants and rigid-

ity to restrain the cable from easily bending. Figure 3-16 shows an example when the test fixture is

connected at the end of a 7 mm-7 mm cable. Calibration should be performed first at the end of the

extension before connecting the test fixture. As the next step the electrical length and open/short

compensations for the test fixture can be performed. (Open/short/load calibration may be per-

formed instead of compensation with working standards connected at test fixture’s measurement

terminals. This method does not require the calibration at the end of the extension.) Detailed dis-

cussion for measurement error sources, calibration and compensation is provided in Section 4.

3-15

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Agilent Technologies 4294A Specifications Page 51