7.1.17 GBAL - General Longitudinal Balance
The general longitudinal balance test is intended 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. To overcome the differences of three standard methods of measuring the CMRR the GBAL dialogue box has a separate group of terminals X and Y for the first and second measurements respectively. The preferred method of measurement is the LBAL test for which Voltech provide a separate test specifically for this measurement.
This test is suitable for audio & telecommunication transformers and checks the effective common mode rejection ratio of the transformer.
MEASUREMENT CONDITIONS AND TYPES
Unfortunately, there are several published specifications for measuring longitudinal balance. These are all different, and potentially give different results for the same transformer.
To allow each user the freedom to test to his preferred method, the GBAL test has been configured with the greatest flexibility. It therefore consists of two separate measurements which the user can program, and the result is the ratio between the two expressed in dB.
For the reasons outlined above, the test consists of two separate user programmed measurements, each with its own ‘energised’ and ‘measured’ terminals. This approach is illustrated below with reference to three standard methods:
1. Preferred method (the method of the LBAL test, used by many transformer manufacturers)
2. IEEE 455 method (used in Canada, Europe and ROW)
3. FCC 68.310 method (used in USA)
The source and load resistors (and the fixture transformer for the IEEE and FCC methods) 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 GBAL test. (The relay patterns associated with inserting and removing fixture resistors have deliberately not been included in the GBAL 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).
After the two measurements have been taken, the longitudinal balance is calculated from the ratio of the two outputs:
GBAL = 20 log | VM1 / VM2 |
+ (optionally) compensation for test fixture scaling