Time-domain Reflectometry (TDR) Measurements Test Methodologies for Today’s Fastest Digital Data Interfaces Intel Developer’s Forum Asia Pacific Spring 2000 Z2 Z1

TDR Test Methodologies for Today's Fastest Digital Data Interfaces Time-Domain Reflectometry (TDR) test methods are used to measure the signal transmission path characteristics in high-speed digital systems. This lab will review the basic concepts of TDR, then present some fundamental TDR measurement methods. These methods will next be applied to some of the fastest digital data interfaces used in today’s computer systems

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

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

When Are TDR Measurements Needed? Typical applications where TDR Measurements are needed include: 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. Distributed systems cause VSWR problems when incident and transmitted energies superpose on the interconnect. Wherever impedance changes within a distributed system, there is a potential for contributing to this problem. Some of the causes of impedance changes are vias, buried versus surface runs, routing under packages, and run constrictions between package pins.

TDR Fundamentals - Applied Step Signal Starting with a transmission line with a characteristic impedance Z equal to Z0 A fast rise-time step signal is applied at the transmission line input point. The step signal will propagate down the line. Propagating Step Step Source Transmission Line Characteristic Impedance Z = Z0

TDR Fundamentals - Impedance Change An impedance change in the transmission line will cause a change in the amplitude of the propagating step. Impedance Change Point Step Source Characteristic Impedance Z2 > Z0 Characteristic Impedance Z1 = Z0

TDR Fundamentals – Transmitted and Reflected Signals The change in impedance causes some of the power to be reflected back to the source. The remainder of the power will be transmitted. Transmitted Step Incident Step Z2 > Z0 Step Source Z1 = Z0 Reflected Step

TDR Fundamentals – Oscilloscope Monitoring Use an oscilloscope to monitor the transmission line signal at the step source input point. The oscilloscope waveform will show the combined sum of the incident and reflected propagating signals. Oscilloscope Measurement Point Step Source Z2 > Z0 Z1 = Z0

TDR Fundamentals – Oscilloscope Waveform The oscilloscope waveform will show the impedance change as a change in signal amplitude occurring in twice the propagation time, Tpd, from the incident step. Propagation time = 2Tpd Time ( Sec/Div ) Propagation time = Tpd Z2 > Z0 Z1 = Z0

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. Transmission Line 50 W ZLoad Step Source SMA Connector Z0 To Oscilloscope Mainframe Sample-Hold Gate TDR Sampling Head

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. TDR Sampling Head Load Z0 ZL

TDR Measurements - Reflection Coefficient – r (rho) r is the ratio of the reflected pulse amplitude to the incident pulse amplitude. Vreflected r = = Vincident r can be expressed in terms of the transmission line characteristic impedance, Z0 , and the load impedance, ZL. Vreflected ( ZL – Z0 ) r = = Vincident ( ZL + Z0 )

TDR Measurements - Reflection Coefficient for Matched Load There are some interesting boundary conditions for the value of the reflection coefficient, r. Load Z0 ZL Reflected Wave is equal to zero. No Reflections. When ZL is equal to Z0 – Matched Load VReflected = 0 and r = 0 Vreflected r = = = Vincident V

TDR Measurements - Reflection Coefficient Boundary Values When ZL is equal Zero – Shorted Load Reflected Wave is equal but negative of incident wave. VReflected = -VIncident and r = -1 Vreflected r -V = = = -1 Vincident V Reflected Wave is equal to the incident wave. When ZL is Infinite – Open Load VReflected = VIncident and r = +1 Vreflected r V = = = 1 Vincident V

TDR Measurements – Open and Shorted Load TDS 8000 Display

TDR Measurements - Reflection Coefficient and Impedance The characteristic impedance, Z0 , or the load impedance, ZL, can be calculated with the value of r. ZL = Z0 * ( 1 + r ) Z0 = ZL * ( 1 - r ) ( 1 – r ) ( 1 + r ) Z0 ZL

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

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

TDR Measurements – Propagation Delay Measurements With Cursors The horizontal waveform cursors can be used to make time and propagation delay measurements. The one-way propagation delay is half the time measured between the incident and reflected waves. Measurement Readouts Ohms/Div Cursor 1 = 50.0 pS Cursor 2 = 201.5 pS Delta 2-1 = 151.5 pS Prop Delay = 75.75 pS Cursor 1 Cursor 2 Sec/Div

TDR Measurements – Propagation Delay and Dielectric Constant The relationship between propagation delay, Using the medium dielectric constant and the fact the measured time is twice the propagation time of the signal. Propagation Delay = TPD = L * ( eEFF )1/2 VC Where: eEFF is the effective dielectric constant which is a function of materials and type of transmission line, strip-line, micro-strip, etc. L is the length of the trace VC is the velocity of light

TDR Measurements – Location Calculation with Propagation Delay The location of a transmission line impedance change can be calculated. Using the medium dielectric constant and the fact the measured time is twice the propagation time of the signal. Location of Impedance Changed = D = TMeas * VC 2 * ( eEFF )1/2 Where: D is the distance to the point of the impedance change that caused the reflection. TMeas Cursor 1 Cursor 2

TDR Measurments – Reflection Diagrams Reflection diagrams are useful for calculating and understanding the propagating wave reflections. Z2 Z1 Transmitted Wave Incident Wave Time Reflected Wave Z1 Z2

TDR Measurement – Reflection Diagram Calculations Reflection diagram calculations determine the magnitude of each reflection using the values of the reflection coefficient, r and the incident voltage. Time Z1 Z2 r Vreflected = Vincident *

TDR Measurements – Diagrams for Multiple Reflections Reflection diagrams are useful for calculating and understanding transmission line with multiple discontinuities. Z2 Z4 Z1 Z3 Time Z1 Z2 Z3 Z4

TDR Measurements – Waveform for Multiple Reflections The TDR waveform for multiple reflections shows the results of the combined reflections from all the impedance discontinuities. Z2 Z4 Z1 Ohms Time

TDR Waveforms - Shorted and Open Terminations Short Circuit Termination TP 2TP V Z0 ZL = 0 Open Circuit Termination TP 2V 2TP V Z0 ZL = Open

TDR Waveforms - Matched and Mismatched Load Terminations TP 2TP V Z0 ZL = Z0 Mismatched Load Termination TP V + VR ZL > Z0 2TP V Z0 ZL <> Z0 ZL < Z0

TDR Waveforms - Capacitor and Inductor Terminations Capacitor Load Termination TP 2V 2TP V Z0 ZL = C Inductor Load Termination TP 2TP V Z0 ZL = L

TDR Waveforms - Shunt Capacitance and Series Inductance Discontinuities Shunt Capacitance Discontinuity TP 2TP V Z0 Z0 C Shunt Inductance Discontinuity TP 2TP L V Z0 Z0

TDR Waveforms - Inductance and Capacitance Discontinuities Series Inductance – Shunt Capacitance TP 2TP L V Z0 Z0 C Shunt Capacitance – Series Inductance TP 2TP L V Z0 Z0 C

TDR Waveforms - Multiple Inductance and Capacitance Discontinuities Capacitance – Inductance - Capacitance TP 2TP L V Z0 Z0 C C Inductance – Capacitance - Inductance TP 2TP L L V Z0 Z0 C

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 Waveforms - PC Board Impedance Measurement The TDR Oscilloscope waveform will display the reflections created by the impedance discontinuities. Measurements can be made to determine how much the impedance deviates from the nominal value.

Rambus TDR Measurements – Channel Impedance Requirements The Rambus channel has an impedance specification of 28 Ohms with a 10% tolerance, +/- 2.8 Ohms. Meeting this requirement means careful design and testing of the PCB transmission lines.

Rambus TDR Measurements – Multiple Simultaneous Data Transfers Multiple bits can be simultaneously propagating on a single Rambus channel data line – multiple clock domains. Unwanted signal reflections from one data transfer would effect multiple data transfers on the channel date line. Assuring PCB impedances meet specifications is a requirement for successful Rambus design. Rambus RDRAM Controller

Rambus TDR Measurements – Channel Timing and Voltage Requirements The Rambus 800Mbps data transfer rate requires timing margins to be maintained with high degree of accuracy. Rambus signals use low voltage swings of 0.8V. Signal reflections caused by impedance discontinuities reduce timing and voltage margins. All the Rambus channel signals and clock lines must also be accurately matched to maintain timing margins. Data 0.8 V Clock 1.25 nS

Rambus TDR Measurements – PCB Transmission Line Design Determine Microstrip and Stripline PCB designs with consideration of the PCB fabrication materials and constraints. Use impedance equations, simulation tools and reference designs for best results – 28 Ohms is low. Include a test coupon for testing impedance and monitoring PCB production lots. Microstrip Stripline

Rambus TDR Measurements – TDR Test Coupon A TDR test coupon can be designed on to PCB for testing impedance and monitoring PCB production lots. The coupon should use the same routing guidelines used for the Rambus channel to represent process. The probe land pattern used with the coupon needs to match the type and method of TDR probing used. The far end of the coupon should be open – no pad or via. Probe pads Open end Ground

Rambus TDR Measurements – Instrumentation Setup The basic TDR instrumentation for Rambus TDR consist of a sampling oscilloscope with a TDR sampling head, with TDR probe and cable. Other methods of connecting to the circuit board can be used including SMA connectors, specialized RIMM modules, probe fixtures, etc.

Rambus TDR Measurements – TDR 28 Ohm Calibration The TDR measurement system should be calibrated with a 28 Ohm standard. This could be a certified test coupon, specialized calibration air line, etc. TDR instrumentation are 50 Ohm systems. So a 28 Ohm impedance will be displayed as a downward step. 50 W TDR Sampling Head 28 W Standard 50 W 28 W

Rambus TDR Measurements – 28 Ohm Calibration TDS8000 Display

Rambus TDR Measurements – TDS8000 Display of Rambus Impedance Range

Rambus TDR Measurements – TDR Probing One method to measure the circuit board trace impedance is to probe the trace with a TDR probe. The probe is used to apply the TDR step and measure the reflections. The probe ground lead needs to be as short as possible and connected to trace ground plane. TDR probe with short ground lead probing trace

Rambus TDR Measurements – Minimizing Probing Aberrations When making 28 Ohm measurements aberrations from the probe launch can be minimized with careful probing techniques.

Rambus TDR Measurements – Probing 28 Ohm Trace TDS8000 Display I used my finger to lower bandwidth and ringing

Rambus TDR Measurements – Rambus Differential Clock There are two pairs of 400MHz differential clock lines, ClockToMaster and ClockFromMaster. Clock and data travel in parallel to minimize timing skew. An RDRAM sends data to the memory controller synchronously with ClockToMaster. The controller sends data to the RDRAMs synchronously with ClockFromMaster. Data and Clock transmission lines must be matched in impedance and length to maintain timing margins.

Rambus TDR Measurements – Differential Clock 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.

Rambus 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 there characteristic impedance as a pair. Proper characterization of the differential impedance of the Rambus clock traces to maintain voltage and timing margins.

Rambus 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.

Rambus TDR Measurements – TDS8000 Differential TDR Display

Rambus TDR Measurements - Propagation Velocity Measurements TDR is used to make propagation velocity measurements to compare electrical signal length of the Rambus data and clock lines. Matching the electrical signal length – propagation time is important to maintain voltage and timing margins. Cursor 1 Cursor 2 Measurement Readouts Distributed systems cause VSWR problems when incident and transmitted energies superpose on the interconnect. Wherever impedance changes within a distributed system, there is a potential for contributing to this problem. Some of the causes of impedance changes are vias, buried versus surface runs, routing under packages, and run constrictions between package pins. Dielectric K = 4.50 Delta 2-1 = 822.6 pS Distance = 58.17 mm Sec/Div

More TDR Measurements – Rambus 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.

Sampling Oscilloscopes - Low Voltage Differential Signaling (LVDS) Low voltage differential signaling (LVDS) is being used in many high-speed interface applications. LVDS using low voltage swing, high-speed, differential signaling – similar to Rambus. High speed buses are formed by using multiple sets of LVDS data lines along with a differential clock. Transmit and Receive pairs for the bi-directional bus. TX RX RX TX

Sampling Oscilloscopes - Low Voltage Differential Signaling (LVDS) Sampling oscilloscopes have the features required to characterize LVDS interfaces. LVDS requires differential transmission line characterization - impedance, coupling, etc. LVDS also requires timing and jitter characterization measurements – eye patterns, noise, etc. Sampling oscilloscopes have very high timing accuracy and low system jitter.

Conclusion – Other TDR Measurement Considerations TDR improved accuracy with baseline level and incident Amplitude correction. Fast TDR step rise-time allows for high resolution measurements to characterize short transmission line segments, connectors, etc. Parametric modeling with third-party software available to develop SPICE model, etc. Easy to determine the transmission line characteristics and Instrumentation for measurements can be applied to manufacturing environments.