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Audio / Telecoms Transformers

Overview

Audio transformers typically operate in the 20 Hz-20 KHz range and provide isolation and impedance matching between different parts of an audio circuit.

Transformers used in such environments are not only subject to safety standards for isolation (eg FCC 68-.304), but also to specific international standards such as FCC 68.310 and IEEE 455 which govern the common mode rejection ratio of transformers used in telephony systems.

The TY-306P is an audio data/voice-coupling transformer manufactured by Triad Magnetics.

Suggested testing

AT Editor Schematic

The AT Editor schematic here represents the transformer.

Measurements of Insertion Loss (ILOS) and Longitudinal balance (LBAL) require source resistors and load resistors to be fitted to the transformer to simulate the impedance of the transmission lines, so that it can be tested under “real world” conditions.
Measurements of Return Loss (RLOS) also require a load resistor to be present across pins 3-2.

In the shown schematic this is achieved by having an additional test leads for pins 7 and 8.
In our example, “7RES” is a test lead, also connected to pin 7, with a 300-Ohm resistor in series.
However, it connects to a different test node on the AT Tester.
This allows us perform “normal” tests on pin “7”, and then use “7RES” when we wish to test using the load resistor in situ.
It is important to note that both “7” and “7RES” connect physically to the same pin on the actual transformer.
When “7RES” is not being used, the internal isolation of the relays on the AT Tester nodes will remove “7RES” from having any effect on the other measurements made.

A similar situation occurs on Pin 6 with the associated “6RES”

There is also an additional 600-Ohm resistor fitted across nodes 2 and 3.
This is to provide the load resistor needed for ILOS, RLOS and LBAL.

The fixture also shorts pin 5 to pin 8 to enable the transformer to be tested as a whole entity.

AT Fixturing

The 6 pins of the transformer are ideally suited for Kelvin pin connections.

As stated before, the pins 5 and 8 are terminated independently on the transformer itself, but the test fixture has a permanent short introduced on these two pins to allow for combined primary testing, whilst still allowing for independent winding testing.

AT Test Program

View a PDF of the completed test program HERE
or download the actual ATP test program HERE
(can be opened with free AT Series Editor software, see DOWNLOADS HERE)

The program first checks the DC resistance of the 3 windings and also one additional check of the DC resistance of the combined primary across pins 7-6 to check correct winding and fixture insertion.

Next the standard TR test checks the whole primary to secondary, then one half of the primary to the other half of the secondary.

We now run a series of Voltech AT tests specifically designed for audio transformers
RLOS (Return Loss) measures the power reflected by the transformer (note that the 600 Ohm Load value is used in the test program)
ILOS (Insertion Loss) measures the power lost inside the transformer (note that the 600 Ohm Load and 600 Ohm Source load values are used in the test program, and nodes 7RES and 6RES are used to enable these resistances)
FREQ (Frequency response) then performs a range of ILOS tests over defined frequencies to check that the dB over frequencies has a flat response
LBAL (Longitudinal balance) then checks the CMRR of the part at the 3 points (200Hz, 1Khz and 4Khz) defined by the part specification.

Finally, the isolation is tested at the nominated 1500 Volts for 1 second.

#	Test	Description	Pins and Conditions	Reason
1	R	DC resistance	pin 2-3, test for < 200 Ohms	To check the winding resistance is below a maximum. Also acts as a check of correct wire gauge and good termination.
2	R	DC resistance	pin 7-8, test for <50 Ohms	To check the winding resistance is below a maximum. Also acts as a check of correct wire gauge and good termination.
3	R	DC resistance	pin 6-5, test for <50 Ohms	To check the winding resistance is below a maximum. Also acts as a check of correct wire gauge and good termination.
4	R	DC resistance	pin 7-6, test for <100 Ohms	To check the winding resistance is below a maximum. Also acts as a check of correct wire gauge and good termination.
5	TR	Turns Ratio	Energize pins 6-7,1V 1 kHz, check Turns ration 6-7:3-2 to be 1.2:1 -+ 2%	To check correct ratio of windings from all of primary to secondary
6	TR	Turns Ratio	Energize pins 6-5,1V 1kHz, check Turns ration 6-5:7-8 to be 1.2:1 -+ 2%	To check correct ratio of windings from P1 to P2
7	RLOS	Return Loss	Pin 7-6 , 5V 3Khz, Real impedance = 600 Ohms (this is the resistor across 2-3), check that RLOS > 10dB as per spec sheet	This test measures the power reflected by the transformer, compared to the transmitted power. As such, Large RLOS values equate to better power transfer and lower losses.
8	ILOS	Insertion Loss	Input pins 6RES-7RES (so that the 2 x 300 ohm resistors are used). Output pins 3-2. Source R = 600 Ohms, Load =600 Ohms, test at 5V, 3kHz. Check ILOS < 1.5dB	This test measures the power lost inside the transformer. It is expressed as theoretical power over real power, so an ideal transformer will have a low ILOS number
9	FREQ	Frequency Response	Input pins 6RES-7RES (so that the 2 x 300 ohm resistors are used). Output pins 3-2. 5V 2 KHz as nominal, check to 500Hz, 1kHz, 3kHz, 4kHz. Limits +/- 0.5dB as per spec.	This test checks the performance if ILOS over a user-selected range of frequencies compared to one nominated frequency. Results are expressed as the largest deviation from nominal, hence a large result signifies poor frequency response.
10	LBAL	Longitudinal Balance	Hi ; 6RES-7RES, LO ; 3-2, Common ; CORE . 5V, 200 Hz. Check for > 60 dB, as per spec.	This test checks the common mode performance of the transformer. The 2x 300 Ohm resistors are used to simulate the source line impedance, and the 600 Ohm is still present as the load impedance. A high figure represents good rejection of common mode signals
11	LBAL	Longitudinal Balance	as above, but at 1Kz, Check for > 60 dB as per spec.	as above to check performance at 1Khz
12	LBAL	Longitudinal Balance	as above, but at 4Kz, Check for > 45dB as per spec.	as above to check performance at 4kHz
13	HPAC	AC Hi-Pot	1kV AC, 1 second, Pins 1&4 High, Pins 2,3 Lo	To check isolation as per datasheet.
				AT5600 Run time 2.60 sec
				(AT3600 Run time 10.20 sec)

AT Test Results

Links / References

- Triad Magnetics
- TY-306P Datasheet
- Voltech Article on audio testing
- Wikipedia article on Return Loss
- Wikipedia article on Insertion Loss
- Wikipedia article on Common mode (Longitudinal Balance)

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Automatic Testers For Transformers : The "AT Series"

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Applications

Real Life Test Examples

AT5600 Guides

Fixturing Examples

Audio / Telecoms Transformers

Overview

Suggested testing

AT Editor Schematic

AT Fixturing

AT Test Program

AT Test Results

Links / References