What is a signal transformer?

 Signal transformer is a term of railway science and technology announced in 1997.

 A transformer consisting of a left wire wrap, a right wire wrap and an iron core, characterized in that: the signal transformer also includes an iron core tie, an iron core tie fastening ring, a stud, a lower bracket, an auxiliary bracket, a resonator Components such as AC capacitors, terminal boards, upper brackets, coil leads, wire wrap skeleton I secondary coils, II secondary coils, and polyester film insulation.



1. The concept and function of signal transformer
  1) The signal transformer is generally called a network transformer, also known as a data pump. In the Ethernet circuit, it mainly connects the PHY chip and the network port, and acts as a bridge between the two.

  2) The main function of the signal transformer:
      1) Transmission signal: The differential signal sent by the PHY chip is filtered by the differential mode coupling coil to enhance the signal, and is connected to the other end of the network cable through the conversion coupling of the electromagnetic              field.
      2) Electrical isolation: It mainly isolates the different levels between different network devices of the network cable. The network cable and the PHY chip do not need a physical level connection to transmit the signal, which cuts off the DC component              in  the signal.
      3) Radiation suppression: Due to the existence of the common mode inductance of the signal transformer, a magnetic field in the same direction will be generated in the coil to increase the inductive reactance of the coil, so that the coil exhibits a high            impedance and produces a strong damping effect to attenuate the common mode current signal. Details can be found later.
      4) Lightning protection: The signal transformer itself is designed to withstand 2KV-3KV high voltage.

2. The structure of the signal transformer

Signal transformer mainly includes center tap, transformer and common mode inductor

The transformer topology is shown below:


1. Isolation transformer:
Mainly the role of secondary isolation, level isolation is realized by this component.
2. Common mode coil:
Differential mode currents flow through the windings of the common mode choke coil in opposite directions, creating equal and opposite magnetic fields, which can cause the outputs to cancel each other out, which makes the common mode choke coil
The impedance to the differential mode signal is zero, and the differential mode signal can pass through the common mode choke coil unimpeded. The impedance of the common mode inductor is related to the core material, core size, and number of turns.


The common-mode current flows through each side of the common-mode choke coil in the same direction, creating a superimposed magnetic field of equal magnitude and phase, presenting a high impedance to the common-mode signal. Common Mode Rejection Capability
Depends on the ratio of common mode choke impedance and load impedance.


Therefore, the role of the common mode coil is to attenuate the common mode signal and improve EMI.
3. Autotransformer:
From a differential mode signal, a center-tapped autotransformer is two bipartite windings that are identical in phase. This means that differential mode currents form a magnetic field in them, making them high impedance to common mode currents
It is equivalent to connecting a high-resistance resistor in parallel to the differential mode signal, which has no effect on the magnitude of the differential mode signal.


From the point of view of the common-mode signal, the center-tapped autotransformer is two bisected windings with opposite phases, which means that the common-mode current will form an equal and opposite magnetic field in it, and this magnetic field will cause the common-mode current to of the outputs cancel each other out. It presents a zero-impedance effect to the common-mode signal, making the common-mode signal short-circuit directly to ground.


Therefore, the role of the autotransformer is to introduce common mode noise into the ground plane and improve EMI. In addition, since the common mode inductance will affect the resistance value between the differential lines when the signal transformer is on the network port side, the autotransformer also has a certain impedance matching function.
However, due to cost reasons, autotransformers are not necessarily used.
The improvement of EMI is generally as follows: three-wire choke coil > double choke coil > three-wire common mode (the input of the center tap is also synchronously wound on the common mode inductance coil to cancel the influence of the signal current in the common mode coil) > single choke
4. Center tap:
(1) Provide a low-impedance return path for common-mode noise on differential lines, reducing common-mode current/voltage
(2) Provide a DC bias voltage or power source, such as POE devices
5. The key parameters of the signal transformer
 (1) Leakage Inductance
The magnetic field lines cannot be fully coupled between the primary and secondary of the isolation transformer, and the past energy cannot be coupled and stored in the "leakage inductance".
 (2) Return loss - Return Loss
The ratio of the reflected signal Vr to the incident signal Vi of the signal transformer inserted in the transmission system
Return loss=20*log(Vr/Vi)
 (3) Insertion Loss - Insertion Loss
The ratio of the output signal Vo of the signal transformer to the incident signal Vi
Insertion loss=20*log(Vo/Vi)
 (4) Crosstalk - Cross Talk
The ratio of the signal V1 in one of the two unit circuits to the signal V2 induced to the other unit circuit
Crosstalk=20*log(V2/V1)
(5) Common Mode Rejection Ratio - CMRR
6. The frequency characteristic of the signal transformer is a band-pass filter, and the frequency characteristic is mainly affected by insertion loss, OCL, leakage inductance and distributed capacitance
7. The connection methods of signal transformers are divided into two types: voltage-driven type and current-driven type. Among them, the center-tapped capacitor to the ground is the voltage-driven type, and the current-driven type is the center-tapped capacitor connected to the power supply, and the differential line pair is connected to 100ohm
Resistors, which can be obtained by connecting two 49.9ohm resistors in series, each on a differential pair.

3. Signal transformer and EMI
1. Causes of EMI problems
(1) Common mode noise is the main and most direct cause of EMI, including signal transformer imbalance, including center tap balance and common mode inductance balance; crosstalk; non-ideal reference surface
(2) Because of the existence of differential mode-common mode conversion, differential mode signal may also be one of the indirect causes of EMI
(3) Magnetic core material, magnetic ring arrangement, winding method, strand density, etc.;
(4) PCB layout and wiring and the overall situation of the PCB, etc.
2. Bob Smith circuit
The Bob Smith circuit comes from Bob Smith, and its main function is to provide 150ohm impedance matching between any two pairs of differential lines of the network port, and at the same time, it can provide a return path for common mode signals.


4. Lightning protection design of signal transformer
The lightning protection of the network port and signal transformer related circuits is very important to ensure the safety of users and products.
There are two main ideas: (1) Provide a discharge path for lightning current
                                           (2) Use the insulation withstand voltage of the signal transformer to isolate the high voltage at the primary of the signal transformer.
General recommendations are as follows:
1. Ensure the impedance matching of the differential line pairs of the network signal, try to be equal in length and equidistant, avoid vias and crosses, etc., and control the characteristic impedance at 100ohm±10%;
2. The high-voltage capacitor in the Bob smith circuit selects a capacitor with a withstand voltage higher than 2KV, and the connection between the resistor and the high-voltage capacitor should be short and thick, ensuring more than 15mil;
3. In order to ensure the withstand capability of common mode isolation voltage, the signal transformer needs to meet the standard of AC insulation withstand voltage between primary and secondary not less than 1.5KV;
4. Since the common mode protection is accomplished through the isolation characteristics of the signal transformer, the high-voltage signal lines (differential lines and Bob Smith circuit wiring) and other signal lines (Rj45 network port)
Sufficient insulation should be ensured between the indicator light control wire), the power wire and the ground wire, and there should be no accidental discharge paths;
5. The distance between Rj45 and the signal transformer should be as short as possible, and the layout of related devices should be placed according to the signal flow direction, do not go around;
6. The filter capacitor from the center tap to the ground should be placed as close as possible to the pin of the signal transformer;
7. The power line and control line of the Rj45 network port indicator should be close to the wiring to minimize the loop area. The control line and the network port wiring should be kept at a distance of 80mil as far as possible.
8. The ground should be divided on both sides of the signal transformer. At the same time, the isolation ground between the signal transformer and the Rj45 side should be prohibited. There should be no power layer and ground layer.