Understanding TDECQ: Transmitter Dispersion and Eye Closure

This article delves into the concept of TDECQ (Transmitter Dispersion and Eye Closure), a crucial metric in the world of optical communication. We’ll explore its definition, its formula, and how it’s used to assess the quality of transmitted optical signals.

What is TDECQ?

Traditionally, eye mask analysis was used to gauge the quality of optical signals. However, a new metric called TDECQ has emerged as a more comprehensive way to characterize transmitted and received signals in the optical domain.

• TDECQ essentially measures the vertical eye closure of an optical transmitter after the signal has traveled through a simulated worst-case optical channel.
• Instead of just looking at the ideal signal, TDECQ considers how signal quality degrades due to dispersion and other channel effects.

The image above shows an example of a stressed eye diagram, highlighting some of the parameters that affect signal quality in optical transmission, for both PAM4 and NRZ optical receivers.

The TDECQ Formula

The TDECQ is calculated using the following formula:

Here’s a breakdown of each component:

• OMA (Optical Modulation Amplitude): This represents the difference between the optical power of the high and low signal levels, measured in microwatts (µW).
• ER (Extinction Ratio): This is the ratio of the optical power of the high signal level to that of the low signal level, typically expressed in decibels (dB) or as a percentage.
• UI (Unit Interval): This refers to the time duration of one bit period.
• A0 (Vertical Eye Opening): This is the measure of the innermost eye opening at the center of the eye diagram, measured in dBm or microwatts (µW). It indicates how clear and open the eye is.
• VECP (Vertical Eye Closure Penalty): This represents the signal degradation due to ISI (inter-symbol interference) measured in dB.
• SEC (Stressed Eye Closure): This represents the vertical eye closure that occurs in a stressed environment.
• TDECQ (Transmitter and Dispersion Eye Closure): This is the final result, indicating the overall signal quality accounting for both transmitter impairments and channel effects.

How is TDECQ Measured?

The measurement of TDECQ involves a conformance test setup, which typically includes the following:

The figure above shows a block diagram for the TDECQ conformance test.

1. The optical lane under test continuously transmits a specific test pattern.
2. An oscilloscope captures the complete pattern for analysis.
3. The captured data is then processed to calculate the TDECQ value. This involves simulating the worst-case optical channel. This process incorporates the effects of dispersion and other optical channel impairments on the transmitted signal.

In summary, TDECQ provides a more robust evaluation of optical transmitter performance by considering not just the ideal signal but also how it degrades under realistic channel conditions. This metric is essential for ensuring reliable high-speed optical communication.