Lbal - Longitudinal Balance - Voltech AT5600 User Manual

C H A P T E R 7 – T E S T S
7.20. LBAL – Longitudinal Balance
Where Used
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.
Measurement Conditions
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 differential
signal, and b) with the input applied as a common mode voltage relative to a
common Low point, which could be the center tap of the secondary winding, the
transformer core or an interwinding screen. In each case the output is measured,
and the longitudinal balance is calculated from the ratio of the two outputs.
Please also see
14.6.16 for specific advice on programming the test, and
10.2.20 for test accuracy specification
AT5600 User Manual 98-119 issue 14
A N D T E S T C O N D I T I O N S
LBAL = 20 Log [Vm1 / Vm2]
Page 129