Cables transit time

If a cable is terminated with an impedance different from Z, a part of the signal is reflected at the load and travels back to the source. If the source is also mismatched, the signal is reflected again and appears at the output after twice the cable transit time. Depending on the relation of the pulse width to the cable length, and the source and load impedance (which may be not purely resistive), the resulting pulse shapes can be very different. 

Fig. 7.49 Delay change for 8 m of cable for a 30°C increase in temperature. Left. RG174, Right RG316, high quality PTFE cable. Total cable transit time is 40 ns.

On the RF time scale, the transit times of electrons in long coaxial cables and the time of flight of photons in optical paths as short as a few centimeters are significant. These effects become more pronounced as the modulation frequency increases. Even simple changes made to a system will affect the resulting measurements. 

In principle, correlation at even shorter time-scales is possible by including the micro times of both modules in the calculation of the correlation function. The problem with this approach is that the micro time scales of the modules may be slightly different, and the macro time transitions may be shifted due to different transit times in the detectors, cables and TCSPC modules. Correcting all these effects is extremely difficult, yet not impossible. Suitable calibration and correlation algorithms were developed by S. Felekyan, R. Ktihuemuth, V. Kudryavtsev, C. Sandhagen, and C.A.M. Seidel, Universitat Dortmund. 

The transit time has to be taken into account when choosing the cable length in the detector and reference signal path of a TCSPC system. More important than the transit time itself is the variation of the transit time with the detector supply voltage. To keep the transit time of a conventional PMT stable within 1 ps a stability of the operating voltage of the order of 0.05% to 0.01% is required. 

The transit time for the commonly used 50 cables (RG58, 4.9 mm diameter and RG174, 2.9 mm diameter) is about 5 ns per meter. 

Reversed start-stop systems require a stop pulse at the end of the recorded time interval. It is therefore often necessary to delay the reference pulses from the laser. The best way to delay the signal is to use a cable, since this does not introduce a noticeable jitter. It is, however, not commonly known that the transit time in a cable depends on the temperature. Figure 7.49 shows the delay change in 8 m of a standard RG 174 cable and RG 316 high-quality cable. 

Fig.11.6a,b. Observed output pulses from an argon laser at X = 488 nm, actively mode locked by an acousto-optic modulator, (a) Detected with a fast photodiode and a sampling oscilloscope, (b) detected by single-photon counting technique using a photomultiplier. The oscillations following the optical pulse in (a) are due to cable reflections of the electric output signal from the diode. The pulse width in (b) is limited by electron transit time variations in the photomultiplier [11.10]... 

The double-Helix Coil transition of synthetic oligonucleotides has been studied [15, 16] with conventional and also with a fast temperature-jump apparatus [17]. In this instrument a short coaxial cable is used as discharge capacitor, resulting in heating times as short as 50 n. Its only disadvantage is common to all Joule-heating type temperature-jump apparatuses the need for supporting electrolytes. 

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