Generate PAM4 signals for receiver compliance testing: Page 2 of 5

September 26, 2016 // By Alexander Katsman, Adsantec
56 Gbit/s links are moving to PAM4 signalling as a way to overcome channel bandwidth issues. As this modulation technique takes hold, you'll need to generate variety of signals to test receivers under varying distortion conditions. Stress testing a receiver can help you understand a receiver’s weaknesses and measure its tolerance to various signal impairments.

To generate a PAM4 test signal, we designate one of the pattern generator/active amplifier pairs as the MSB, which provides an output amplitude twice as large as that from the other pair (designated as LSB). The resulting signal will meet all the electrical PAM4 signalling and transmitter performance parameters as specified in the standards for different symbol/data rates. Figure 3 shows the physical setup diagrammed on Figure 2.

Figure 3: The direct connection between the test-setup components minimizes signal loss.

The use of two serial pattern generators lets us combine two uncorrelated data signals together, providing the flexibility to separately program the data patterns for the MSB and LSB. The active programmable pre(de)-emphasis amplifiers provide an easy way to introduce required (for PAM4 receiver testing purposes) signal alterations to both the MSB and LSB. The passive combiners are a straightforward way to generate a PAM4 signal from two input serial data sources. Here are some of the tests you need to perform.


Tolerance to jitter and noise

With four separate symbol levels to ascertain, PAM4 receivers must tolerate poor (compared to PAM2) signal-to-noise ratios and small symbol-to-symbol voltage swings. The amplitude of each eye in a PAM4 signal is roughly one-third that of an NRZ signal. An easy method to judge a receiver's sensitivity is to lower the input signal amplitude until the BER (bit-error rate) reaches an unacceptable level. You can accomplish that by properly reducing the output amplitudes of both active amplifiers in parallel.

You can also achieve eye closure by using the pre(de)-emphasis of the active amplifiers to introduce ISI-type characteristics. The amplifiers can model a full passive frequency-dependent loss profile, thus removing the need to have any physical reference channels (e.g., compliance test boards) or programmable attenuators on hand.

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