Transition time measurement of PAM4 transmitters

摘要

Methods, apparatus and systems for measuring signal transition times for a four-level pulse modulated amplitude (PAM4) transmitter. During a test procedure, a PAM4 transmitter is configured to repetitively transmit a PAM4 symbol test pattern, which is captured as a signal waveform. The test pattern includes at least one rising signal sequence having a PAM4 symbol pattern of at least three −1 PAM4 symbols followed by at least three +1 PAM4 symbols and at least one falling signal sequence having a PAM4 symbol pattern of at least three +1 PAM4 symbols followed by at least three −1 PAM4 symbols. A voltage modulation amplitude (VMA) level for each of a −1 and +1 PAM4 signal level is measured for at least one rising signal sequence and falling signal sequence to derive 20% and 80% VMA levels. A rise transition time is then determined by measuring the time interval between when a rising signal crosses the 20% and 80% VMA levels, and a fall transition time is determined by measuring the time interval between when a falling signal crosses the 80% and 20% VMA levels.

主权项

1. A method for measuring signal transition times for a four-level pulse modulated amplitude (PAM4) transmitter, comprising:
transmitting a test pattern with the PAM4 transmitter, the test pattern having at least one rising signal sequence having a PAM4 symbol pattern of at least three −1 PAM4 symbols followed by at least three +1 PAM4 symbols and at least one falling signal sequence having a PAM4 symbol pattern of at least three +1 PAM4 symbols followed by at least three −1 PAM4 symbols; capturing a waveform of the test pattern, the captured waveform at least including portions of the test pattern proximate to at least one rising sequence and at least one falling signal sequence; determining, using the captured waveform, a voltage modulation amplitude (VMA) level for each of a −1 and +1 PAM4 symbol to determine a 0% and 100% VMA level; deriving a 20% VMA level and an 80% VMA level as a function of the VMA 0% and 100% VMA levels; determining a rise transition time by measuring a time interval between when a rising PAM4 signal from a −1 PAM4 symbol to a +1 PAM4 symbol for a rising signal sequence crosses the 20% VMA voltage level and crosses the 80% VMA voltage level; and determining a fall transition time by measuring a time interval between when a falling PAM4 signal from a +1 PAM4 symbol to a −1 PAM4 symbol for a falling signal sequence crosses the 80% VMA voltage level and crosses the 20% VMA voltage level, wherein determining the 0% VMA voltage level comprises measuring the transmitter signal voltage level over the center 20% of the portion of the waveform comprising at least three consecutive −1 PAM4 symbols, and wherein measuring the 100% VMA voltage level comprises measuring the transmitter signal voltage level over the center 20% of the portion of the waveform comprising at least three consecutive +1 PAM4 symbols immediately before or after that at least three consecutive −1 PAM4 symbols.