How to Make PAM4, PAM8, and PAM16 Signals

Yesterday I wrote about how it was possible to create a PAM4 signal using a Wilkinson power divider. Our Wilkinson product line also includes more rare 3 and 4 way power dividers, which means that we can combine more than two signals together, making higher order amplitude modulation possible.

To review, here is how to make a four level pulse amplitude modulated (PAM4) signal from a single PRBS generator:

You just take a PRBS signal, decorrelate it from itself, attenuate one signal by 6 dB, phase align them, and then recombine them. It’s really pretty easy if you have all the components. The other option is to use two data sources with the amplitudes already set to the correct value, with common clocks to align the signals in time. Here is what you get:

So how to make a PAM8 signal? Just add another PRBS signal, but this time it has to be attenuated by another 6 dB. To recap, a PRBS signal, a different (or uncorrelated) PRBS signal with 6 dB lower amplitude, and another different PRBS signal with 12 dB lower amplitude walk into a bar, and out comes a PAM8 signal:

As long as the bar is a three way wilkinson power divider, you get this:

Not that great. It is a PAM8 eye diagram for sure, but it doesn’t look that nice. This is because the phase decorrelators I was using had a lowpass response, as well as the multiple reflections present in my combination system. So it’s possible to do some crude testing using this setup, but I wouldn’t recommend it for any postdeadline OFC papers.

Personally I don’t like PAM modulations, especially above PAM4. At least with PAM4 you get twice the number of bits, but with PAM8 or god forbid PAM16, you are sacrificing a big chunk of your noise margin (remember the spread on each of the eyes is gaussian) for an improvement of only 50% or 33%, not an order of magnitude or anything like that. I think it is much more promising to use either the phase of the optical carrier itself or an electrical carrier on the signal for doing IQ modulation in interesting ways. Speaking of IQ modulation, we sell some excellent IQ mixers that would be perfect for that. Contact [email protected] to learn more.