Cables impedance matching

The transmitter amplifier chain consists of a linear, three-stage transistor amplifier from Amplifier Research (10 W), a class C single-stage field effect transistor (FET) amplifier (120 W) custom-built by H. Bonn GmbH, Munich, and a final tube amplifier with two tetrodes 4CX 350A that deliver more than 1.5 kW of pulse power. A special effort was made to match the input and output impedances of this tube amplifier to the characteristic impedance (50 ( ) of the cables connecting it with the probe and the driver, respectively. This impedance matching resulted in the virtually complete disappearance of antisymmetric phase transients (for a discussion of the effects of such phase transients on m.p. spectra, see Haeberlen, 1976, Appendix D). 

Every electronic component has a characteristic impedance. When a signal is transmitted from a unit with a high-impedance output to a low-impedance input, there is going to be a loss in the signal unless an impedance-matching device is used to couple the two units. Manufacturers of preamplifiers and amplifiers quote the impedance of the input and output for their products. Coaxial cables have an impedance between 90 and 100 ft. 

Like all high-speed electronics, TCSPC uses impedance-matched cables for the signal eonneetions. These connections are relatively immune from capacitive noise piekup. Therefore, inductive coupling is the dominating effect that introduces noise into the system. 

An isolator is not an impedance matching device. Proper cable lengths and the possible use of Kne matchers are means to match other devices to the characteristic design impedance of an... 

Source terminated A circuit whose output is terminated for correct impedance matching with standard cable. 

Connect the amplifier UNIpolar output to the MCA input. Again, it will be neater to use the socket on the back panel of the amplifier. It is definitely recommended that the output be taken from the rear panel if impedance matching is necessary as that socket can be matched to the cable (usually 93 fi). 

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. 

Ferroelectric composites can be used as underwater transducers [22] to detect (passive mode) and/or generate (active mode) sound. Transducers made from ceramics have the disadvantage that their density makes it difficult to obtain good impedance matching with water. One of the problems with PVDF (aside from the problems associated with poling) is its low permittivity. This will produce a low element capacitance, which will load the output. The equivalent circuit for a flank array transducer linked to an amplifier via a cable is shown in Fig. 6.15. The open-circuit voltage sensitivity Moc/ is loaded by stray capacitance Cg, and/or cable capacitance Q. The end-of-cable sensitivity Mgc is given by... 

Current mode preamplifiers can provide reliable counting up to 10 counts per second. They are specially suited for use with fission chambers.Voltage sensitive preamplifiers, with low gain, have high input impedance and low output impedance. They are useful for use with scintillation detectors, and also for matching of cable impedance. 

Operated broadband amplifier 1, has uniform (not worse than 1 dB) frequency feature within the range of 1 to 50 MHz and the range of reinforcement from 0 to 90 dB. The input cascade has an impedance switch that enables to matching of sensors of different types and to avoid signals and distortions caused by the cable. 

Impedance. Impedance defines the relationship of voltage and current in a coaxial cable. The electrical requirements of the hardware dictate the impedance values for the interconnecting cables. Most coaxial cables are designed to match the impedances required by electronic hardware. 

The probe uses a single-coil for 20 mm spinning samples at room temperature. There are no tuning elements in the probe -- rather the tuning is by a variable inductor at the end of a A./4 cable as described in the discussion of quarter wave cables in V.C.8. The transformer at the end of the second quarter wave cable matches the impedance between the tank circuit and the broadband preamplifier. The output... 

Now at frequencies below a few hundred MHz, the typical capacitor used in NMR is virtually loss free and r is equal to the resistance of the wire in the coil and possibly of a cable between L and C. Since this is a small number, the series tank circuit requires additional circuitry to raise the impedance to match, for example, to a preamplifier with a 50 ohm input. [The need to present a certain impedance arises because the S/N of the preamplifier depends on this impedance Thus, this is an important matching condition.]... 

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. 

The elements in an array will usually be fed via a harness of one sort or another. That opens up the possibility of reradiation via the feed cables unless special precautions are taken. As a simple but very fundamental example, consider Fig. 2.17. Here two subarrays are fed via a simple parallel T connector. Note that the characteristic impedance of the main feed line is Zq and how it splits up into two branchlines each with characteristic impedance 2Zq and terminated with the subarrays each with input impedance 2Zq. Thus, we will have a perfect match at all frequencies whether we hansmit or receive. 

The signal output — this will need a cable with BNC connectors and usually of 93 ft impedance. It would be stamped RG 62/U. Systems matched to 50 ft or 75 ft would require different cables (see Table 11.2). It is important that cable of the correct impedance is used, particularly if long cable runs are necessary. 

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