Why is ECL faster ?

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yann-guidon-ygdesYann Guidon / YGDES 07/12/2018 at 11:259 Comments

Lately I was looking for very fast diodes to design faster DTL/TTL discrete gates.

@Al Williams just released an article on :

The recovery time makes a difference in several designs including switching power supplies. If you dig into the physics, there is a usually a trade-off between several other parameters and recovery time. Just to give you an idea, the datasheet for a BAT42 Schottky diode says the reverse recovery time at 10mA is no more than 5 ns."

I don't know the exact figures in practice in a logic gate but 5ns of recovery is not good. That's about the propagation time of a DCTL inverter in the CDC6600.

Rectification speed was already a burning subject in the 40s because it was essential to the war effort (faster diodes means higher carrier frequencies, shorter wavelengths and better radar resolution)

ECL prevents all these issues because

  1. there are only NPN transistors
  2. no diode (no recovery time)
  3. no saturation

However there are more transistors... so maybe DCTL is an interesting alternative ?


Julian wrote 07/13/2018 at 23:58 point

Although, the BAT42 is a pretty old diode -- we can do a little bit better these days.  ON Semiconductor's NSR02F30MX has 2.4ns typical / 3ns max recovery time.  And that's to start conducting 10mA ... if you only needed, say, 1mA to cause your gate to switch on, that'd be substantially faster.

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Yann Guidon / YGDES wrote 07/17/2018 at 03:04 point

I haven't seen them in the wild... but you make an interesting point, about the recovery time dependency on the current.

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K.C. Lee wrote 07/17/2018 at 03:18 point

Even my dusty reel of silicon switching diode BAV99 is faster than the BAT42.  The recovery time at 10mA is 4ns max.

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Yann Guidon / YGDES wrote 07/17/2018 at 12:57 point

Now it would be awesome if someone tested the recovery time of many types of diodes at different driving levels :-D

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Yann Guidon / YGDES wrote 07/17/2018 at 14:26 point

@K.C. Lee  that's the principle of the #The Diode Clock  :-P

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Julian wrote 07/13/2018 at 23:46 point

If only they weren't so damned expensive, I'd love to play around with tunnel diodes.  They have basically no recovery time (at least if you use them in their tunnelling voltage regime, rather than pushing higher voltages that cause them to start working like a regular junction diode) because all that's happening is your applied voltage prevents the charge carriers already in the component from tunnelling the wrong way across the junction, so they stop moving randomly and start flowing in the right direction.  They can be made to self-resonate in the range of hundreds of gigahertz (!).  E.g. - -- looks like this one would most likely resonate at almost exactly 100GHz. 

Unfortunately the cheapest I've been able to find them is ~$10 *each*. :(

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K.C. Lee wrote 07/12/2018 at 12:15 point

I highly recommend: ''mecl system design handbook'' from Motorola.  First result on google returns the On Semi pdf.  Also google for ''MECL DATA book DL122/D'' first result for pdf.

You probably want to stick with the10K design as it is a simpler design. (page 42 figure 1 of data book)

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Yann Guidon / YGDES wrote 07/12/2018 at 12:29 point

the original version is cool as well

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