Physics and Engineering of Radiation Detection

8.2. Signal Transport 477

copper is 1. 69 × 10 −^8 Ωcm. Compute the resistance the cable offers to the

signal current.

Solution:

We can use equation 8.2.1 to compute the resistance.

R = ρ

Lc

Ac

=

(

1. 69 × 10 −^8

) 50

0. 5 × 10 −^6

=1.69 Ω

A.1 CoaxialCable..........................

As shown in Fig.8.2.1(a), a typical coaxial cable has the following components.

A central conductor, mostly made of copper.

Two insulators surrounding the central wire. One thin and other thick.

Metallic shield surrounding the outer insulator.

Outermost insulator surrounding the metal shield.

CL

LL RL

Insulator

Insulator Shield Insulator

Conductor

(a)

(b)

Figure 8.2.1: (a) Sketch of a

coaxial cable. This type of ca-

ble is commonly used for signal

transport due to its low atten-

uation and high shielding capa-

bilities. (b) Model of a typical

coaxial cable having inductance,

resistance, and capacitance per

unit length of the cable.

The purpose of the shield is to alienate the central conductor from the outside
electromagnetic field. However there is no coaxial cable that provides perfect shield-
ingandthedecisiontoeitheruseitortogoforsomeothertypeofcableisbased
on the relative strengths of the signal and the background radiation.
The insulators in between the central conductor and the shield provide a low
distortion path orwaveguideto the signals. This ensures minimal dispersion and
hence small attenuation of signals specially the short duration pulses.