ECL vs PECL vs LVPECL: Key Differences Explained

This article compares ECL, PECL, and LVPECL, highlighting the key differences between these logic families, along with their advantages and disadvantages.

ECL: Emitter Coupled Logic

ECL emerged as an alternative to the TTL logic family, primarily due to its high-speed data transmission capabilities. This speed stems from the fact that transistors in ECL circuits always remain in the active region, allowing them to switch states very quickly.

Here are some key characteristics of the ECL logic family:

• Propagation Delay: Approximately 1 to 2 ns
• Noise Immunity & Power Dissipation: Considered the worst among all logic families.
• Voltage Levels: High Level is -0.8V, and Low Level is -1.8V
• Internal Design: Features a differential input amplifier, an internal temperature and voltage-compensated bias network, and emitter follower outputs.
• Output Flexibility: ECL gates provide both true and complemented outputs.

Disadvantages of ECL:

• Requires relatively high currents.
• Relies on a negative power supply, making it difficult to interface with positive supply-based devices.

PECL: Positive Emitter Coupled Logic

• PECL, along with LVPECL, was introduced in the 1960s as an offshoot of the earlier ECL logic family.
• PECL operates using a positive supply voltage of 5V.
• PECL outputs are commonly used in high-speed clock circuits.

Advantages of PECL:

• High noise immunity.
• Ability to drive high data rates over longer distances.
• Good jitter performance due to a larger voltage swing.

Disadvantages of PECL:

• Large power consumption due to the use of a 5V supply and external DC biasing compared to a single-ended supply.

LVPECL: Low Voltage Positive Emitter Coupled Logic

• LVPECL circuits use a lower power supply voltage (3.3V or 2.5V) compared to the 5V used by PECL. This voltage is often the same as that used by CMOS devices.
• The electrical specifications of LVPECL are similar to the LVDS interface, but it operates at a larger differential voltage swing.
• It’s less power-efficient compared to LVDS.
• It can operate very fast, with frequencies up to 10 Gbps.
• LVPECL output currents are approximately 15 mA, derived from an open emitter. A resistive load is terminated to produce a voltage. It utilizes a 50 Ohm impedance trace and a 50 Ohm Thevenin equivalent load, as shown in the figure below.
• It’s recommended that all unused outputs be terminated using the same method and not left floating.
• Gigabit Ethernet and Fiber Channel protocols are often implemented using LVPECL.

Advantages of LVPECL:

• Low power consumption due to the use of a lower voltage.
• High-speed operation.