Specifications

In addition, electrical length of the extension cable will vary with environmental temperature,
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 application the port extension should be minimized.
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, consideration should be given to temperature coefficients of propagation constants and
rigidity to restrain the cable from easily bending. Figure 3-20 shows an example of the test fixture
connected at the end of a 7 mm-7 mm connector cable. Calibration should be performed first at the
end of the extension before connecting to the test fixture. Next, the electrical length and open/short
compensations for the test fixture can be performed. (Alternatively, instead of the compensation,
the open/short/load calibration may be performed with working-standards connected at the test
fixture's measurement terminals. This method does not require the calibration at the end of the
extension.) A detailed discussion on measurement error sources, calibration, and compensation is
provided in Section 4.
Figure 3-20. Practical calibration and compensation at extended test port
3-20