Time Domain Reflectometry In Ohm Systems

The -hp- TDR system1 has a characteristic impedance of 50 ohms because this is the most common impedance for cables and microwave systems. There are, however, other common impedances, and in the communication industries, in television broadcasting, and in community-antenna television systems, 75 ohms is widely used.

Calibration and interpretation of the TDR display are simplest when the characteristic impedance of the TDR system is the same as the characteristic impedance of the system being tested. The reason for this is that, when these impedances are matched, energy reflected from the system being tested is terminated in the proper impedance, and no re-reflections occur at the source.

The -hp- TDR system can now be converted easily for testing 75-ohm systems by means of new 50-to-75-ohm adapters recently developed by the -hp-Colorado Springs Division (Fig. 1).

The adapters are 25-ohm resistors with connectors which are compatible with the -hp- TDR system and with common 75-ohm systems. Fig. 2 is a schematic diagram of a typical TDR setup for testing a 75-ohm cable. Energy returning to the TDR system from the cable is properly terminated in 75 ohms by the series combination of the 25-ohm adapter and the 50-ohm TDR system. There is no source mismatch to cause re-reflections.

Fig. 3 is a photograph of the TDR display for the setup of Fig. 2 with an i -hp- Model 1415A TDR Plug-in for the -hp- Model 140A Oscilloscope.

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Fig. 1. New -hp- Models 10457A-8A 50-to-75-ohm adapters simplify testing of 75-ohm systems with -hp- Model 1415A Time Domain Reflectometer. Adapters are 25-ohm resistors with one GR874 connector and one type 'N' or 'F' connector.

Fig. 2. Block diagram of setup for TDR testing of 75-ohm system using new 50-to-75-ohm adapter. Energy reflected from 75-ohm system is properly terminated by series combination of 25-ohm resistor (adapter) and 50-ohm TDR system, thereby eliminating source re-reflections.

-hp- 140A -hp- 1415A OSCILLOSCOPE TDR SYSTEM

-hp- 140A -hp- 1415A OSCILLOSCOPE TDR SYSTEM

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-hp- 10457A 75 ohm 50-75 ohm SYSTEM ADAPTER UNDER TEST

Sampler input

50-ohm line

Sampler input

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-hp- 10457A 75 ohm 50-75 ohm SYSTEM ADAPTER UNDER TEST

open circuit at the end of the 75-ohm cable (ZL = oo). The display shows an initial mismatch at point 'A', which is the result of the 50-ohm TDR system's being terminated in the 100-ohm series combination of the adapter and the 75-ohm cable. This mismatch does not hinder operation, however. Level 'B' is now the 75-ohm reference level (reflection coefficient p = 0) and level 'C' is now the open-circuit level (p = 1). The system can be calibrated to read reflection coefficient directly for 75-ohm operation by adjusting the step cal control of the TDR plug-in (screwdriver adjustment) so that the difference between levels 'B' and 'C is ten centimeters on the display when the reflection coefficient control is in the 'p/cm = 0.1' position. The reflection coefficient control is then calibrated for 75-ohm systems instead of for 50-ohm systems.

When the -hp- TDR system is operated with a 50-to-75-ohm adapter, the system rise time increases to about 170 picoseconds (instead of 150 ps) and the internal noise level is increased by a factor of 1.5. Other specifications of the TDR system are unchanged. A set of 75-ohm impedance overlays is included with each adapter.

The 50-to-75-ohm adapters may also be used with any signal generator which has a 50-ohm source impedance. The adapter converts the generator into a 75-ohm source so that it can be used in 75-ohm systems without reflections from the source and without any loss of pulse amplitude. If the generator has a calibrated pulse amplitude control, this control retains its calibration when the adapter is used.

— Charles A. Donaldson

Fig. 3. Oscillogram of TDR display for test setup of Fig. 2. with cable open-circuited (Zl = &>). Initial reflection at 'A' is due to impedance mismatch between 50-ohm TDR system and 100 ohms of adapter plus cable. Level 'B' is new reference level for 75-ohm measurements (reflection coefficient p = 0). Level 'C' is open-circuit level (p = 1).

Fig. 3. Oscillogram of TDR display for test setup of Fig. 2. with cable open-circuited (Zl = &>). Initial reflection at 'A' is due to impedance mismatch between 50-ohm TDR system and 100 ohms of adapter plus cable. Level 'B' is new reference level for 75-ohm measurements (reflection coefficient p = 0). Level 'C' is open-circuit level (p = 1).

CHARLES A. DONALDSON Chuck Donaldson graduated from the University of Colorado in 1962 with a BS degree in electrical engineering and business. After graduation, he spent three years in the evaluation and design of pulse generators and oscilloscope circuitry. Chuck joined -hp- in 1965 as an applications engineer, and since then he has been working with the sampling and pulse-generator design groups of the -hp- Oscilloscope Division in Colorado Springs.

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