We use cookies to improve your online experience. By continuing browsing this website, we assume you agree our use of cookies.

What is Noise Conduction, Common Mode Rejection and the Bob Smith Circuit

Views : 334
Author : CETUS INTL
Update time : 2023-08-01 10:59:51
Partial explanation of the principles of network transformers: noise conduction, common-mode rejection, and the Bob Smith circuit.


Network Transformer and Noise Conduction
An ideal transformer is a theoretically perfect circuit element that transfers electrical energy between primary and secondary windings with perfect magnetic coupling. An ideal transformer can only carry alternating differential mode currents. It cannot transmit common-mode current, because the potential difference of common-mode current at both ends of the transformer winding is zero, and it cannot generate a magnetic field on the transformer winding. There is a small but not equal to zero coupling capacitance CWW between the primary and secondary windings of the actual transformer. This capacitance is created by the presence of non-dielectric and physical gaps between the windings. The value of the inter-winding capacitance can be reduced by increasing the gap between the windings and filling the space between the windings with a material with a low dielectric constant. Capacitor CWW provides a channel for the common-mode current to pass through the transformer, and its impedance is determined by the size of the capacitor and the frequency of the signal.

Common Mode Rejection
The common mode conduction noise is driven by the noise voltage in the equipment, passing through the parasitic capacitance between the ground and the equipment, and the noise current flowing between the ground and the cable. The method to reduce the common-mode conduction noise is to connect the common-mode choke coil in series in the signal line or the power line, connect the capacitor in parallel between the ground and the wire, form an LC filter for filtering, and filter out the common-mode conduction noise. The common mode choke coil is formed by winding the signal wire and the ground wire in the same direction on the ferrite core. The common-mode current impedance flowing between them is very large.
Common-mode radiation noise is caused by the common-mode voltage at the cable port, driven by it, and the common-mode current flowing from the ground to the cable. The radiated electric field strength is inversely proportional to the distance from the cable to the observation point, and (when the cable length is shorter than the wavelength of the current) is proportional to the frequency and the length of the cable. The methods to reduce this radiation are: reduce the impedance of the ground wire by using a ground plane on the circuit board, and use an LC low-pass filter or a common mode choke coil at the port of the cable. In addition, minimizing the length of the cable and using shielded cables can also reduce radiation.

The common mode inductor integrated in the network transformer can effectively suppress the EMI problem caused by the common mode current, but special attention should be paid to the position of the common mode inductor:
1- If it is placed on the chip side, it is not suitable for current-driven chips. When the current flows through the common-mode inductor in the same direction, there is no magnetic field line in the common-mode inductor core to cancel each other out. At this time, the impedance of the common-mode inductor hinders The change of the current will affect the normal working signal.
2- If it is placed on the cable side, in order to ensure the matching effect of the Bob Smith circuit, it is necessary to add another autotransformer. The above solution needs to add an additional magnetic core, so the current application is more to use three-wire common mode inductors. This solution can be applied to current-driven chips, and only two magnetic cores are needed, which reduces the cost.

Bob Smith circuit
This circuit has two functions: providing 150ohm impedance matching between any two pairs of differential signals on the network port; providing a return path for common mode signals.
Considering the first function, we can clearly see why the common mode inductor cannot meet the matching requirements of the Bob smith circuit when placed on the cable side:
At this time, the matching resistance is not 150ohm, but becomes Z=2×75+2×Zcmc, which cannot meet its impedance matching requirements, so the two-wire common mode inductor cannot be placed on the cable side. If it is placed on the cable side, an additional one is required. Autotransformer.
Consider the second function, the impedance it can provide:
In order to obtain low impedance in a wide frequency range, it is necessary to control the connection impedance to ensure the low impedance connection of the Bob-smith circuit. According to different problem frequency points, the capacitance value of the capacitor in the circuit can also be adjusted appropriately. Its function is similar to that of the center tap capacitor, but because there is a 75ohm series resistance on its path, and this capacitor is a high-voltage capacitor, it is difficult to choose a large value, so its impact on the radiated emission of the network port is not as obvious as that of the center tap. , but it is also an aspect that we can adjust to solve the network port problem.