Dealing with impedances on the test bench
Impedance fluctuations are often not tolerable
In addition to material and production specific variations, design specific ones (e.g. layer changes, too small distances to GND-planes, PCB borders, or other transmission lines) may occur as well, which eventually result in intolerably fluctuating transmission path impedances. In consequence, clock edges degrade and inter-symbol interferences occur which, in turn, cause inacceptable bit error ratios and, finally, performance degradation or even system malfunctions.
Figure 1: Block diagram of a TDR-based impedance measurement system.
Line impedances can be determined with a high degree of precision by means of a time domain reflectometry (TDR). TDR technology has already been used for detecting faults in underground or submarine cables since the 1970s, where faults can simply be interpreted as large impedance variations. Since then, a lot of applications for fields as different as geology and food technology have been addressed.
Figure 1 shows the block diagram for a TDR-based impedance measurement setup. The TDR itself only consists of a voltage step generator and broadband sampler accompanied by a data acquisition unit.
The basic measurement principle is as follows: The generator emits a step signal travelling via adapters, cables and a probe to the device under test (DUT). While interacting over the entire length of the DUT, the signal experiences partial reflections, which travel back to the detector and thus allow the spatial determination of the DUT's wave impedance. Many people know this basic principle from radar applications, which is also the reason, why TDRs are frequently called Cable Radars.
The rise time tr of the step signal determines the spatial resolution and should thus be as short as possible (for Sequid DTDR-65, this is tr 65ps, allowing a spatial resolution of approx. 5mm). The synchronisation between the generator and the sampler (which should feature an analogue input bandwidth of at least 10GHz) is crucial for low-noise operation, i.e. for jitter values of only some picoseconds. Ideally, a "real-thru" sampler is used; hence no external signal dividers or couplers are necessary. This is highly beneficial, since broadband signal dividers are usually built resistively and thus would add insertion loss and noise. Finally, a TDR features a data recording unit, usually implemented by a microprocessor or FPGA.
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