IPC-TM-650 2.5.5.11

Description / Abstract: This method specifies time domain reflectometry (TDR) methods for measuring and calculating the propagation delay of uniform, controlled impedance transmission lines fabricated in printed board (PB) technology. The method defines a propagation delay per unit length tD by specifying how to measure the time it takes a signal to propagate a given length of transmission line. This method describes methods that utilize TDR measurements of multiple, unterminated test lines that are designed to differ only in length. A TDR signal, usually a step waveform1, is injected into a transmission line or lines and the reflection response is measured some time later. This method shows how tD is determined as the difference between the time it takes a TDR pulse to reflect from the unterminated ends of two transmission lines divided by the length difference of the two lines.

Applicability: Engineering development of high-speed and high-frequency electronic circuits and systems requires detailed information on the electrical performance of PBs to assure that transmission line designs yield the expected performance characteristics. Detailed analysis of the design and fabrication variations expected throughout manufacturing assures that a proposed design can be manufactured at a useful quality level. Measuring and characterizing propagation delay on transmission line test structures is a direct means of assessing the success of the PB transmission line model. Since transmission line measurements are affected by impedance conditions at the

transmission line boundaries, propagation measurements specified here may not return the actual delay observed for a given application. The procedures test whether uniform, impedance controlled PB transmission lines exhibit the expected propagation delay based on an electrical model or reference test structures. This method is generally applicable to uniform transmission lines fabricated with commercial PB processes (see IPC- 2141), and is also useful for various transmission lines and material systems studied at the research and development stages.

The method is applicable when:

Electrical contacts (connectors or probes) are readily made to the transmission lines test structures

Transmission line characteristic impedance is neither extremely high nor low compared to the instrument's test port impedance

Transmission line propagation loss sets acceptable signalto- noise ratios for the measured signals

The current version of this method specifies singled-ended TDR measurements of unbalanced transmission lines, though the method is sufficiently general to be extended to differential TDR measurements of balanced lines.

Measurement System Limitations: Applying a specified test method helps assure accurate and consistent propagation delay results, however measurements of propagation delay can vary depending on equipment used. Known measurement system limitations include:

a. Electrical noise of the TDR receiver, limiting propagation delay accuracy and repeatability when signal levels are low

b. Trigger, source, and receiver jitter in the TDR instrument, limiting temporal resolution

c. Drift in the trigger point of the TDR sources limiting, temporal resolution

d. Slow TDR pulse rise times, limiting temporal resolution

e. Waveform distortion induced by the low-quality test set-up cables, connectors, and the signal launch points, inducing errors in the reported propagation delay

Further measurement system considerations and notes are provided in Section 6.

Sample Limitations: The type of test sample used may also impact propagation delay accuracy. The sample-based limitations include:

a. Lines on a fabricated PB deviating significantly from design. For example, microstrip lines longer than 15.0 cm [5.91 in] on PBs with plated-through holes (PTH) often have variations in line width due to nonuniform plating and/or etching. This makes the uniform transmission line