PAM4 and 400G Ethernet - EEs Talk Tech #18 - Electrical Engineering Podcast скачать в хорошем качестве

PAM4 and 400G Ethernet - EEs Talk Tech #18 - Electrical Engineering Podcast

Double the data rate - why PAM4 is tomorrow's NRZ Click to subscribe! ► http://bit.ly/Scopes_Sub ◄ Watch more at https://eestalktech.com Learn how PAM4 is allowing some companies to double their data rate - and the new challenges this brings up for engineers. Daniel Bogdanoff and Mike Hoffman sit down with PAM4 transmitter expert Alex Bailes and PAM4 receiver expert Steve Reinhold to explore the trends, challenges, and rewards of this technology. 1:00 PAM stands for Pulse Amplitude Modulation NRZ is essentially PAM2 We are reaching the limit of NRZ communication capabilities. 2:10 PAM has been around for a long time, since 1000BASE-T, 10GBASE-T has PAM16, 16 different voltage levels A bit like an analog to digital converter. 2:55 PAM4 specs have a voltage swing of 600 to 800 mV 3:15 What does a PAM4 reciver look like? 3:40 You add multiple slicers and do some post processing or put an ADC and do the receiving all at once PAM4 communicates 2-bits per clock cycle 4:40 With communications going so fast, the channel bandwidth limits the ability to transmit data. PAM4 allows you to effectively double your data rate 5:05 What's the downside of PAM4? The Signal to Noise Ratio (SNR) is not as good as NRZ. The ideal SNR is 9.6 dB by factoring in four levels instead of two. 5:30 Eyes may not be the same height, so that effects SNR 6:05 What is the bit error ratio (BER) of an NRZ vs. PAM4 signal in the same channel? 6:45 The channels were already impaired already, even for NRZ 7:00 PAM4 is designed to operate at a high BER NRZ often required 1E-12 or 1E-15 BER, but some PAM4 standards are targeting 1E-4 or 1E-5 and correct with forward error correction 7:50 Companies are having to design more complex receivers and more robust computing power to make PAM4 work. But, it's worth it because they don't have to boost their existing hardware 8:45 PAM is being driven by ethernet. The goal is to get to 1 Tb/s 10:25 In Steve's HP days, the disk salesmen would e-mail large files to him to try to fill up his disk. 11:10 Is there a diminishing return for going to higher PAM levels? PAM3 is used for automotive Ethernet, 1000BASE-T uses PAM5. Broadcom helped develop PAM3. They want to have just 1 pair of cables running through a vehicle instead of 4 pairs in typical Ethernet cabling. Cars tend to be an electrically noisy Ethernet. Ethernet is very popular for Ethernet. Essentially, Ethernet is replacing FlexRay. There was a technology battle for different automotive communication techniques. 14:45 In optical communications, they modulate much more but don't have the same noise concerns. For digital communications PAM8 is not possible over current channels because of noise 15:20 PAM4 is the main new computing scheme for digital communications 15:50 Baseband digital data covers a very large frequency range. It goes from DC (all zeroes or all ones) to the baud rate/2 (101010) 16:15 This causes intersymbol interference (ISI) that has to be corrected for - which is why we use transmitter equalization and receiver equalization 16:55 PAM4 also requires clock recovery, and it is much harder to recover when you have multiple levels 17:35 ISI is easier to think about on an NRZ signal. If you have ten 0s in a row, then it transitions up to ten 1s in a row, loss will be minimal. But, if you put a transition every bit, the attenuation will be much worse. 19:15 To reduce ISI you use de-emphasis or pre-emphasis on the transmit side, and equalization on the receiver side Engineers essentially boost the high frequencies at the expense of the low frequencies. It's very similar to Dolby audio. 20:40 How do you boost only the high frequencies? There are circuits you can design that react based on the history of the bit stream. At transition bits, this circuitry puts out a higher amplitude than a normal bit. 22:35 Clock recovery is a big challenge, especially for collapsed eyes. In oscilloscopes, there are special techniques to recover the eye for analysis. With different tools you can profile an impulse response, and detect whether you need to de-emphasize or modify the signal for transmission. Essentially engineers get the transfer function of their link. 23:45 For Ethernet, there are typically three equalization taps. Chip designers can modify the tap coefficients to make their chips work. You have to design in enough compensation flexibility 25:00 PAM vs. QAM? Is QAM just an RF technique, or can it be used in a digital setting? Steve suspects QAM will break into the digital communication space instead of just being used in coherent coms 26:30 PAM4 is mostly applicable to the 200 GbE and 400 GbE, and something is going to have to happen 26:48 A number of other technologies are starting to look into PAM4. InfiniBand, Thunderbolt, PCIe, etc. #PAM3 #PAM4 #QAM #Electronics #Digital #Communications #Podcast #electricalengineeringpodcast #engineeringpodcast #Technology #Jitter #Ethernet #ElectricalEngineering #SNR