TDT Method for Printed Circuit Board Reverse Engineering

The best accuracy for measuring velocity characteristics of Printed Circuit Board Reverse Engineering with a TDR is by using the TDR in TDT mode.

The TDT is completed by launching the pulse on one end of the test coupon with a 50 Ω probe and capturing the signal at both the launch point and open end with a low capacitance, high impedance probe of Printed Circuit Board Reverse Engineering.

The advantage of the TDT over the TDR is that the captured signal has propagated only once down the coupon, yielding an improved rise-time response. This improves the “guess work” involved for determining the measurement positions on the response curves (i.e., the voltage level at which the delay measurement will be taken).

The TDT requires a 50 Ω launch probe and an high impedance probe to capture the transmitted signal. It is recommended that the high impedance probe should be a low capacitance, 10X or 20X microprobe to provide minimum loading and maximum bandwidth for Printed Circuit Board Reverse Engineering.

Complete measurements by connecting the 50 Ω probe to a sampling head with the TDR/TDT mode “ON”. The setup is the same as with TDR measurements and is only used to launch the signal (driver). The high impedance probe should be connected to a separate channel and with the head function’s TDR/TDT setting mode to “OFF”. This enables the high impedance probe to act like a normal oscilloscope probe (receiver) with the sole purpose to capture the launched pulse after Printed Circuit Board Reverse Engineering.

Once the instrument is setup, position the high impedance probe as close as possible to the launch point from the TDR probe. The TDR 50 Ω probe should be launching the pulse and the captured response from the high impedance probe should be visible on the screen. Adjust the scaling on the screen to position the high impedance response to the left side and maximize the voltage scale if Printed Circuit Board Reverse Engineering.

The time base may need to be adjusted to the minimum time scale that will allow both the transmitted and received signals to be displayed on the same screen as illustrated in Figure 21. Store both the transmitted and received signals and measure the difference between curve 1 and curve 2 to get the transmitted delay from PCB Reverse Engineering.