Voltech AT3600 - Page 317

VOLTECH AT3600 USER MANUAL TRANSFORMER TESTS
TESTS AND TEST CONDITIONS VPN 98 - 024 PAGE 7.1.37.
1.18. 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 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.
R
S
R
S
= V
I
LBAL = 20 log | V
O2
/ V
O1
|
R
S
R
L
R
S
V
O1
R
L
V
O2
V
I