The Circuit Designer's Companion - diagramas.diagram...

Grounding and wiring 27Shielding and microphonyShielding of data and multicore falls into three categories:• copper braid. This offers a good general-purpose electrical shield but cannotgive 100% shield coverage (80–95% is typical) and it increases the size andweight of the cable.• tape or foil. The most common of these is aluminised mylar. A drain wire isrun in contact with the metallisation to provide a terminating contact and toreduce the inductance of the shield when it is helically-wound. Thisprovides a fairly mediocre degree of shielding but hardly affects the size,weight and flexibility of the cable at all.• composite foil and braid. These provide excellent electrostatic shielding fordemanding environments but are more expensive – about twice the price offoil types.For small-signal applications, particularly low-noise audio work, another cableproperty is important − microphony due to triboelectric induction. Any insulatorgenerates a static voltage when it is rubbed against a dissimilar material, and this effectresults in a noise voltage between conductor and screen when the cable is moved orvibrated. Special low-noise cable is available which minimises this noise mechanismby including a layer of low resistance dielectric material between braid and insulator todissipate the static charge. When you are terminating this type of cable, make sure thelow resistance layer is stripped back to the braid, otherwise you run the risk of a nearshort circuit between inner and outer.1.2.5 RF cablesCables for the transport of radio frequency signals are almost invariably coaxial, apartfrom a few specialised applications such as hf aerial feeder which may use balancedlines. Coax’s outstanding property is that the field due to the signal propagating alongit is confined to the inside of the cable (Figure 1.21), so that interaction with its externalenvironment is kept to a minimum. A further useful property is that the characteristicimpedance of coax is easily defined and maintained. This is important for RFapplications as in these cases cable lengths frequently exceed the operating wavelength.sheath screen or dielectricouter conductorfield is confined withinouter conductorFigure 1.21 Coax cableinner conductorThe generic properties of transmission lines − of which coax is a particular type −will be discussed in section 1.3. The parameters that you will normally find in coaxspecifications are as follows:• characteristic impedance (Z o ): the universal standard is 50Ω, since thisresults in a good balance between mechanical properties and ease of circuitapplication. 75Ω and 93Ω are other standards which find application in