TDS8000 and TDR Considerations to Help Solve Signal Integrity Issues

Control of transmission impedance is becoming more important as digital system designers seek faster speeds. Mismatches and variations of impedance cause reflections that add to system noise and jitter, especially with fast signals. This can lead to data and logic errors and severe, hard-to- identify reliability problems

–Time Domain Reflectometry - a measure of reflection in an unknown device, relative to the reflection in a standard impedance. –Compares reflected energy to incident energy on a single-line transmission system –Known stimulus applied to the standard impedance is propagated toward the unknown device –Reflections from the unknown device are returned toward the source –Known standard impedance may or may not be present simultaneously with the device or system under test TDR Definition

Typical TDR Applications –Printed Circuit Boards –Connectors –IC Packages –Cables and Interconnects TDR Measurements Are Used to Characterize the Signal Transmission Properties Of:

Typical TDR Measurements –Signal characteristic impedance –Differential signal characteristic impedance –Signal-signal Crosstalk –Signal propagation delay –Inductance and capacitance characterization Types of TDR Measurements used to characterize signal transmission properties include:

When Are TDR Measurements Needed? –To characterize electrical transmission properties in high bandwidth and high data-rate applications. –To guarantee the transmission properties meet the system performance requirements. –To verify manufacturing processes of PC boards, IC packages and connectors. Typical applications where TDR Measurements are needed include:

TDR Fundamentals - Typical TDR System –A Typical TDR system consists of TDR sampling head that generates fast step signals, samples the incident and reflected signals that are digitized by the oscilloscope. Load Z Load Transmission Line 0Z00Z0 50  Sample-Hold Gate To Oscilloscope Mainframe SMA Connector TDR Sampling Head Step Source

TDR Measurements – Typical System –TDR measurements provide a means to get quantitative characterization data of the transmission system. –The measurement comparison of the incident and reflected signals provide the data for analysis. LZLLZL LoadTDR Sampling Head Z0Z0

TDR Measurements - Reflection Coefficient –  (rho)  is the ratio of the reflected pulse amplitude to the incident pulse amplitude. = V reflected V incident  =  can be expressed in terms of the transmission line characteristic impedance, Z 0, and the load impedance, Z L. = V reflected V incident  = ( Z L – Z 0 ) ( Z L + Z 0 )

TDR Measurements - Reflection Coefficient for Matched Load There are some interesting boundary conditions for the value of the reflection coefficient, . V reflected V incident  = LZLLZL Load Z0Z0 –When Z L is equal to Z 0 – Matched Load V Reflected = 0 and  = 0 = 0 V =0 Reflected Wave is equal to zero. No Reflections.

TDR Measurements - Reflection Coefficient Boundary Values V reflected V incident  = –When Z L is Infinite – Open Load = V V =1 V reflected V incident  = –When Z L is equal Zero – Shorted Load V Reflected = -V Incident and  = -1 = -V V = Reflected Wave is equal but negative of incident wave. Reflected Wave is equal to the incident wave. V Reflected = V Incident and  = +1

TDR Overview TDR Waveforms - Open, Short and 50  terminations Amplitude Open (Z =  ) (Z = 50  ) Short (Z = 0) Time Reflected + 1  0  - 1  t0t0 t1t1 Incident

Measuring Impedance

TDR Measurements – Open and Shorted Load TDS 8000 Display

TDR Measurements – Oscilloscope Waveform Measurement Units –Oscilloscope TDR Measurements can use units of Volts, Ohms or  (Rho) for the vertical magnitude scale. –The horizontal axis represents unit of time. Time Units - Sec/Div Magnitude Units Volts/Div Rho/Div Ohms/Div -or-

TDR Measurements – Impedance Measurements With Cursors –Using a properly calibrated TDR Oscilloscope the horizontal waveform cursors can be used to make impedance measurements. Sec/Div Ohms/Div Cursor 1 Cursor 2 Measurement Readouts Cursor 1 = 50.0  Cursor 2 = 95.3  Delta 2-1 = 4.7 

TDR Waveforms - Shorted and Open Terminations –Short Circuit Termination –Open Circuit Termination V 0 2T P TPTP Z0Z0 Z L = 0 V 0 2T P Z0Z0 Z L = Open TPTP 2V

TDR Waveforms - Matched and Mismatched Load Terminations –Matched Load Termination –Mismatched Load Termination V 0 2T P TPTP Z0Z0 Z L = Z 0 V 0 2T P Z0Z0 TPTP Z L <> Z 0 Z L < Z 0 Z L > Z 0 V + V R

TDR Waveforms - Capacitor and Inductor Terminations –Capacitor Load Termination –Inductor Load Termination V 0 2T P TPTP Z0Z0 Z L = C V 0 2T P Z0Z0 TPTP Z L = L 2V

TDR Waveforms - Shunt Capacitance and Series Inductance Discontinuities –Shunt Capacitance Discontinuity –Shunt Inductance Discontinuity V 0 2T P TPTP Z0Z0 C V 0 Z0Z0 TPTP L Z0Z0 Z0Z0

TDR Waveforms - Inductance and Capacitance Discontinuities –Series Inductance – Shunt Capacitance –Shunt Capacitance – Series Inductance V 0 2T P TPTP Z0Z0 C V 0 Z0Z0 TPTP L Z0Z0 Z0Z0 C L

TDR Waveforms - Multiple Inductance and Capacitance Discontinuities –Capacitance – Inductance - Capacitance –Inductance – Capacitance - Inductance V 0 2T P TPTP Z0Z0 C V 0 Z0Z0 TPTP L Z0Z0 Z0Z0 C L C L

TDR Waveforms - PC Board Transmission line –A typical PC board will have impedance controlled PCB micro-strip and strip-line transmission lines. –The transmission lines will have components, vias, connectors, etc., that will create impedance discontinuities. Input

TDR Waveforms - PC Board Impedance Model –These impedance discontinuities can be modeled as inductors, capacitors and resistors. Input

TDR Response of Simple Components –Observe high frequency behavior –Discern lumped versus distributed –Derive equivalent C, L, Z 0, t values to put into simulation –Verify physical location of discontinuity Limits: –First or most significant discontinuity only

Shunt C and Series L Discontinuities Shunt Capacitance Discontinuity Series Inductance Discontinuity Z0Z0 Z0Z0 Z0Z0 Z0Z0 L thru C

Shunt C and L Terminations Capacitor Load Termination Inductor Load Termination Z0Z0 Z0Z0 open short C L

Distributed Discontinuities Capacitive Discontinuity Inductive Discontinuity Z Time incident Z0Z0 Z0Z0 Z0Z0 Z1Z1 Z2Z2 t1t1 t2t2 Z1Z1 Z2Z2

TDR Resolution Insufficient TDR resolution Several factors affect a TDR system’s ability to resolve closely spaced discontinuities. If the TDR system has insufficient resolution, small or closely spaced discontinuities may be smoothed together into a single aberration in the waveform. This effect may not only obscure some discontinuities, but it may lead to inaccurate impedance readings.

TDR Resolution –Resolution Factors –Rise Time –Settling Time –Foot and Preshoot Settling Time Foot Rise Time Preshoot

TDR Resolution –TDR resolution is related to system risetime –Reflections last as long as the incident step and display as long as the system risetime Z 1, t D Z0Z0 Z0Z0 Displayed Time  t 01 t 12 t r(system) 2t D

TDR Measurements – Differential TDR Measurement With the signal integrity issues many designs have gone to differential transmission lines to achieve: –Higher noise immunity due to common mode rejection –Less radiated noise due to canceling fields –More precise timing characteristics –Less crosstalk due to noise immunity and less radiated energy –Less power supply noise due to current transients.

TDR Measurements – Differential Clock Coupling –Attempting to measure the two halves of the differential pair separately can produce misleading results. –Two traces in close proximity tend to read a lower impedance than their characteristic impedance as a pair. –Proper characterization of the differential impedance of the transmission line to maintain voltage and timing margins.

TDR Measurements – Differential TDR Measurement –A differential TDR measurement is performed much like a single-ended TDR measurement. –Use two TDR sampling head channels with the step generators set to opposite polarities.

TDR Measurements – Differential TDR Step Timing Skew –Another important consideration when making differential TDR measurement is the alignment of the TDR step pulses. –The positive and negative going TDR steps must be adjusted so there is not any time skew between them at the transmission line launch point.

TDR Measurements – TDS8000 Differential TDR Display Before and after deskew

More TDR Measurements – Transmission Line Coupling and Crosstalk –Mutual coupling and crosstalk between signal lines can be characterized with TDR measurements. –Apply the TDR step on one signal line and measure the signal strength on the other.

Conclusion –TDR is used to find Signal Integrity Problems at design level and is an easy way to isolate where the problems are. –Fast TDR step rise-time allows for high resolution measurements to characterize short transmission line Parametric modeling with third-party software available to develop segments, connectors, etc. –Easy to determine the transmission line characteristics especially when designing differential and close coupled pcb traces.

Questions?