Oscilloscope impedance matching

The output from this impedance matching device is fed directly into a storage oscilloscope (Tektronix Type 564). It was possible to read the oscilloscope output to about 0.01 pH unit, limited by 60 cps noise. 

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. 

AC bridge measurements provide only an absolute impedance and phase shift, so there is no way to distinguish between either a series or parallel R-C combination by use of a pure sinusoidal waveform. Thus the series R-C combination of the cell may be matched by the series combination shown in Figure 6.24b or the parallel combination of the Wien bridge shown in Figure 6.24c. Either an oscilloscope or phase-angle voltmeter100 are recommended as null indicators because they allow both the phase shift and the imbalance potential to be nulled. 

As already described, a magic-tee is a four port device in which two of the ports are isolated, i.e., have infinite impedance between them if the impedances at the other two ports are matched. Thus, the device can be used to match a resonant circuit to 50 Q or, for that matter, to measure the impedance at resonance of the tank circuit. The tank circuit under question will have impedance R at resonance when the output of the magic-tee, easily monitored on an oscilloscope, is an absolute minimum. (See the figure under magic-tee on p. 393.) Unfortunately, this is easier said than done because there are relative minima when the capacitors are adjusted to give an impedance which is, for example, equal to R in magnitude but with an imaginary component. When you play around with a scheme like this for the first time, it will be useful to let R be variable so that it can be adjusted for a... 

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... 

Up to the present time, cavitation tests in various liquids indicate this frequency range covers practically all cavitation phenomena. It is a simple matter to extend either the lower or upper range of the response if it should become desirable. The power supply for these electronic circuits consists of easily replaceable mercury batteries, thus making the cavitation detector controller a small and self-contained unit which may be used in various locations. The output of this controller is so designed that it will match the input impedance of various types of oscilloscopes and recorders. The output circuitry is so designed that an oscilloscope and recorder may be driven simultaneously, thus giving both an indication and record of the cavitation being studied. 

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