Choke filtering, EMI and ethernet
Common mode chokes filter common mode EMI currents without causing signal degradation and without derating under high currents. These components are applied to pairs of conductors - supply and return.
The addition of a common mode choke before the connector, filters common mode currents while allowing the desired signals to pass unaffected. The result is effective communication between devices, reliable product and system operation and a product that complies with EMI regulations.
These chokes are highly suitable components for EMI filtering of power and signal lines. They withstand high DC currents without degradation of filtering performance that can occur with differential mode filters, like small chip beads.
Stable chokes allow most signals to pass unaffected, yet filter the noise (EMI) from these circuits.
Most well-designed power and signal circuits present no EMI that is caused from intended currents. For example, at 30 MHz, a pair of traces or wires 1 m long, separated by 1.3 mm, requires over 20 mA differential current imbalance to exceed 100 mV/m radiation 3 m away.
However, unintended, unforseen common mode currents can exceed 100 mV/m radiation with only 8 mA common mode current flow.
Suppression of these tiny common mode currents is often crucial to ensuring EMI regulatory compliance and reliable product performance.
Accompanying the rapid increase of electronic circuit speed, the signal frequency in a circuit is already higher to the EMI noise frequency range. The EMI design becomes a critical issue.
The traditional parallel signals possess some disadvantages and cannot normally work at the high frequency. Featuring high anti-interference with low EMI, series signals like USB, SATA PCIe, etc are popular alternatives for replacing parallel signals that are used in current designs.
The differential signal has high anti-interference and low EMI performance. It is now widely used in high speed digital systems like USB, SATA, and HDMI. Common mode and differential mode currents and voltages are always defined relative to the ground. Generally, a signal contains both differential and common mode signals.
Diagram 1 shows a clock driver output of a differential signal to a receiver which contains a common mode signal in the same time.
|
|
Common mode currents flow with the same voltage and direction and go to ground by the parasitic capacitances between the lines and ground. There is no common mode current flow through the load of the receiver because there is no voltage difference on the load.
Diagrams 2 and Diagram 3 show a differential mode signal that contains useful information. The current phase difference is 180°, where the current fully flows through the load, and where there is no current going to ground.
|
|
|
|
Because field strength in the two lines creates equal and opposing electromagnetic fields, they tend to cancel each other out and reduce crosstalk and EMI emission.
Differential mode current, flowing in opposite directions through the choke windings, creates equal and opposite magnetic fields, represented by the black lines in Diagram 4. Cancelling each other out results in the choke presenting zero impedance to the differential mode signal, which passes through the choke unattenuated.
|
|
Common mode current, the unintended component of signal current, is the main source of EMI and should be suppressed. The common mode current flows in the same direction through each of the choke windings, represented by the red lines in Diagram 4.
This creates equal and in-phase magnetic fields which add together and result in the choke presenting high impedance to the common mode signal, which passes through the choke heavily attenuated.
Diagram 5 is an ethernet transformer operation principle. The winding polarity of a centre tapped transformer is as shown in the diagram.
|
|
The transformer is a 1:1 for signal transfer between two sides. For the differential signal of Tx+ and Tx-, the current Tx+ is equal to Tx- with opposite direction. But for the common mode current, the I com will flow into the ground through the centre-tap because of no impedance in the opposite direction winding of the transformer for the common mode current.
So the transformer has two functions: the first is isolation of the input and output for safety purposes; the second is rejection of the common mode signal, which may create EMI noise, but does not affect the useful differential signal.
Because of the non-ideal isolation and un-balance of the transformer, it still has some leakage when the common mode current flow crosses the transformer. The I gcom represents the leakage current as it crosses the transformer, which may create EMI noise that should be equal to zero under ideal circumstances.
Common mode rejection rato (CMRR) is defined as the rejection ability for common mode noise: CMRR=20 log V2/V1. With a larger CMRR, the noise is attenuated, resulting in the need for larger suppression of the noise. The CMRR test is shown in Diagram 6.
|
|
Diagram 7 is a common mode choke put in the ethernet differential signal lines. For the differential signal of Tx+ and Tx-, the current Tx+ is equal to Tx- with opposite direction. The common mode choke will not affect the differential signal because the flux is opposite and will be cancelled.
|
|
For the common mode current, the high frequency I com noise is first attenuated by the transformer. I gcom leakage flows cross the transformer and the I gcom will be attenuated by the common mode choke again before flow to the cable side. The CMRR cut-off frequency can be changed by common mode choke selection and through the adjustment of the C.
The Laird Technologies CH and CM series common mode choke can be used for this application and also for high speed ethernet communication application. The common mode choke will help to improve the CMRR by reducing EMI noise with the transformer altogether.
As communication technology growth increases, network speeds become higher. Laird Technologies has also developed three-line common mode chokes, which are used in the ethernet transformer cable side to increase the attenuation in all three of the lines for 10G ethernet applications, thereby further improving CMRR.
|
|
Suppression of EMI has become a major concern in the transmission, reception, and processing of electronic signal and data. Modern systems are frequently an interconnection of functional blocks and connections made by cables or wiring harnesses.
These interconnections often present the opportunity for common mode current loops between devices that can lead to EMI regulatory failure.
The addition of a common mode choke before the connector filters these common mode currents while allowing the desired signals to pass unaffected.
The result is effective communication between devices, reliable product and system operation and product EMI regulatory compliance.
Laird Technologies
www.lairdtech.com
Novel technique to shield 2D materials from vibrations
Researchers from Monash University have demonstrated a new way to protect atomically thin...
What's the difference between traditional resins and bio-based resins?
Two Electrolube resin specialists have collaborated to explain how introducing bio-based resin...
Waterproof connectors and enclosures: check what protection you need
Every electronic device needs some kind of protection, depending on where and how it will be used.
