Q13: What happens if I Over Drive an ECL Input?

A13: If one refers back to Fig. 1, the collector voltage of the "on" transistor Q₁ is V_CC-0.8 V. 

If the base of Q₁ is raised above V_CC - 0.2 V, the base to collector diode of Q₁ will be forward biased, resulting in increased turn off delay of Q₁. Excessive overdrive applied to any ECL input above V_CC or below V_EE can damage the device.

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

Q14: Why can’t I get any output from an ECL Output Connector, and how should I measure it?

A14: In Fig. 1 above, it was shown that ECL outputs are open emitters. Without pull-down resistors, the outputs are turned off and therefore, there is no output voltage. Even if the output has an internal pull-down resistor, it may still not be possible to measure the true output signal either, unless the measurement device is impedance-matched to the ECL output structure. The reason for this problem is that the internal connection between the output ECL device pin and the output connector is most likely a "long line," and neither the scope probe nor the high impedance scope input represents an impedance match to the ECL output structure.

If one was to connect the ECL output directly to the 50 Ω oscilloscope input, there would no output either, because the output emitter will be turned off by the ground-referenced 50 Ω input, even if the output has a 200 Ω pull-down resistor. However, AC coupling an ECL output with an internal 200 Ω pull-down resistor to a 50 Ω input instrument is OK (See Question 12).

So much for not being able to measure an ECL signal, now we shall show how it can be measured using an ECL Terminator.

ECL/PECL output circuits are designed to drive 50 Ω loads terminated into a terminating voltage V_TT= V_CC-2 V. For ECL, V_CC = 0 V, and V_TT = -2 V. For PECL, V_TT = +3 V. If the input of a measurement instrument is made to look just like a 50 Ω/V_TT termination, then all should be well. In fact, that is exactly what an ECL or PECL Terminator is.

An ECL Terminator is basically a biased 50 Ω microwave attenuator. The input has an equivalent 50 Ω/-2 V terminatation, and the output is suitable for driving a ground referenced 50 Ω input instrument. Similarly, the input of a PECL Terminator has an equivalent 50 Ω/3 V termination. In order to protect sensitive instruments, however, a properly designed ECL/PECL terminator should have a near ground level output (See the PRL-550NQ4X data sheet).

For measuring a differential ECL output using a Twinax or Triax connector, either an instrument with a differential input and the proper termination or a differential to single-ended converter is required.

Caution! Do not connect the output of a PECL device to an ECL terminator or to a ground-referenced 50 Ω input instrument. This could spell instant disaster for the PECL device or the instrument (See Question 15). Although connecting an ECL output to a PECL Terminator may not destroy the ECL device, it could cause gradual degradation of the output emitter follower, due to possible excessive reverse bias voltage developed across the base to emitter junction.

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

Q15: What happens if I short an ECL or PECL Output to Ground?

A15: In Fig. 1 above, it is shown that the collectors of the ECL output emitter followers are connected to V_CC. When V_CC is ground, shorting the emitter to ground merely turns off the emitter follower and no damage will occur.

This is not the case when V_CC is = +5 V. The transistor output current is limited only by b times its base current, which is supplied by R₁ or R₂ connected to V_CC. In most cases, the device is destroyed instantly. In fact, connecting a PECL output device to a ground-referenced 50 Ω load often destroys the device instantly as well.

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

Q16: How do I tap into an ECL I/O Line to look at the signal?

A16: As discussed in Question 9, fast ECL signals need to have their I/O lines terminated. A 10X coaxial probe, as shown in Fig. 8, allows the extraction of a part of the signal for measurement and still maintains the impedance match at the I/O ports.

The 10X coaxial probe splits a signal into a 90% amplitude output and a 10% amplitude, or 10X, output, which can be connected to the 50 Ω input of an oscilloscope. This 10X coaxial probe is designed for use in a 50 Ω environment, where every port is to be terminated into 50 Ω . When properly terminated, it has the property that the input impedance of the signal input port and the output impedance of the 90% signal output port are both 50 Ω .

Although the output resistance of an ECL device is not 50 Ω, the input port of the 10X probe does look into a 50 Ω impedance when the other two ports are terminated into 50 Ω. Therefore, when an ECL output circuit is connected to the input port of this probe, it sees a 50 Ω impedance when the other two ports are terminated into 50 Ω/-2 V and 50 Ω, respectively. This satisfies the condition that the ECL output is properly terminated.

This probe arrangement works quite well with either a single-ended ECL or PECL signal, although the 10% signal loss in the through path may be of concern as it reduces the noise margin of the signal as well. However, by using a dual channel probe, such as the PRL-860D, for transmitting both the true and complement output signals, the noise margin will be improved.

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-434A 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