7.1.18 LBAL - Longitudinal Balance
The longitudinal balance test is Voltech's preferred method to measure what is effectively the Common Mode Rejection Ratio of a transformer designed to connect to a balanced line. Two measurements are performed each by applying a voltage to the transformer and measuring the resulting voltage to calculate the CMRR. This test is suitable for audio & telecommunication transformers and checks the effective common mode rejection ratio of the transformer.
LBAL = 20 log | VO2 / VO1 |
The source and load resistors are assumed to be on the fixture, but are not shown on the editor schematic. They are switched in circuit using an OUT test that must be inserted in the program before the LBAL test. (the relay patterns associated with inserting and removing fixture resistors have deliberately not been included in the LBAL test dialogue for two reasons: a) it would make the dialogue too complicated, and b) it is not always necessary - e.g. in the case of a fixture where the resistors are permanently fitted in circuit).
The test consists of two measurements: a) with the input applied as a common mode voltage relative to a common lo point, which could be the centre tap of the primary winding, the centre tap of the secondary winding, the transformer core or an interwinding screen, and b) with the same input applied as a differential signal. In each case the output is measured, and the longitudinal balance is calculated from the ratio of the two outputs.