Longitudinal (Transverse) Balance is a figure of merit of a transformer winding's symmetry with respect to ground which results in the hum, imbalance related noise and crosstalk rejection of a transformer on balanced lines. It is the ratio, expressed in decibels, of the metallic (differential) signal with respect to the longitudinal (common mode) signal appearing between the transformer's winding and ground. Good balance is an integral part of analog and digital telecommunications worldwide. Transformer design and manufacturing techniques critically determine balance performance. (Balance is, however, not a strong function of a transformer's DC current.)

Two methods commonly used to measure balance are ANSI/ IEEE Standard 455-1985, (American National Standards Institute/Institute of Electrical and Electronic Engineers) seen in Figure 1, and FCC 68.310 (Federal Communication Commission), as in Figure 2. CCITT Rec.0.121 (International Telephone and Telegraph Consultative Committee) embraces both methods, and offers some additional definitions of balance. Equipment specifications should reflect the balance test method to be used as the dB figures are different for each procedure. Specifications should also include ground connection details.

Historically, in Canada, Europe, and most of the world, a method similar to the ANSI/IEEE 455 "L  M" method has prevailed. It applies a longitudinal signal simulating an undesired common mode signal then measures the conversion within the EUT (Equipment Under Test) to metallic, "L  M" (longitudinal to metallic). Wurth Electronics Midcom's 671-0244 may be used to assemble the ANSI/IEEE 455 circuit illustrated in Figure 1, for voice-band measurement. Wurth Electronics Midcom's 671-5767 may be substituted for the 671-0244 for measurements with this circuit at higher frequencies.

In the USA, the FCC Method "M L" (metallic to longitudinal) applies a metallic reference signal, then measures the conversion within the EUT to longitudinal, expressing the undesired result as an absolute dB ratio with respect to the reference. FCC Part 68.310 shows this as "M  L" (metallic to longitudinal). Figure 68.310(a) or Figure 2, when assembled with Wurth Electronics Midcom's 671-0323, may be used for voice-band measurement. This method is also known as transverse balance, and will become standard in both Canada and the USA via recent US/Canadian harmonizations of telecom requirements.

Wurth Electronics Midcom has developed a high-frequency version of the Figure 68.310(a) circuit using Wurth Electronics Midcom's 671-5767 for use from upper voiceband through 1.544 MHz. (See Wurth Electronics Midcom application note AD-671-5767 for details).

*Figure 2 DC Loop Bias (if required) may be sourced through a balanced battery feed inductor such as Wurth Electronics Midcom 671-4130 from an unbalanced lab power supply. Floating battery power (independent of mains and preferably chemically derived) may be required for extremely critical balance measurements.