Utilizing Time Domain (TDR) Test Methods
20Pages

The Performance Leader in Microwave Connectors SOUTHWEST MICROWAVE Utilizing Time Domain (TDR) Test Methods For Maximizing Microwave Board Performance S11 Re www.southwestmicrowave.com ■ Tempe, Arizona USA

Southwest Microwave, Inc. Utilizing Time Domain (TDR) Test Methods For Maximizing Microwave Board Performance Bill Rosas, Product Engineering Manager, Southwest Microwave, Inc. Copyright © 2009 by Southwest Microwave, Inc. and Bill Rosas. All rights reserved. CST Microwave Studio® (CST MWS), Teflon® (DuPont) and X-Acto® are registered trademarks of their respective companies. Southwest Microwave, Inc. • Tempe, Arizona 85284 USA • 480-783-0201 • www.southwestmicrowave

Table of Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 What is TDR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Relationship Between Time Domain and Frequency Domain . . . . . . . . . . . . . . . . . . . 3 Review of Fourier Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 How Does the TDR Function on a VNA Work ? . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Bandwidth vs. Rise...

Introduction Time Domain Reflectometry (TDR) is a very useful tool for designing transitions such as the transition between coaxial connectors and PC boards. The TDR function will display the impedance over time showing the place and nature of discontinuities that are due to impedance changes. This paper discusses the aspects of TDR measurements that are relevant to the analysis of a transition. The complete range of TDR measurements and applications will not be explored. What is TDR? True time domain measurements are made with a pulse generator and an oscilloscope. The TDR tester injects a...

Third Harmonic Fifth Harmonic Relationship Between Time Domain and Frequency Domain Review of Fourier Analysis Fourier’s theory is that any repetitive signal can be expressed as a summation of single frequency signals. Take a square wave for example; it is the infinite summation of harmonics of a sine wave with the same frequency as the square wave. SUM – Approximation of (Square Wave) rise time Fundamental frequency and harmonics create an approximation of a square wave. There are no true square waves in electronics. The rise time of a square wave determines the bandwidth or the highest...

Bandwidth vs. Risetime In frequency domain the highest frequency harmonic determines the bandwidth. With modern synthesizers, VNA’s can make higher bandwidth TDR measurements than is practical with true TDR measurement systems. For example, a 50 GHz bandwidth measurement corresponds to a 7 psec risetime pulse. It is very difficult to create real pulses with this small a risetime. Real Units Real units are on a logarithmic scale normalizing a short (zero impedance) to minus one and an open (infinite impedance) to plus one with the nominal impedance being zero. In the case of a network...

How to Analyze a Transition Using TDR VNA Calibration To make a TDR measurement the calibration of the VNA has to be done with a harmonic sweep. Any calibration method can be used and for TDR it can be single port. There is a minimum number of points that are required and this will be explained in later sections. TDR Set-up The measurement channel should be S¹¹ The TDR settings to get impedance over distance/time are: low pass step, real units at 10 milli-units per division, and minimum window. The resolution can be adjusted for larger discontinuities. 1” Test Board Trace = 0.045” Ground =...

Calculating Electrical Lengths To interpret the TDR plot the electrical lengths of each section need to be calculated. Formula for Calculating Electrical Lengths In Time t = - JTr C C is the speed of light The connector is .308” and it has an air dielectric. This calculates to an electrical length of 26 picoseconds (psec). Calculation of Electrical Lengths ► Connector is .308” and the dielectric is mostly air —> 26 psec ► Launch pin is .167” and the dielectric is PTFE —> 20 psec ► Total length to discontinuity is roughly (26+20) x 2 = 92 psec The actual electrical length of the connector is...

Inductive Point: VNA generated TDR graphs are not direct measurements so errors can occur in the computation. Windowing is used to reduce the effects of these computational errors from anomalies in the reverse Fourier transform. With this test board there is nothing in the structure that corresponds to the inductive spike circled in blue. To confirm that this was a “questionable” inductive point, capacitance was added by extending the Teflon out over the pin on the board. The inductive spike did not change which indicates it is not an accurate point in the TDR. Changing the windowing to...

How to Optimize a Transition Using TDR 3-D Simulation 3-D electromagnetic simulation can be used to predict the results of these types of structures. Then changes can be made and the results of the changes can be viewed without having to fabricate and test actual hardware. Decent correlation of the known performance of this test board was achieved with CST Microwave Studio® (CST MWS) simulation. CST provided the simulations. CST MWS Model Without Connectors CST MWS Model The 3-D simulation model is created by looking only at the transition blocks and the test board. Simulation results...

Maximizing the Test Board Performance Inductance needs to be added to the board layout to offset the added capacitance of the launch pin. This is done with a taper and the taper is designed such that boards can be modified with an X-Acto® knife for those that do not have a simulation tool. The optimized taper design was developed using CST Microwave Studio’s optimization routine. The simulation results show the capacitive dip is reduced producing excellent results for S¹¹, much better than could be realized in practice and the insertion loss is very smooth up to the normal 45 GHz glitch...