High-Speed Logic Families: Which family supports the highest practical operating frequency in standard digital design?

Difficulty: Easy

Correct Answer: ECL

Explanation:


Introduction / Context:
Different logic families trade off speed, power, noise margin, and voltage swing. This question asks which family, in practice, reaches the highest operating frequency, a key consideration for high-speed clocks, serializers, and RF-adjacent digital interfaces.


Given Data / Assumptions:

  • ECL (emitter-coupled logic) uses differential pairs and avoids transistor saturation.
  • TTL subfamilies like 74S/74AS are fast but still involve TTL totem-pole stages.
  • HCMOS emphasizes low power and wide supply ranges rather than absolute maximum speed.
  • 54S is the military-rated counterpart of Schottky TTL, not fundamentally faster than its commercial sibling.


Concept / Approach:
ECL maintains transistors in their active region and uses small voltage swings, minimizing charge storage and reducing delay. TTL variants improved speed with Schottky clamping but still cannot match the ultra-low propagation delays of classic ECL families at comparable process nodes.


Step-by-Step Solution:
Identify the fastest architecture: ECL with non-saturating differential pairs.Compare with TTL (74S/74AS): faster than standard TTL, but still slower than ECL at top end.Compare with CMOS (HCMOS): lower power, excellent density, but historically slower than ECL in pure speed contests.Conclude that ECL enables the highest operating frequency among the options.


Verification / Alternative check:
Classic ECL data sheets (e.g., 10K/100K families) show propagation delays in the sub-nanosecond to low-nanosecond range, outpacing contemporary TTL and CMOS families designed for general logic.


Why Other Options Are Wrong:

  • 74AS: Very fast TTL, but still slower than ECL at the extreme.
  • HCMOS: Prioritizes low power and high integration; not the fastest at top frequency limits.
  • 54S: Military-temperature Schottky TTL; speed on par with 74S, not exceeding ECL.


Common Pitfalls:
Confusing modern high-speed CMOS I/O standards with general HCMOS logic families; also, assuming military rating (54xx) implies a speed advantage.


Final Answer:
ECL

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