ECL vs PECL vs LVPECL-Difference between ECL PECL LVPECL

This page compares ECL vs PECL vs LVPECL and mentions difference between ECL, PECL and LVPECL. It mentions advantages and disadvantages of ECL, PECL and LVPECL logic families.

ECL-Emitter Coupled Logic

ECL was introduced as alternative of TTL logic family due to its high speed data transmission. This is due to the fact that transistors remain in active region always. Hence they can change the state very fast.

Following are characteristics of ECL logic family.
• propagation rate is about 1 to 2ns
• Noise immunity and power dissipation is worst of all the logic families.
• High Level is -0.8V and Low Level is -1.8V
• It has differential input amplifier, Internal temperature and voltage compensated bias network, emitter follower outputs
• ECL gates provide both true as well as complemented outputs.

There are two disadvantages of ECL as follows.
➨It requires relatively high currents.
➨It relies on negative power supply. This makes it difficult to interface with positive supply based devices on the system.
Refer Advantages and disadvantages of ECL for more information.

PECL-Positive Emitter Coupled Logic

• Both PECL and LVPECL have been introduced in the year 1960 as offshoots of earlier ECL logic family.
• PECL operates using positive supply voltage of 5V.
• The PECL outputs are used in high speed clock circuits.

Following are the benefits or advantages of PECL.
➨It offers high noise immunity.
➨It can drive high data rates over longer distance.
➨It offers good jitter performance due to larger voltage swing.

Following are the drawbacks or disadvantages of PECL.
➨It has large power consumption due to use of 5V supply and external DC biasing compare to single ended supply.

LVPECL-Low Voltage Positive Emitter Coupled Logic

• LVPECL circuits use 3.3V or 2.5V power supply which is lower compare to 5V used by PECL. This voltage is same as used by CMOS devices.
• LVPECL electrical specification is similar to LVDS interface but it operates at large differential voltage swing.
• It is less power efficient compare to LVDS.
• It can operate very fast with frequencies upto 10 Gbps.
• LVPECL output currents are about 15 mA, which is derived from open emitter. Resistive load is terminated to produce voltage. It uses 50 Ohm impedance trace and 50 Ohm thevenin equivalent load. The same is shown in the figure-1 below.
• It is recommended that all the unused outputs should be terminated with same method and should not be left floating.
• Gigabit Ethernet protocols and fiber channel based protocols are implemented using LVPECL.

Following are the advantages of LVPECL.
➨Low power consumption due to use of less value voltage.
➨It can operate very fast.