Differential TDR

Open the TDR Schematic project from its file in the Examples directory.

  1. From the File menu, select Open Examples to open an explorer window.
  2. Open Circuit, then Signal Integrity, and select the file TDR_Example.aedt. Click Open.

    Expand the TDR_Example icon and the 2_Differential icon. The differential TDR schematic appears in the design window:

    Differential Time DOain Reflectometer Circuit Schematic

    The differential TDR consists of two channels, each with a pulse source, an internal impedance equal to 0.5Z0 ohm, a transmission line with 0.5Z0-ohm impedance and with 0.5-ns delay, and two voltage probes, Vexcited_pos or Vexcited_neg for the step voltage and Vdetected_pos or Vdetected_neg for the reflected voltage. (Z0 is the TDR component parameter.)

    The TDR component provides two options for the pulse type: Trapezoidal and SmoothStep. SmoothStep provides a more realistic step input and allows control of the rise interval.

  1. Click Tools > Project Tools > Update Definitions.
  2. In the Update Definitions dialog, select Show All Items, Select All, then click Update . Close the dialog once updates are successful.Update Definitions Dialog, all items highlighted.

  3. Right-click the TDR model symbol and click Properties to open the properties dialog.

  4. Click the Value field for ChoosePulseType to view the two options. Select SmoothStep, click Apply, then OK to close the dialog.

    TDR properties dialog. SmoothStep pulse type selected.

The Device Under Test (DUT) is microstrip coupled line pair and a termination network. The transient analysis sweeps the spacing between the coupled lines, which shows how the spacing affects the differential impedance, Zdiff.

  1. Select the coupled lines. The Property window shows that the spacing parameter SP has been defined with a local variable.
    2. Properties window, Param Values tab.
  2. Expand the Analysis icon, double-click on the Transient analysis setup, and verify the sweep of the spacing variable.

    Transent Analysis window.

  3. In the Project Tree, right-click Transient and select Analyze. The analysis runs to completion.
  1. Expand the Reports icon and right-click on the Zdiff report setup. Select Modify Report to open the Report Setup.

    Report window

  1. Click New Report to generate the report:

    Zdiff plot.

The TDR analysis shows how the spacing affects the differential impedance. Using the Trapezoidal waveform, the reflection timings are expected at 2*(Total Delay).

Note: If SmoothStep pulse type is selected, the reflection timings are expected at T50 + 2*(Total Delay). See Circuit > Nexxim Components > Probes > Time Domain Reflectometer for more information about TDR properties, including the T50 SmoothStep parameter.

Zdiff Plot using SmoothStep waveform.