Application Note

Generating PAM-4 Signals Using the ONT CFP2 Module

4-level pulse amplitude modulation (PAM-4) will be a core enabling technology for 400 Gbps Ethernet. As of yet, very few practical solutions exist to generate known good PAM-4 signals, especially for the critical and complex pseudo random quaternary sequence (PRQS) required for robust testing and evaluation of 400 Gbps-ready components.

This application note outlines how the industry-reference Viavi ONT CFP2 solution can generate accurate, yet complex PAM-4 signals vital for the evaluation of components and systems such as:

y PAM-4 amplifier ICs

y Laser drivers and modulators

y PAM-4 receivers including continuous-time linear equalizer (CTLE) and decision feedback equalizer (DFE) systems

Detail

The Viavi ONT CFP2 module, together with the CFP2 electrical adapter, lets users fully test and validate 25 Gbps NRZ-based technology critical to bring 100 Gbps technology to the market. By adding a few simple external components, a crisp and accurate PAM-4 signal can be generated while providing full control of the pattern, timing, and electrical parameters.

The method used to form a PAM-4 signal is to “add up” two independent but well-aligned 25 Gbps NRZ PRBS signals, where one of the signals is attenuated by 6 dB so that the signal amplitude is just half of the original signal. The superposition of these two signals will create a signal that can have four different amplitude levels, depending on the input value from the two PRBS sequences:

A1: (0.5 * 0) + 0 = 0.0A2: (0.5 * 1) + 0 = 0.5A3: (0.5 * 0) + 1 = 1.0A4: (0.5 * 1) + 1 = 1.5

These NRZ signals can be taken from any two of the four electrical adapter TX lanes that the ONT provides. In our example, we will use Tx0 and Tx1. The only extra components needed are:

y 2 x 6 dB in-line attenuators with 2.9 mm connectors (to attenuate the Tx0 signal to half the amplitude of the Tx1)

y 2 x power splitters/combiners with 2.9 mm connectors (to add the attenuated Tx0 to the Tx1 signal in order to generate a 4 level signal)

These extra components are available from many vendors; for best results, we recommend those that have an operating frequency up to 40 GHz.

2 Generating PAM-4 Signals Using the ONT CFP2 Module

Figure 1 shows the block diagram of the test setup, while Figure 2 shows the actual attenuators/combiners used for our test.

Tx1 will be the MSB and Tx0 (now half the amplitude behind the attenuator) will form the LSB.

Tx1 (the MSB) and the attenuated Tx0 (the LSB) are now directly connected to the two input ports of the power combiner. Ensure that the two +V signals both go to the one combiner, and that the two –V signals go to the second combiner. The combiners’ output now forms a PAM-4 signal.

Note that this is a ‘raw’ PAM-4 signal that has not been gray-encoded.

Figure 1. Schematic test setup for PAM-4 generation

Figure 2. Attenuators and combiners connected to the ONT electrical adapter

After this initial physical setup, you are by not guaranteed a crisp and clear PAM-4 signal as both sub-UI skew and pattern offset (whole UI steps) may mean the output signal is distorted or misaligned (see Figure 3). Fortunately, the ONT CFP2 applications allow both these issues to be corrected to give a known good PAM-4 signal without the aid of any external test equipment.

Figure 3. The initial, “bad” PAM-4 signal caused by a ~500 mUI alignment error between Tx0 and Tx1

We can use the following features of the ONT CFP2 module to calibrate out any skew and pattern errors in the PAM-4 TX signal:

y Dynamic skew generation — this allows the ONT to adjust the relative timing of any two 25G NRZ lanes in fraction UI steps. We can use this feature to null out any timing differences between Tx0 and Tx1 to ensure the PAM-4 crossing is adjacent for the LSB and MSB.

y Synchronized patterns — the ONT can generate an identical PRBS signal per lane with zero bit offset between lanes; with this, we can ensure that Tx0 and Tx1 can be fully aligned (see Figure 4).

y Sensitive and high performance NRZ receiver — we can directly connect the PAM-4 output signal into Rx0 of our NRZ receiver, since the output signal from the power combiners (our PAM-4 source) would actually be a valid PRBS NRZ signal when the contents of both Tx0 and Tx1 are identical (see Figure 4). Using this special case signal, we can, in fact perform a BERT test directly using our NRZ receiver.

3 Generating PAM-4 Signals Using the ONT CFP2 Module

To find the ideal values for offset and skew between Tx0 and Tx1, one can use the automatic dynamic skew feature to generate a ramping skew between +10,000 mUI and –10 000 mUI on the LSB (TX0) lane. During this ramp (best to choose a slow rate of ramp, perhaps around 50 mUI/s), you will notice the RX error detector starts counting errors. This should happen at two extremities of skew. The mid-point of this range should provide the optimum setting for best alignment. This can also be verified using an oscilloscope. The skew range (±10 UI) is large but will help guarantee the very best alignment.

Figures 3 through 5 actually show the different stages of the calibration process, where Figure 5 is the “artificial” PAM-4 signal that can be used for testing PAM-4 components:

Figure 4. Well-adjusted PAM-4 signal with no skew and zero UI pattern offset. Now that both the LSB and MSB carry the same PRBS, the resulting

signal is still an NRZ PRBS and can be used for BERT using the ONT CFP2 NRZ receiver.

Figure 5. The pattern offset between the LSB and MSB is changed to a non-zero (but full UI) value. We can see a classic PAM-4 signal with four distinct levels, and critically, the user has full control over the driving patterns and

sequencing using the ONT applications. These known PAM-4 sequences can now be used to fully test PAM-4 components, modules, and systems with a

high degree of confidence.

The following traces show the relative alignment of the NRZ Tx0 and Tx1 signals forming the PAM-4 LSB and MSB respectively:

Figure 6. PRBS7, synchronous (= zero UI offset), skew adjusted

© 2016 Viavi Solutions Inc. Product specifications and descriptions in this document are subject to change without notice. pam4withONT-an-opt-nse-ae 30179684 000 0316

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Figure 7. PRBS7, auto staggered (= fixed UI offset), skew adjusted

Figure 8. ONT-603 CFP2 with electrical adapter fitted

Figure 9. Detailed view of CFP2 electrical adapter

ConclusionPAM-4 modulation will become the dominant format used in emerging high-speed communication systems at 100 Gbps and beyond. Until recently, it has been relatively complex to generate known pattern PAM-4 electrical signals for designing, developing, and validating components, modules, and systems. The ONT CFP2, with its advanced applications such as dynamic skew generation, coupled with the electrical adapter and a few external components, can quickly and precisely generate known PAM-4 signals, accelerating product evaluation and development.

Components

y Viavi ONT CFP2 PHY module with dynamic skew option

y Viavi CFP2 electrical adapter

y Power combiner such as an Aeroflex/Weinschel Model 1575

y Attenuator such as an Inmet 40AH-6dB