IEEE CX4 Quantitative Analysis of Return-Loss Aaron Buchwald & Howard Baumer Mar 003
Return Loss Issues for IEEE 0G-Base-CX4 Realizable Is the spec realizable with standard packages and I/O structures Define guideline for package requirements to meet specifications Consider bondwire and trace inductance Impact on Performance How does return loss effect margin What is minimum short-channel
Example Differential RX Termination IC Implementation Single-Ended Lumped Model V COM V in V out to RX + to RX - 50Ω 50Ω C = pf ~ 50Ω RX Data Lumped capacitance of Pad, ESD structure and input devices
S-Parameters for Capacitive Termination S-parameters Single-Ended Lumped Model S = s + s s V in C = pf V out Where, = RC S ( s) = TX ( s) RX S = s + s Reflection Coefficient / Return-Loss is high-pass function with unity-gain at high frequencies
Reflection Coefficient for DC Mismatch Reflection Coefficient Reflection Coefficient vs. Frequency Γ = where, Γ 0 = R R L L Γ 0 + R + R s = s ( R ) L R C Reflection Coefficient (db) = 75W = 0. CX4 Template = 33W = -0. Frequency (GHz) = 50W = 0 Reflection Coefficient (Return-Loss) is a high-pass function with unity-gain at high frequencies and DC gain determined by mismatch in
Reflections in Time Domain Impulse Response [ + Γ ] t 0 h( t) = δ ( t) + e u( t) Step Response step ( t) = Γ0 0 t t [ + Γ ] e u( ) Amplitude (V) Step Response (TDR) = 50W = 0 = 75W = 0. = 33W = -0. Time (ps)
Reflections in Time Domain: Pulse Response Amplitude (V) = 33W = -0. Amplitude (V) = 50W = 0 Unit Interval (30ps) Amplitude (V) = 75W = 0. C = pf ~ 50Ω Unit Interval (30ps)
Return Loss Issues for IEEE 0G-Base-CX4 Realizable Is the spec realizable with standard packages and I/O structures Define guideline for package requirements to meet specifications Consider bondwires and trace inductance Impact on Performance How does return loss effect margin What is minimum short-channel
RX with Bondwire & Trace Inductance IC Implementation Single-Ended Lumped Model to RX + to RX - 50Ω 50Ω V COM V in V out L = 5nH C = pf ~ 50Ω RX Data Lumped inductance of package trace and bondwire
S-Parameters: Bondwire & Trace Inductance S-Parameters: Bondwire & Trace Inductance S-Parameters: Bondwire & Trace Inductance L = 5nH C = pf + + + + + + = LC s R L RC s LC s R L RC s LC s R L RC s S [ ] [ ] ) ( s s s s s RX + + + + = S RC R L = = Where, Reflection Coefficient / Return-Loss is high-pass function with unity-gain at high frequencies
Reflection Coefficient with DC Mismatch Γ RX ( s) = Γ 0 + + s [ ( ) ( )] ( ) Γ0 + Γ0 + s + Γ0 [ ( ) ( )] ( ) s Γ 0 + + Γ 0 + s + Γ 0 where, Γ 0 = R R L L L = R RC = R + R L = 5nH C = pf ~ 50Ω
Reflection Coefficient for pf & 5nH Termination Network Reflection Coefficient vs. Frequency L = 5nH C = pf ~ 50Ω Reflection Coefficient (db) = 33W = -0. CX4 Template = 50W = 0 = 75W = 0. Frequency (GHz) Return-Loss violates template for pf and 5nH for wide range of values for RL
Reflection Coefficient for 0.75pF & 3nH Termination Network L = 3nH C = 0.75pF ~ 50Ω Reflection Coefficient (db) Reflection Coefficient vs. Frequency = 33W = -0. CX4 Template = 50W = 0 = 75W = 0. Frequency (GHz) Return-Loss meets template for 0.75pF and 3nH for wide range of values for RL. Notice resonant null for 75Ω load. Spec is achievable, but care must be taken in package
Reflections in Time Domain Termination Network Step Response (TDR) L = 3nH C = 0.75pF ~ 50Ω Amplitude (V) = 50W = 0 = 75W = 0. = 33W = -0. Step response shows open circuit at high-frequency due to inductor and some ringing due to nd-order dynamics Time (ps)
Reflections in Time Domain: Pulse Response Amplitude (V) = 33W = -0. Amplitude (V) = 50W = 0 Unit Interval (30ps) L = 3nH Amplitude (V) C = 0.75pF ~ 50Ω = 75W = 0. Unit Interval (30ps)
Return Loss Issues for IEEE 0G-Base-CX4 Realizable Is the spec realizable with standard packages and I/O structures Define guideline for package requirements to meet specifications Consider bondwires and trace inductance Impact on Performance How does return loss effect margin What is minimum short-channel
Definition of Terms: Frequency Domain S CH (s) Reflection at TX-end of Channel with 50-Ohm line S CH (s) Reverse transmission on channel with 50-ohm source and load S CH (s) Reflection at TX-end of Channel with 50-Ohm line TX RX RX TX TX S CH (s) RX S (s) Reflection at TX with 50-Ohm line Forward transmission on channel with 50-ohm source and load S (s) Reflection at RX with 50-Ohm line
Channel S-Parameters: 3m InfiniBand Cable Amplitude (db) S S Amplitude (db) S S Frequency (GHz) Frequency (GHz)
Definition of Terms: Time Domain h CH Reflection at TX-end of Channel with 50-Ohm line h CH Reverse transmission on channel with 50-ohm source and load h CH Reflection at TX-end of Channel with 50-Ohm line TX RX RX TX TX h CH RX h Reflection at TX with 50-Ohm line Forward transmission on channel with 50-ohm source and load h Reflection at RX with 50-Ohm line
Reflections in Time Domain: Pulse Response P P Amplitude (V) Amplitude (V) P P Unit Interval (30ps) Unit Interval (30ps)
First-Order Reflection Analysis: Path h CH Reflection at TX-end of Channel with 50-Ohm line Path : h CH *h *h CH Path : h CH *h *h CH TX RX RX TX TX h CH RX h Reflection at TX with 50-Ohm line Forward transmission on channel with 50-ohm source and load
First-Order Reflection Analysis: Path Path : h CH *h *h CH Path : h CH *h *h CH h CH Reflection at TX-end of Channel with 50-Ohm line TX RX RX TX TX h CH RX Forward transmission on channel with 50-ohm source and load h Reflection at RX with 50-Ohm line
First-Order Reflection Analysis: Path 3 h CH Reverse transmission on channel with 50-ohm source and load TX RX RX TX TX h CH RX h Reflection at TX with 50-Ohm line Forward transmission on channel with 50-ohm source and load Path 3: h CH *h *h CH * h * h CH Path 3: h CH *h *h CH * h * h CH h Reflection at RX with 50-Ohm line
First-Order Reflection Performance: 0m TX RX RX TX TX RX Margin @ BER 0-7 Blue: Signal, & Jitter Perfect Termination: 000.0-mV RL = 33Ω: 48.0-mV RL = 50Ω 35.7-mV RL = 75Ω 39.0-mV Yellow: Reflections
First-Order Reflection Performance: m TX RX RX TX TX RX Margin @ BER 0-7 Perfect Termination: RL = 33Ω: RL = 50Ω RL = 75Ω 545.0-mV 3.0-mV 43.9-mV 468.0-mV Blue: Signal, & Jitter Yellow: Reflections
First-Order Reflection Performance: 5m TX RX RX TX TX RX Margin @ BER 0-7 Perfect Termination: RL = 33Ω: RL = 50Ω RL = 75Ω 87.0-mV 44.7-mV 68.-mV 79.6-mV Blue: Signal, & Jitter Yellow: Reflections
Return Loss Conclusions Return-Loss specs can be achieved with standard packages and ESD structures DC matching is not critical Signal has no DC content 4-dB or 50% DC deviation can be easily tolerated Resonant nulls could improve return-loss but can not be controlled accurately in production Long channel margin degrades approximately 30-mV due to reflections Short channel margins are acceptable with 0m channel In practice the package and board trace will provide filtering for 0m channel and reflections will be much less severe (similar to m channel case)
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