Single ended/Differential line monitoring and auto matching
Original Publication Date: 2003-Oct-25
Included in the Prior Art Database: 2003-Oct-25
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Signal edge rates define the transition point from normal traces to transmission lines and are important for transferring information. As CPU (Central Processing Unit) frequencies increase to allow faster data rates, edge rates become faster which worsens the transmission line effects. One kind of those effects are reflections which appear when a signal encounters a mismatch in impedance. This mismatch can be the result of trace geometry, capacitive loading or improperly terminated traces. Up to now there are several well known solutions like the series source resistor, AC termination, parallel termination or Thevenin termination. All of these methods may work well in certain cases but resistor and capacitor values always have to be predetermined and then are fixed. There will be no automatic tuning so they cannot react to temperature changes and variation of traces on different PCB bards. A tunable solution has been proposed in US patent 5,726,583 but it still has a number of disadvantages. First the user has to know the line impedance and enable a number of parallel elements. He also has to program the elements as they do not adjust themselves to changes of the impedance. Additionally the circuit is not very accurate and becomes more complex if the line impedance is low.