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Agilent AN 346-2 Balanced Circuit Measurement with an Impedance Analyzer/LCR Meter/Network Analyzer Application Note Introduction Measuring a balanced circuit with an Selecting a Balun How a balanced circuit differs from an unbalanced measurement instrument There are several types and brands of unbalanced circuit A balanced circuit cannot be directly balun transformers. When selecting a A balanced circuit has its electrical measured with an unbalanced meas- balun, ensure that frequency is com- midpoint grounded. An unbalanced urement instrument because of the patible with your measurement circuit, however, has one side ground- difference in their configuration. requirements. When you measure the ed. A balanced circuit is typically impedance parameters of a balanced used in communications equipment When measuring balanced circuits, circuit, you don't have to use the because a balanced circuit has the the unbalanced measuring instrument balun which has the same impedance advantage of better spurious noise requires a balun (balanced to unbal- with the circuit under test. However, suppression. anced) transformer. A balun is a type when you measure the transmission of impedance-matching RF trans- or reflection of it, you have to use a Figure 1 shows a balanced cable former. balun which has the same impedance which is an example of a balanced with the circuit under test to keep circuit. The voltages of the cable's two Figure 3 shows the configuration for impedance matching. Table 1 shows conductors are at every point equal measuring a balanced circuit with an recommended balun transformers. in amplitude and opposite in phase. unbalanced instrument. Figure 2 shows an unbalanced cable which is an example of an unbal- Note: In balanced cable measure- anced circuit. Most measurement cir- ments, residual current in the balun cuits in Agilent Technologies' imped- or the measuring instrument can ance analyzers and LCR meters are cause measurement errors. To reduce unbalanced. the degree of error, perform open/short and load compensation at the measurement terminals of the +V balun. -V +V