Chord Signaling for High-Speed Chip-to-Chip Applications

Chord Signaling for High-Speed Chip-to-Chip Applications

Case Studies

Kandou is an interface technology company that specializes in the invention, design, license, and implementation of industry-leading chip-to-chip link solutions. Kandou’s patented Chord™ signaling architecture is a generalization of differential signaling. While differential signaling delivers one bit on two correlated wires, Chord signaling delivers n bits on n+1 correlated wires. An associated “code” is the combination of the signaling on the wires and the comparators used to identify the bits.

According to Kandou, this new category of signaling can provide two to four times the bandwidth of conventional serializer/deserializer (SerDes) technologies at half or less the total power consumption. New system architectures for products ranging from mobile devices to hyperscale data centers, artificial intelligence (AI), and machine learning (ML) can benefit from the use of Chord signaling.

Chord signaling types include, amongst many others:

  • ENRZ: Ensemble NRZ transmits three bits on four correlated wires
  • CNRZ-5: Chord NRZ transmits five bits on six correlated wires
  • ENRZ and CNRZ-5 enable significant power savings depending on the application. For example, with CNRZ-5 enabled Ultra Short Reach (USR) SerDes, it is possible to save from three to ten times the power in some applications versus other traditional signaling methods.

The Key Issues

Technology advancements, such as AI and machine learning, require faster communication over more challenging channels. As a result, data centers need to evolve from 100 gigabit Ethernet (100GE) data speeds to 400GE and beyond. 400GE speeds require moving from non-return-to-zero (NRZ) to new modulation technologies such as four-level pulse amplitude modulation (PAM4), or the use of Chord signaling.

As data rates increase, channel loss between the transmitter and the receiver becomes more important, as well as minimizing power consumption. Printed circuit board traces, connectors, and cables all contribute to loss in the signal path. This channel loss results in intersymbol interference (ISI) that depends on the channel material and dimensions, the data rate, and the bit pattern. High-speed digital receivers tolerate a certain amount of total jitter, which typically includes some ISI caused by channel loss. According to Kandou, its SerDes technology provides high-system bandwidth, much lower vulnerability to ISI and to the reflection form of ISI than PAM4, and low power consumption via a much lower need for equalization through the use of Chord signaling.

Figure 1 shows one of Kandou’s multi-chip modules with six dye connected over multiple traces. One of the dye has two breakout channels connected to Keysight’s sampling oscilloscope. The scope allows Kandou to take linear combinations of the wire values and display the binary eyes that their receiver will sample. Figure 2 shows a screenshot from the sampling oscilloscope displaying the measurements of the wire values (incoming signals) and the opening of the eye at the receiver (decoded sub-channels).

Kandou needed a way to emulate transmit (signal generation) and receive (signal capture) to validate that its Chord signaling algorithms perform as expected. Validation required complex and time-consuming measurements, so it was essential to Kandou to find a way to simplify and reduce test setup time as well.

The Solution

Using Keysight test equipment, Kandou was able to validate its Chord signaling algorithms. Using similar test methods, Kandou’s customers can also validate the use of Chord signaling in their designs for today’s 400GE speed class, as well as for next-generation speeds even before digital signal processing (DSP) silicon is available. The following test equipment is required:

  • M8196A arbitrary waveform generator (AWG)
  • N1000X DCA-X mainframe with N1045B 60 GHz remote head module and N1010A FlexDCA sampling oscilloscope software

The M8196A AWG provides a sampling rate up to 92 gigasamples per second (GSa/s) on up to 4 differential channels simultaneously, 8-bit resolution, and an analog bandwidth of 32 gigahertz (GHz). The AWG connects to any channel of choice such as a PCB, connector, etc. Chord signaling delivers n bits on n+1 correlated wires. Encoded waveforms using Chord signaling are loaded into the AWG to emulate the Kandou transceiver (Tx). Four AWG channels are required to transmit ENRZ and six channels for CNRZ-5.

A digital communications analyzer (DCA) is Keysight’s term for instruments otherwise known as sampling oscilloscopes or equivalent-time sampling oscilloscopes. Sampling oscilloscopes display the time-domain waveforms of optical and electrical transmitters used in high-speed serial communications systems. They feature superior accuracy, dedicated analysis features, and lower cost.

The N1000X DCA-X mainframe with the N1045B remote head module provides 60 GHz bandwidth and two or four channels per module. The DCA captures the waveforms generated by the AWG and displays the linear combinations according to Chord signaling. The quality of the signal is determined after it has passed through the particular channel at the desired speed. The scope includes the N1010100A FlexDCA R&D software that provides math functions (supplied by Kandou) for CNRZ-5 and ENRZ, including automated de-skew capability.

Conclusion

Driven by 5G technology, data-intensive applications such as virtual reality (VR), AI, ML, Internet of Things (IoT) and the roll-out of autonomous vehicles will create a surge in computing, storage, and performance demands on data centers. Chord signaling provides an alternative to PAM4 modulation to reach the hundreds of gigabits per second to terabits per second speeds required to support these new applications. Chord signaling is a multi-wire signaling approach that generalizes differential signaling. According to Kandou, Chord signaling delivers faster, denser, simpler, more robust, and lower power interconnects compared to PAM4 and NRZ technologies.

With Keysight’s M8196A AWG, users can generate the multi-wire Chord signaling CNRZ-5 and ENRZ encoded waveforms used in receiver test applications to validate their designs. Keysight’s transmitter test solutions leverage the multi-channel acquisition system of the N1000A DCA-X wide-bandwidth oscilloscope and the N1010100A FlexDCA R&D software to decode and analyze Chord signaling applications quickly. Test setup times were reduced from hours to minutes. Keysight’s first to market solutions enabled Kandou to accurately validate its Chord signaling technology. Kandou’s customers can also use Keysight’s test solutions to validate Chord signaling in their designs for 400GE and beyond, even before next-generation DSP silicon is available.