HP a-210 free download

File name a-210.pdf

FliOW HEWLETT ~e.. PACKARD DIFFERENTIAL TDR USING A SINGLE STEP GENERATOR Interest has been on the rise for making differential TDR measurements on balanced systems like twisted pair cables, both shielded and unshielded, on dual trace micros trip lines, and on dual conductor stripline. Under certain conditions, such balanced transmission paths offer improved performance over unbalanced lines, particularly in terms of common mode noise rejection and immunity to crosstalk. The question arises on whether standard TDR can be used effectively to analyze balanced systems, and if so, how? This note explores these questions and will describe how differential TDR is easily accomplished using a standard TDR if a few special considerations are made. These are different considerations than those required with dual step source differential TDR. Equivalent circuits of dual and single step source TDR's are compared along with SPICE model simulation results to support this claim. Finally, actual measurements are performed on a number of standard devices and on a twisted pair cable to verify the technique. A view of the twisted pair cable using a dual pulse differential TDR is compared to these results and shown to have very close agreement, while standard TDR with no special considerations is shown to be error. Dual pulse generator method: Differential TDR can be accomplished by using two step generators to stimulate the DUT. One step generator outputs a pulse from 0 volts to some positive DC level with a fast edge. The other step generator has opposite polarity, and steps from 0 volts to a negative level. Each generator is single ended coaxial, but a differential pulse exists between the center conductors of the output cables of these generators. This differential signal is applied to the differential inputs of the DUT, a scope channel is attached to each input, and the difference of the channels is displayed on screen. A block diagram of the differential stimulas is shown in Figure 1. This technique relies on step generators being synchronized in time, and identical in pulse shape. 50 OHM CH 1 SAMPLER BALANCED LINE OUT ..lac/IL37 Figure 1. Dual generator differential stimulus Single generator method: Differential TDR is also possible using a single pulse generator and taking www.HPARCHIVE.com advantage of its common mode and differential mode components. Waveform math in the oscilloscope can be used to subtract the common mode component to view the differential mode reflections. The HP 54l20T TDR step is produced by a current source attached to the scope input sampler wh