JESD204B

JESD204B guarantees a fixed and repeatable latency from the sampling edge of the ADC to the arrival of data at the FPGA fabric. This is achieved through the Subclass 1 mechanism, which uses a common SYSREF signal distributed to all converters and logic devices. Deterministic latency is non-negotiable for DPD systems, where the time alignment between the transmitted reference and the observed feedback signal must be sample-accurate to extract a valid predistortion model. Without it, phase coherence across multiple converter channels and power-up cycles cannot be maintained.

Each serial lane uses 8B/10B encoding to embed the clock into the data stream, eliminating the need for a separate forwarded clock. This encoding ensures DC balance and provides sufficient transition density for the receiver's Clock and Data Recovery (CDR) circuit. During the Initial Lane Alignment Sequence (ILAS), the transmitter sends a known comma character (K28.5) that the receiver uses to identify symbol boundaries and align all lanes into a coherent multi-lane data frame. This is essential for aggregating the high sample-rate, multi-bit feedback data from wideband observation ADCs.

JESD204B replaces wide, parallel LVCMOS or LVDS buses with a small number of high-speed SERDES lanes. A single JESD204B lane operating at 10 Gbps can carry the equivalent of dozens of parallel I/O pins. For a DPD observation receiver with a 16-bit ADC sampling at 2.5 GSPS, JESD204B can transport the full data rate over just 4-8 differential pairs, compared to over 64 pins for a parallel interface. This drastically reduces PCB routing complexity, crosstalk, and FPGA I/O bank consumption, freeing resources for the predistorter core logic.

JESD204B Subclass 1 enables the synchronization of multiple data converters across a system. The SYSREF signal acts as a system-wide timing reference that resets the internal Local Multi-Frame Clock (LMFC) counters in all devices simultaneously. This allows an FPGA to coherently process data from multiple ADCs—such as in a MIMO DPD system or a dual-polarization observation path—knowing that samples from different converters are precisely aligned in time. The protocol also handles multi-chip synchronization for converters that exceed the bandwidth of a single device.

To avoid electromagnetic interference (EMI) and spectral lines caused by repetitive data patterns, JESD204B applies an optional self-synchronous scrambler to the data stream. The scrambling polynomial (1 + x^14 + x^15) whitens the serial data, spreading its energy evenly across the spectrum. This is particularly important in sensitive RF environments where the DPD feedback path sits close to the receiver front-end; concentrated spectral tones on the digital interface could couple into the analog domain and degrade the Error Vector Magnitude (EVM) of the overall system.

JESD204B supports configurable link parameters to match the specific converter and FPGA requirements:

• L: Number of lanes per link (1 to 8)
• M: Number of converters per device
• S: Samples per converter per frame
• N': Sample resolution in bits
• K: Frames per multi-frame This parameterization allows a single FPGA SERDES quad to interface with converters of varying resolution and sample rate. For DPD, a common configuration is L=4, M=1, S=1, N'=16, mapping a single 16-bit ADC to four lanes for a 10 Gbps aggregate throughput.

Multi-lane JESD204B links introduce lane-to-lane skew that must be compensated at the receiver. The protocol handles this through:

• ILAS (Initial Lane Alignment Sequence): Transmitted during link establishment, containing comma characters and lane identification data
• Elastic buffers in the receiver FPGA fabric absorb individual lane delays
• Multiframe boundaries marked by K28.5 control characters enable realignment
• For wideband DPD feedback paths using 4-8 lanes, proper de-skew ensures the complex I/Q sample ordering is preserved across the parallel data interface