LVDS Pre-Emphasis Buffer for Stable Common-Mode Transmission
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Solution Overview
Problem
In low-voltage differential signaling (LVDS) systems, high-speed, long-distance signal transmission is hindered by increased bit error rates due to signal attenuation and common mode potential fluctuations, which are exacerbated by the inversion of current directions in differential transmission lines, leading to electromagnetic interference (EMI) noise.
Innovation Solution
A transmitting apparatus incorporating a main buffer circuit and a pre-emphasis buffer circuit with switch circuits and current sources, which temporarily enhances the current signal intensity during level changes, and a voltage-holding circuit to stabilize common mode potential, thereby reducing signal transition time and minimizing potential fluctuations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the magnitude of current signal is increased during transition to reduce signal establishment time, then the signal transition speed is improved, but common mode potential fluctuates and EMI noise increases
Solution Approach 1:
The patent divides the current signal generation into two independent parts: a main buffer circuit that provides the primary current signal, and a pre-emphasis buffer circuit that provides an additional current signal only during transitions. This segmentation allows the pre-emphasis circuit to boost transition speed without requiring the main buffer to continuously operate at high current levels, thereby reducing overall common mode potential fluctuation and EMI noise.
Solution Approach 2:
The pre-emphasis buffer circuit is configured to activate before the main buffer circuit during signal transitions, providing an initial current boost that accelerates the signal establishment. This preliminary action reduces the time required for the current signal to reach its steady-state value, improving transition speed while limiting the duration of high current operation to minimize harmful effects.
2Speed
If high-speed signal transmission is implemented in long-distance LVDS systems, then transmission speed is improved, but bit error rate increases due to signal attenuation
Solution Approach 1:
The pre-emphasis buffer circuit activates before the main buffer during signal transitions, providing an initial current boost that accelerates signal establishment. This preliminary action ensures that the current signal reaches its steady-state value faster, reducing the time window during which attenuation can degrade the signal, thereby maintaining reliability at high transmission speeds over long distances.
3Loss of time
If current signal magnitude is increased to reduce transition time, then signal establishment time is reduced, but power consumption and EMI increase
Solution Approach 1:
The patent segments the current signal provision into a main buffer and a pre-emphasis buffer with separate control. The pre-emphasis buffer operates only during transitions, consuming power only when needed to reduce establishment time. The main buffer operates continuously but at lower current levels during steady states. This segmentation ensures that high power consumption occurs only briefly during transitions rather than continuously, reducing overall power consumption while maintaining fast signal establishment.
Solution Approach 2:
The pre-emphasis buffer circuit operates periodically only during signal transitions rather than continuously. This periodic activation reduces the average power consumption while still providing the current boost necessary to reduce signal establishment time during the critical transition periods.
Data Source
AI summary
To provide a transmitting apparatus capable of suppressing the fluctuation of a common mode potential and performing high-speed, long-distance signal transmission. The transmitting apparatus has a main buffer circuit and a pre-emphasis buffer circuit 20. The pre-emphasis buffer circuit 20, which has a switch circuit 21, a first current source 22, and a second current source 23, uses the switch circuit 21 to output a current signal having the same direction as an output current of the main buffer circuit 10 during a certain time interval starting from a time point when the level of data to be transmitted changes, and brings the output terminals 201, 202 to a High-Z state during a time interval when the level is constant after a lapse of the abovementioned certain time interval. The output of the pre-emphasis buffer circuit 20 has no influence on the common mode potential of the output of the main buffer circuit but has influence only the amplitude of the current signal output to a differential transmission line. In this manner, the transmitting apparatus can suppress the fluctuation of the common mode potential and can perform high-speed, long-distance signal transmission.