Summary: <p>Today’s systems simply can’t communicate any faster. Learn how some companies are getting creative and doubling their data rates using PAM4 – and the extra challenge this technology means for engineers.</p> <p>Mike Hoffman and Daniel Bogdanoff sit down with PAM4 transmitter expert Alex Bailes and PAM4 receiver expert Steve Reinhold to discuss the trends, challenges, and rewards of this technology.</p> <div class="jetpack-video-wrapper"></div> <p> </p> <audio class="wp-audio-shortcode" id="audio-1508-17" style="width: 100%;"><a href="https://eestalktech.com/wp-content/uploads/2017/09/pam4-and-400g-ethernet-ees-talk-tech-electrical-engineering-podcast-18.mp3">https://eestalktech.com/wp-content/uploads/2017/09/pam4-and-400g-ethernet-ees-talk-tech-electrical-engineering-podcast-18.mp3</a></audio> <p>1:00<br> PAM isn’t just cooking spray.</p> <p>What is PAM4? PAM stands for Pulse Amplitude Modulation, and is a serial data communication technique in which more than one bit of data can be communicated per clock cycle. Instead of just a high (1) or low (0) value, a in PAM4, a voltage level can represent 00, 01, 10, or 11. <a href="http://www.electronicdesign.com/communications/what-s-difference-between-nrz-and-pam">NRZ</a> is essentially just PAM2.</p> <p>We are reaching the limit of NRZ communication capabilities over the current communication channels.</p> <p>2:10 PAM has been around for a while, it was used in 1000BASE-T. 10GBASE-T uses PAM16, which means it has 16 different possible voltage levels per clock cycle. It acts a bit like an <a href="http://eestalktech.com/2017/07/13/fast-adc/">analog to digital converter</a>.</p> <p>2:55 Many existing PAM4 specifications have voltage swings of 600-800 mV</p> <p>3:15 What does a PAM4 receiver look like? A basic NRZ receiver just needs a comparator, but what about multiple levels?</p> <p>3:40 Engineers add multiple slicers and do post-processing to clean up the data or put an ADC at the receiver and do the data analysis all at once.</p> <p>PAM4 communicates 2-bits per clock cycle, 00, 01, 10, or 11.</p> <p>4:25 Radio engineers have been searching for better modulation techniques for some time, but now digital people are starting to get interested.</p> <p>4:40 With communications going so fast, the channel bandwidth limits the ability to transmit data.</p> <p>PAM4 allows you to effectively double your data rate by doubling the amount of data per clock cycle.</p> <p>5:05 What’s the downside of PAM4? The Signal to Noise Ratio (SNR) for PAM4 worse than traditional NRZ. In a perfect world, the ideal SNR would be 9.6 dB (for four levels instead of two). In reality, it’s worse, though.</p> <p>5:30 Each eye may not be the same height, so that also has an effect on the total SNR.</p> <p>6:05 What’s the bit error ratio (BER) of a PAM4 vs. NRZ signal if the transmission channel doesn’t change?</p> <p>6:45 The channels were already challenged, even for many NRZ signals. So, it doesn’t look good for PAM4 signals. Something has to change.</p> <p>7:00 PAM4 is designed to operate at a high BER. NRZ typically specified a 1E-12 or 1E-15 BER, but many PAM4 specs are targeting 1E-4 or 1E-5. It uses forward error correction (or other schemes) to get accurate data transmission.</p> <p>7:50 Companies are designing more complex receivers and more robust computing power to make PAM4 work. This investment is worth it because they don’t have to significantly change their existing hardware.</p> <p>8:45 PAM is being driven largely by Ethernet. The goal is to get to a 1 Tb/s data rate.</p> <p>9:15 Currently 400 GbE is the next step towards the 1 Tbps Ethernet rate (terabit per second).</p> <p>10:25 In Steve’s HP days, the salesmen would e-mail</p>
