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Design Notes

TIME DOMAIN REFLECTOMETRY

This example demonstrates how the  AWR Design Environment can be used for Time Domain Reflectometry by using the built-in TDR measurements:

TDR_LPS - Low-pass Step Response

TDR_LPI - Low-pass Impulse Response

Overview

This example uses simple ideal transmission lines to demonstrate the TDR measurements in AWR.  Ideal lumped components are added to show the effects of capacitive and inductive discontinuities.

Ideal_TLIN

The "Ideal_TLIN" schematic contains 3 ideal transmission lines with different characteristic impedances.  The variation in impedance over time is displayed in the "Ideal TLIN Impedance Variation" graph.  In the "Ideal TLIN Lowpass Response" graph, the peaks of the "Impulse Response" trace identify the location of the change in characteristic impedance.  Press the Tune button in the toolbar to open the tuner.  Move the sliders and observe the peaks of the Impulse response moving as the lengths are changed.  To find the physical location, divide the time at the "Impulse Response" peak by 2 and multiply by the speed of light in the transmission medium.  For ideal transmission lines we use c = 3e8 m/s.  For microstrip or stripline, d = (time/2) * c / sqrt(Er).   See the online MWO/AO Measurement Reference for additional information.

Ideal_TLIN with Discontinuities

The "Ideal_TLIN with Discontinuities" schematic modifies the "Ideal_TLIN" schematic by inserting a series inductance between TL1 and TL2, and a shunt capacitor between TL2 and TL3.  Looking at the "Discontinuities Lowpass Response" graph, an inductive overshoot is observed at 500 ps and a capacitive rise is seen at 1000 ps.  By looking at the step response, the user is able to determine the type of discontinuity and the location.

Line_Trace

The "Line_Trace" schematic uses the MTRACE element to implement a routed microstrip line on a board.  The layout consists of 3 straight sections and 2 bends.  The "Line Return Loss" graph shows that the input return loss of this line is not very good past 5 GHz.  The "Line Lowpass Response" shows the impulse and step repsonse of the line using the TDR measurements.  The first peak of the Impulse response is at 250 ps.  In the "Line_Trace" schematic we have used equations to compute the location of the first discontinuity, 995 mils.  The layout view shows this is at the first 90 degree bend.  By comparing the Step response in "Line Lowpass Response" with the Step response in the "Discontinuities Lowpass Response" graph, one can see that the bend is a capacitive discontinuity.  To compensate for the capacitive discontinuity, chamfer the corner of the bend by changing the M parameter of the MTRACE from 0 to 0.6.  Notice how the magnitude of the step response has decreased and the input return loss has improved across the band.  However, the step response has changed from capacitive to inductive, indicating that too much metal was trimmed from the bend.  Change M from 0.6 to 0.4.  Notice how the magnitude of the step response has decreased and the input return loss is now better than -20 dB across the band. 

This simple example has shown how AWR can be used with Time Domain Reflectometry to find and identify discontinuities.

Schematic - Ideal_TLIN

Schematic - Ideal_TLIN_with_Discontinuities

Schematic - Line_Trace