Q9: Why do ECL Signals need to be terminated and where do I get this 50 Ω/-2 V Termination?

A9: ECL signals have very fast transition times. For devices in the 10KH and 100KH series, the output transition times are in the 1.5 to 2 ns range. For devices in the 10EP/100EP series, however, transition times can be in the 150-250 ps range. When dealing with transmission of signals having these short transition times, terminating the signal into the Characteristic Impedance, Z_o, of the interconnecting device is mandatory, not an option. For coaxial cables, common values for Z_o are 50 Ω, 75 Ω, and 93 Ω. Other Z_o values such as 78 Ω and 124 Ω are available in shielded twisted cables.

The general theory is that when the transition time of the signal is "short" compared to the interconnect propagation delay, then a "long line", or transmission line, environment exists, and transmission line treatment of the circuit must be used. Typical propagation delay of a 50 Ω cable, such as the RG178, is 1.65 ns/ft. A short discussion providing an intuitive picture of what the transmission line effect is all about will soon be available via our FAQ section under the category "Transmission Line".

There is no hard and fast rule which separates a "short line" from a "long line," although numbers from 2 to 5 for the ratio of the interconnect delay to the signal transition time have been suggested for the definition of a "long line". More discussion on this subject can be found in Motorola MECL Systems Design Handbook and in a number of GaAs device data books. Rigorous treatments of the transmission theory can be found in the classical text book, Walter C. Johnson, "Transmission Lines and Networks", McGraw-Hill, 1950, Part I, Chapters 1-9.

For a listing of PRL products using ECL Circuits, click here

Q10: What are reflections and how do I get rid of them?

A10: Reflections are energies not absorbed at the load that must be returned or reflected back to the source. There will be no reflection when the load impedance is equal to the Characteristic Impedance Z_o of the transmission media.

For a listing of PRL products using ECL Circuits, click here

Q11: Can a Differential Input ECL device be used as a Voltage Comparator?

A11: Yes. Many devices in the newer ECLinPS and ECLinPS Lite families have a typical common mode range of V_CC–0.4 V maximum to V_EE +2.5 V minimum. This relatively wide common mode range enables a differential input ECL receiver device, shown in Fig. 2A, 2B or 2C to be used as a comparator. 

Either input in Fig. 2A can also be driven by a single-ended AC coupled sine or square wave signal, because such a signal would swing symmetrically with respect to the non-driven input which is now biased at V_TT.

In fact, the PRL-430 Differential ECL Receiver is used as a receiver/comparator for detecting weak signals in a number of satellite communications systems. The complementary outputs of the PRL-430 are suitable for driving up to 200 feet of 50 Ω cables.

For a listing of PRL products using ECL Circuits, click here

Q12: Can an ECL Output Drive an AC coupled 50 Ω load?

A12: When driving an AC coupled 50 Ω load, the ECL output emitter follower must obtain its DC supply current from the pull-down resistor. If the logic swing is to be maintained at 800 mV, the discharge current flowing out of the 50 Ω load resistor during the downward swing of the signal must be equal to (0.8 V/50 Ω)=16 mA, and this current flows into the output NPN emitter, as shown in Fig. 7. 

Since the NPN emitter can only source current, the current in the pull-down resistor must be made larger then 16 mA so that the emitter current I_E is always positive. Assuming that the "lo" state output emitter voltage is –1.6 V, the maximum value of the pull-down resistor is calculated to be equal to (5.2 V-1.6 V)/16 mA=225 Ω. Typically, a 200 Ω pull-down resistor is used when AC coupling to a 50 Ω load is required.

For power consumption reasons, most equipment with ECL outputs do not include internal pull-down resistors. A 200 Ω pull-down resistor draws an average current of 20 mA. In a system where 64 output drivers are required, for example, this translates to an additional current drain of 1.28 A if every output has a 200 Ω pull-down resistor.

PRL products using ECL Circuits:

Our line of Logic Level Translators

The following Logic Function Modules:

• PRL-170 ECL Xtal Clock Source
• PRL-255 ECL Frequency Divider
• PRL-258 4 Phase ECL Clock Generator
• PRL-430 Dual Channel Differential ECL Receiver
• PRL-431 1:2 Differential Fan Out ECL Buffer
• PRL-434 1:4 Differential Fan Out ECL Buffer
• PRL-435 Differential Input AND Gate
• PRL-470 High Speed, Line Driver-Level Translator
• PRL-480 Dual Channel Programmable ECL Delay Module