Agilent Technologies 4294A Specifications download pdf (Page 99)

5-10. Balanced device measurement

When a balanced DUT (such as balanced cable or the balanced input impedance of a differential

amplifier) is measured, it is necessary to connect a “balun” (balance-unbalance) transformer

between the instrument and the DUT. Looking from the DUT side, the UNKNOWN terminals of the

impedance measurement instrument are in an “unbalanced” configuration. Figure 5-35 (a) shows

an example of an auto balancing bridge instrument. Its Low terminal is considered a “virtual

ground” because it is held at approximately a 0 V potential. When a 1:1 balun transformer is con-

nected as shown in Figure 5-35 (b), the instrument can measure a balanced DUT directly.

An actual balun transformer has a limited frequency range. The measurement must be made within

its frequency range. In addition to Agilent’s balanced/unbalanced converters, various types of com-

mercial bulan transformers are available for various frequency ranges. To select the appropriate

balun transformer, check the frequency range and the impedance of the transformer’s balanced

(DUT) side. Its impedance should be close to the characteristic impedance of the DUT. The imped-

ance of the unbalanced side should be 50 Ω or 75 Ω as appropriate for the measurement instrument.

Open/short/load compensation for the balun transformer is required when the turns ratio of the

balun transformer used is not 1:1, or when an accurate measurement is needed. Open/short com-

pensation is not adequate because the balun transformer will produce both magnitude (|Z|) and

phase errors due to its transfer function characteristic. The terminal connectors of balanced side

should be connectable for both the standard devices used for open/short/load compensation and the

DUT. Figures 5-36 (a) through (d) show an example of an actual balun configuration and compensa-

tion.

Figure 5-35. Balanced device measurement

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Agilent Technologies 4294A Specifications Page 99