Audio and Telecommunications Transformer Testing - Common Questions
My audio and telecommunications transformers need to be connected to a
balanced line. What test do I need to perform to ensure the necessary
What is needed here is a test for longitudinal balance, a measure of the
common mode rejection ratio (CMRR) of a transformer. This is defined as
the ability of the transformer to reject unwanted noise signals that are
common to both input terminals with respect to a common point. In an ideal
world, a transformer would have infinite CMRR. However, in practice,
differences in symmetry of transformer construction mean that input common
mode signals appear as unwanted output voltages.
How does a longitudinal balance test work?
Figure 1 shows a basic longitudinal balance test in which a load
resister, RL is placed across the transformer output and two source
resistors (RS) are placed at the transformer inputs. First the test
voltage Vin is applied as a common mode signal and the output voltage
Vout1 is recorded. The same Vin is then applied as a normal input voltage
and the output Vout2 is recorded. Longitudinal balance is then calculated
as the ratio:
LBAL = 20 log |(Vout2/Vout1)|
Would the standard longitudinal balance test be suitable to confirm the
CMRR of audio and telecommunications transformers required to meet
standards such as IEEE455 and FCC68310?
In order to assess transformers to the IEEE455, FCC 68.310 and other
similar standards, a slightly different version of the basic longitudinal
balance test is required. This is known as the general longitudinal
balance test. In the case of IEEE455, the source and load resisters RS and
RL are connected in the same way as in the previous test but a measuring
(fixture) transformer is also added (see Figure 2). A measuring
transformer and load resistor are also used for testing against FCC 68.310
(Figure 3), but the source resistors are not required.
In each case, voltage Vout1 and Vout2 are measured while Vin is kept
constant. The ratio of these two voltages reflects the ability of the
transformer under test to reject common mode voltages. General
longitudinal balance is calculated as the ratio:
GBAL = 20 log |(Vout2/Vout1)|
I have been told I need to conduct an insertion loss test as part of our
quality process. Why is this?
This test is often specified as it helps to ensure that the transformer
has been assembled using the correct core and winding materials. Insertion
los is a measure of the power that is lost by a transformer relative to
the maximum theoretical power that the device should transmit to a given
load. Core and winding resistance losses mean that some power is always
consumed by a transformer, reducing the power available to the load from
the theoretical maximum.
How do I conduct an insertion loss test?
Source and load resistors (Rs and RL) are connected to the transformer
under test as shown in Figure 4. The Specified voltage, Vin is then
applied to the transformer and the output voltage, Vout is measured. The
insertion loss (ratio of the actual to the theoretical power loss) can
then be calculated using the formula:
ILOS = 10log (Vin2RL/4Vout2RS).
A complementary measurement that may often be required at the same time as
insertion loss is a test for return loss (RLOS). Unlike insertion loss
where power lost within the transformer is measured, the return loss test
is used to assess the power that is returned to the input by the
Are there any other tests that might be particularly applicable to audio
and telecommunications transformers?
Where transformers are to be used with transmission lines of specified
impedance it will often be necessary to conductance an impedance test. The
impedance of a transformer is usually complex as it consists of real
(resistive) and imaginary (inductive or capacitive) elements. The total
impedance at a specified frequency is the vector sum of these parts and is
expressed as Z=√(R2 + X2) where R and X are the real and imaginary
In the production environment I will need to carry out my tests at high
speed – how can I automate the testing process?
The advent of modern, single-station wound component testing systems has
helped to automate and simplify the transformer testing process, while
eliminating the need for OEMs to purchase, configure and maintain a
variety of disparate test equipment. To date, however, many audio and
telecommunications tests have not been available on such platforms.
Voltech addressed this situation by launching a number of tests that are
specifically designed for line isolation transformers and that can be used
on the company’s ATi and AT3600 single-station transformer testers. These
tests allow users to measure longitudinal and general balance in a
measurement range of 0 to 100dB, with test voltages of 1mV to 5V and test
frequencies ranging from 20Hz to 1MHz. Insertion losses can be measured
within the same test voltage and frequency boundaries in a measurement
range of -100dB to 100dB, while the impedance test range is 1mohm to
1Mohm. Basic accuracy for these measurements is as good as 0.05%.
All of the tests can be specified with new AT5600 testers or can be
purchased as simple software upgrades by existing users.
All tests can be configured and executed via PC-based transformer test
editor software and test programs can be archived onto a server’s disk for
The versatile AT5600 transformer tester already provides an unrivaled
range of tests for checking the construction and performance of a wide
range of coils and transformers. By following the above guidelines, users
can also seamlessly integrate the testing of cores and transformers under
constant ac current conditions into the AT3600 environment, providing
high-speed PASS / FAIL testing and accurate, detailed test results for