Schottky Ttl 1970s Measure Noise Margin at Higher Switching Speed
# Schottky TTL (early 1970s): Step 13 — Measure Noise Margin at Higher Switching Speed
Measure Noise Margin at Higher Switching Speed. Schottky TTL's static noise margins — the DC voltage gap between a driving gate's worst-case output level and a receiving gate's input threshold, for both the high and low states — stayed close to standard TTL's, typically around 0.4 V on the low side (V_OL max around 0.4-0.5 V against V_IL max around 0.8 V) and a comparable margin on the high side, because the logic-level conventions were deliberately kept compatible with standard TTL.
## 1. Static Noise Margin Holds, Dynamic Margin Tightens
Schottky TTL's static noise margins — the DC voltage gap between a driving gate's worst-case output level and a receiving gate's input threshold, for both the high and low states — stayed close to standard TTL's, typically around 0.4 V on the low side (V_OL max around 0.4-0.5 V against V_IL max around 0.8 V) and a comparable margin on the high side, because the logic-level conventions were deliberately kept compatible with standard TTL. What changed was the dynamic picture: a gate switching three to four times faster (74S) or at comparable speed but lower drive impedance (74LS) produces faster edge rates, and faster edges couple more effectively into adjacent signal traces and power/ground planes through parasitic capacitance and inductance, so transient noise — crosstalk and ground bounce specifically — became a proportionally larger share of the total noise budget even though the static DC margin on the data sheet looked unchanged.
## 2. Real Diagram: Static vs. Dynamic Noise Margin at Increasing Switching Speed
## 3. The Practical Design Consequence
The practical upshot was that Schottky TTL, especially 74S, demanded more careful attention to decoupling capacitor placement, ground-plane continuity, and trace routing than standard TTL had, even though the data-sheet noise margin numbers barely moved — a lesson that generalized directly into the ground-bounce engineering discussed in the next step, and that paralleled, at a smaller scale, the transmission-line discipline the ECL series' Step 12 describes as necessary at ECL's still-higher speeds.
## 4. Companion Diagram: Stage Sequence and Trend Summary
## Measure Noise Margin at Higher Switching Speed's Place in the Process Lineage
This is step 13 of 20 in the Schottky TTL (early 1970s) series, distinguishing Schottky TTL's largely unchanged static noise margin from its tightened dynamic (transient) noise behavior.