Lowpass-bandstop common mode filter for differential lines carrying high rate digital signals
View Patent ↗The present invention provides a novel structure that can be used to make a common mode filter. Only the common mode will be attenuated and the differential mode will not be attenuated. This structure can be implemented in a number of ways, a specific embodiment using strip-line and slot-line junctions is very compact and well-suited to use with multilayer PCBs, and does not require any extra components. It can be designed to attenuate certain discrete frequencies, by designing the poles of the transfer function to be at these frequencies.
1. A common mode filter for high speed data communications over differential signal lines, comprising:
signal splitting means to split signals transmitted from a first port connected to a first line of a pair of differential signal lines and a second port connected to a second line of the pair of differential signal lines;
phase shifting means communicating with the signal splitting means to invert one of each of the split signals;
signal combining means communicating with the signal splitting means and the phase shifting means to combine the inverted signals from each of the first and second ports with the split signals from the other of the first and second ports to provide combined signals, thereby attenuating common mode signals carried on the pair of differential signal lines, and to transmit the combined signals to third and fourth ports.
2. The common mode filter of claim 1 , wherein the signal splitting means and the signal combining means are implemented as single functional blocks.
3. The common mode filter of claim 2 , wherein the single functional blocks are junctions between a plurality of slot-lines, shorted at both ends, and the pair of differential signal lines, and wherein each said shorted slot-line provides phase-shifting at a different common mode discrete frequency.
4. The common mode filter of claim 2 , wherein the single functional blocks are junctions between a slot-line shorted at both ends and the pair of differential signal lines.
5. The common mode filter of claim 4 , wherein the slot-line is formed in a ground plane.
6. The common mode filter of claim 4 , wherein the pair of differential signal lines are strip-lines.
7. The common mode filter of claim 4 , wherein the pair of differential signal lines are microstrip lines.
8. The common mode filter of claim 4 , wherein the shorted ends of the slot-line provide the phase shifting means.
9. The common mode filter of claim 8 , wherein the shorted ends of the slot-line appear as an effective slot-line open to the common mode signals.
10. The common mode filter of claim 8 , wherein the shorted ends of the slot-line have a length equal to one quarter wavelength of a common mode discrete frequency.
11. The common mode filter of claim 10 , wherein the shorted ends of the slot-line are one of circular stubs, radial stubs or diamond stubs.
12. The common mode filter of claim 1 , wherein the phase shifting means are balanced-to-unbalanced (BALUN) transformers connected to a differential signal driver.
13. The common mode filter of claim 1 , wherein at least one of the signal splitting means, the phase shifting means and the signal combining means is implemented in a digital signal processor.
14. A high speed communication system using differential transmission lines, comprising:
a differential input driver driving a pair of differential signal lines; and
a common mode filter having a signal splitter to split signals transmitted from a first port connected to a first line of the pair of differential signal lines and a second port connected to a second line of the pair of differential signal lines; a phase shifter communicating with the signal splitter to invert one of each of the split signals; and a signal combiner communicating with the signal splitter and the phase shifter to combine the inverted signals from each of the first and second ports with the split signals from the other of the first and second ports to provide combined signals, thereby attenuating common mode signals carried on the pair of differential signal lines, and to transmit the combined signals to third and fourth ports.
15. A method of filtering common mode signals in high speed data communications over differential signal lines, comprising:
splitting signals transmitted from a first port connected to a first line of a pair of differential signal lines and a second port connected to a second line of the pair of differential signal lines;
inverting one of each of the split signals;
combining the inverted signals from each of the first and second ports with the split signals from the other of the first and second ports to provide combined signals, thereby attenuating common mode signals carried on the pair of differential signal lines, and
transmitting the combined signals to third and fourth ports.