Adoption of high data rate transceivers like 400G has led to changes in how optical modules are tested. Standards bodies like IEEE have defined measurement metrics to quantify the performance of an optical transceiver and ensure inter-operability. TDECQ — Transmitter and Dispersion Eye Closure Quaternary — is the key metric for PAM4 transmitter qualification and is now a mandatory compliance measurement for 400G and higher-speed optical modules.
Table of Contents
6 sections — jump to any topic1. Transceiver Transmitter Testing
Traditional Mask and Transmitter and Dispersion Penalty (TDP) tests used for NRZ (non-return-to-zero, PAM2) signals were extremely useful for low-speed signals. The disadvantage of TDP measurements was that they were slow and required expensive test equipment.
As a result, TDEC (Transmitter and Dispersion Eye Closure) testing evolved to replace the TDP test. Both TDP and TDEC compare the transmitter under test to an ideal or reference transmitter. While TDP uses an actual reference transmitter, the ideal signal in TDEC is simulated. TDEC gives a measure of eye shape and includes the effects of Inter-Symbol Interference (ISI) and modal dispersion caused by system impairments. TDEC provides a direct correlation with BER — for a given BER target, you can determine the maximum TDEC penalty the system can tolerate.
TDP — Transmitter and Dispersion Penalty
- Designed for NRZ (PAM2) signals
- Uses an actual physical reference transmitter
- Slow measurement process
- Requires expensive test equipment
TDEC — Transmitter and Dispersion Eye Closure
- Replacement for TDP for NRZ signals
- Ideal reference transmitter is simulated (not physical)
- Faster and more cost-effective
- Direct correlation with BER
2. From TDEC to TDECQ
With data rates increasing, the industry shifted from NRZ to PAM4 modulation. PAM4 has the advantage of packing 2 bits into a symbol, reducing the required bandwidth by half compared to NRZ at the same data rate.
In PAM4 signaling, TDEC was replaced by TDECQ — Transmitter and Dispersion Eye Closure Quaternary. TDECQ is a key optical measurement for equalized optical links and is used to measure the performance of a transmitter under test. It quantifies the combined degradation from noise, ISI, and modal dispersion that affects the transmitter signal, and measures the Vertical Eye Closure across all three PAM4 eye openings.
4 amplitude levels, 2 bits per symbol, half the bandwidth of NRZ at equal data rate
PAM4 produces 3 distinct eyes — TDECQ measures vertical closure across all three simultaneously
TDECQ penalty maps directly to SER and BER — a lower penalty means a better transmitter
3. Test Setup
The test setup for measuring TDECQ is the same as that for TDEC with two important additions: a 4th order Bessel-Thomson filter (in the sampling oscilloscope) and a reference equalizer.
The noise generated by the O/E converter and oscilloscope introduces ISI and modal dispersion to replicate worst-case conditions. A waveform after passing through this oscilloscope has completely closed eyes — the reference equalizer is a mandatory addition to the setup specifically to reopen those eyes before the TDECQ measurement is taken.
TDEC Setup (NRZ)
- O/E converter
- 4th order Bessel-Thomson filter
- Sampling oscilloscope
TDECQ Setup (PAM4) — Adds:
- Same as TDEC setup, plus:
- Reference equalizer (mandatory for PAM4)
- Histogram sampling at 0.45UI and 0.55UI
4. TDECQ Calculation
To compute TDECQ, 8 vertical histograms — 4 above and 4 below the average power level — are sampled at 0.45UI and 0.55UI as reference measurement windows within each symbol period.
In the test, Gaussian noise is added to the signal until the targeted SER (Symbol Error Ratio) is reached. This process is performed on both the Device Under Test (DUT) and a virtual ideal transmitter. The difference in noise levels required to reach the same SER represents the TDECQ power penalty for the DUT.
5. IEEE Limits and Compliance
IEEE defines multiple TDECQ limits depending on the application. The limits differ across optical reach categories because longer-reach links face greater impairments and the transmitter must meet tighter specifications to ensure the link budget is preserved end-to-end.
For example, for 400GBASE-DR transmit characteristics, the IEEE 802.3bs TDECQ limit is 3.4 dB. A module that measures above this limit fails compliance for that application and cannot be used in a DR-reach 400G deployment.
| Application | Standard | TDECQ Limit |
|---|---|---|
| 400GBASE-DR4 | IEEE 802.3bs | 3.4 dB |
| 400GBASE-FR4 | IEEE 802.3bs | 3.4 dB |
| 400GBASE-LR4 | IEEE 802.3bs | 3.4 dB |
| Varies by application | IEEE 802.3 series | Defined per spec — always confirm module datasheet |
6. Conclusion
TDECQ is the measurement metric for PAM4 transmitter qualification and will continue to grow in importance as transceivers at 400G, 800G, and beyond increasingly rely on PAM4 modulation. It provides a more accurate and efficient testing method compared to traditional TDP tests — and its direct mapping to SER and BER makes it a practical compliance gate for production module qualification.
Key points to remember: TDECQ is expressed in dB, lower is better, 0 dB represents an ideal transmitter, and IEEE defines application-specific limits that all compliant modules must meet.
