Trace Canceller AGC Equalization for Unclipped Training Pulses
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Solution Overview
Problem
Transmission lines are difficult to drive with integrated circuit (IC) buffers due to relative mismatch between IC transistor size and transmission line length, leading to distortion and ringing, especially with the insertion of redrivers that clip training pulses and introduce amplitude and phase distortions.
Innovation Solution
A trace canceller with an equalizer and automatic gain control loop that adjusts the gain of a variable-gain amplifier to compensate for phase and amplitude distortions, preventing waveform clipping and allowing full transmission of training pulses without distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a non-linear limiting amplifier is used in the redriver to amplify signals, then the output swing is fixed and phase distortion is canceled, but the signal amplitudes are clipped and input information is lost
Solution Approach 1:
The patent applies dynamics by making the amplifier gain variable rather than fixed. The variable gain amplifier adjusts its gain based on the input signal amplitude, allowing it to operate in a linear region for small signals and provide sufficient drive for large signals without clipping. This dynamic adjustment resolves the contradiction between maintaining fixed output swing for phase distortion cancellation and preserving input amplitude information.
Solution Approach 2:
The patent changes the parameter of amplifier gain from a fixed value to a variable value that adapts to different signal conditions. By controlling the gain parameter dynamically, the system can prevent clipping while maintaining adequate drive strength, thus preserving signal amplitude information while still canceling phase distortion.
2Length of stationary object
If the transmission line length is increased to extend communication distance, then the coverage area is improved, but amplitude and phase distortions increase
Solution Approach 1:
The patent introduces a trace canceller circuit as an intermediary device placed at the midpoint of the transmission line. This canceller generates an opposite-phase signal to cancel the distortions accumulated over the extended transmission line length. By using this intermediary cancellation mechanism, the system can achieve longer communication distances while maintaining signal quality.
Solution Approach 2:
The trace canceller performs preliminary anti-action by pre-compensating for the amplitude and phase distortions that will occur over the extended transmission line. The canceller is configured with equalizer settings matched to the trace length, allowing it to proactively counteract distortions before they degrade the signal quality at the receiver.
3Length of stationary object
If a redriver is inserted to extend transmission distance, then the drivable distance is improved, but the training pulses are clipped and amplitude information is lost
Solution Approach 1:
The variable gain amplifier in the trace canceller dynamically adjusts its gain during training sequences to prevent clipping of training pulses. The automatic gain control mechanism ensures that even during amplitude variation in training sequences, the output remains within the linear region, preserving the amplitude information needed for receiver training and equalization.
Data Source
AI summary
Distortions of both amplitude and phase along a transmission line are compensated for by a trace canceller inserted between a transmitter and a receiver. The trace canceller has an equalizer that compensates for a trace length between the transmitter and the trace canceller. A variable gain amplifier between the equalizer and an output buffer has its gain controlled by an automatic gain control circuit that compares low-frequency swings of the input and output of the trace canceller. The gain of the variable gain amplifier is reduced to prevent the output buffer from saturating and clipping peak voltages on its output. Thus both the variable gain amplifier and the output buffer remain in the linear region. Training pulses from the transmitter are passed through the trace canceller without clipping of peak voltages, allowing the transmitter and receiver to adjust transmission parameters to best match the transmission line.