Cable reflections/matching

Cables are available for Z= 50, 60, 75 and 100 Q. For measurement equipment and other wide-band systems only Z = 50 Q is used. The CFD inputs of TCSPC modules, amplifiers, or routers have internal matching resistors of 50 Q. However, the input impedance of amplifiers or of the pulse shaping network used in CFDs is often far from being ideally resistive. Moreover, PMTs and photodiodes are current sources. Matching at the detector side is avoided because it would decrease the signal amplitude. The resulting reflections at the input cables of a TCSPC device can normally be tolerated, especially if some precaution is taken in adjusting the CFD thresholds. 

A is the reflected signal amplitude, is the incident signal amplitude, and Zq and Zj are the impedances on both sides of this point. Such condition is generated when cables of different impedances are joined or when the input resistor of the amplifier does not match the impedance of the signal cable. Let us discuss the consequences of Equation 143 in somewhat more detail. A preamplifier is connected to the Y-amplifier of an oscilloscope. The signal coming from the preamplifier travels in a coaxial cable with an impedance of Zq = 50 Q The input resistor of the oscillo-scopic amplifier is Z = 1 Inserting these values into Equation 143 yields for... 

In experiments with voltage pulses, the impedance matching of cable and test cell is of great importance in order to avoid voltage reflections (Arii et al, 1979). A test cell used for time lag to breakdown measurements and a typical oscilloscopic trace obtained are shown in Figure 46. 

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