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NRZ (Non-Return-to-Zero)

Table of Contents

  1. Definition
  2. How it Works
  3. Features
  4. Benefits
  5. Enabling Technologies

What is NRZ?

Non-Return-to-Zero (NRZ) is a binary encoding scheme used in digital communication systems to transmit data over serial links. In NRZ signaling, logical ‘1’s and ‘0’s are represented by two distinct voltage levels, and the signal does not return to a baseline (zero) between bits. This method is widely used in high-speed interfaces such as PCI Express (PCIe), Ethernet, and USB, particularly in earlier generations before the adoption of more advanced modulation schemes like PAM4.

How NRZ works

NRZ transmits data by maintaining a constant voltage level throughout the bit interval. A logical ‘1’ might be represented by a high voltage, and a logical ‘0’ by a low voltage (or vice versa). Since the signal does not return to zero between bits, NRZ is simple to implement and requires less bandwidth than some other encoding schemes. However, long sequences of identical bits can cause synchronization issues due to lack of transitions, which is why clock recovery and encoding techniques like 8b/10b or 128b/130b are often used alongside NRZ to ensure sufficient signal transitions.

What are the key features of NRZ?

  • Two-level signaling (high and low voltage)
  • No return to zero between bits
  • Often paired with line coding for clock recovery
  • Limited bandwidth efficiency compared to multi-level schemes
  • Susceptible to signal degradation over long distances or high data rates
 

What are the benefits of NRZ?

  • Simplicity: Easy to implement with minimal hardware complexity.
  • Efficiency: Uses fewer transitions, conserving power in some applications.
  • Compatibility: Supported by many legacy and current high-speed interfaces.
 

Enabling Technologies

NRZ is foundational in:

  • PCIe Gen 1–3
  • USB 2.0 and 3.0
  • SATA and SAS interfaces
  • Ethernet standards up to 10GBASE-T
  • SerDes (Serializer/Deserializer) PHYs for chip-to-chip communication

As data rates increase, NRZ is being replaced by PAM4 (Pulse Amplitude Modulation with 4 levels) in newer standards like PCIe 6.0, 100G Ethernet, and DDR5, which double the bits per symbol to improve bandwidth efficiency.

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