MODELLING OF NONUNIFORM RC STRUCTURES FOR COMPUTER AIDED DESIGN

A simple model for nonuniform distributed RC structures is presented. The model consists of three passive elements only and can be used for modelling nonuniform distributed RC structures involving exponential, hyperbolic sine squared, hyperbolic cosine squared and square taper geometries. The model can be easily implemented for computer-aided analysis and design of circuits and systems comprising nonuniform distributed RC structures.


INTRODUCTION
Nonuniform distributed parameter RC structures are widely used in many diverse fields; for example in integrated circuits, thin films as well as monolithics.This is attributed to the flexibility in tailoring and shaping their frequency response and time domain characteristics by exploiting their variable taper-geometry.For ar- bitrary taper-geometries, there is no general closed-form analytical solution for the transfer functions of these structures.Closed-form analytical expressions are avail- able only for a few particular taper-geometries involving exponential, hyperbolic sine squared, hyperbolic cosine squared and square functions .The use of these expressions, however, poses a formidable problem and involves considerable math- ematical intricacy and, therefore, can not be used for obtaining the time-domain characteristics of these nonuniform RC structures.This is attributed to the difficulty in calculating the pole values of their transfer functions.Recourse to approximate models for the transfer functions of nonuniform distributed tapered RC structures is, therefore, inevitable.In this regard, Ghausi and Kelly used the m-pole and dominant-pole and excess-phase approximation methods for obtaining the unit- step response of exponentially tapered RC structures.Although, in principle, these methods can be used for obtaining the unit-step response of other taper-geometries, Ghausi and Kelly did not present such results; probably because of the rather tedious mathematical manipulations required to obtain the m-pole approximation and consequently the dominant-pole and excess-phase shift approximation.
In this paper, a general procedure will be presented for obtaining approximate closed-form analytical expressions for the transfer functions of the nonuniform distributed parameter RC structures with exponential, hyperbolic sine squared, hyperbolic cosine squared, and square taper-geometries.Using these expressions the unit-step response of these nonuniform distributed RC structures can be easily predicted.Moreover, using these expressions a simple model for these structures can be derived.Such model using three passive elements only, can be easily im- plemented for computer-aided analysis and design of circuits and systems com- prising nonuniform distributed RC structures with exponential, hyperbolic sine squared, hyperbolic cosine squared and square taper geometries.

ANALYSIS
It is well known that the open-circuit voltage transfer function of two-port networks is given by y(s) Tv The short-circuit admittance parameters y2(s) and y22(S) where O (B + ZoY0)1/2, B kd, Z0 rod, Y0 sc0d, k is a constant equal to the taper factor and d is the length of tapered RC structure.The prime denotes differentiation with respect to the distance variable x along the tapered structure.P(0) and P(d) are the terminal values of the electrical taper function P(x) of the tapered RC structure.The value of P(x) for various kinds of taper are listed in Table I. r0 and Co are the resistance and capacitance per unit length.Using eqns (1-3) it is easy to show that the transfer function of the special taper-geometries under consideration can be written as follows In general, the parameters 3'n can be obtained, for different values of kd and kc, by using standard curve-fitting subroutines available in most mainframe computers.However, for second-order polynomials, with N 2, the parameters 3' and 3'2 can be obtained by hand calculation using the Lagrange interpolating polynomial3.This procedure yields a family of parameters 3' and 3'2 that depend on kd and kc.
These parameters are fitted to simple closed-form analytical expressions.These expressions are listed in Table I.
Using eqns (4, 5) calculations are performed, using N 2, and are shown in Fig .(1) from which it is obvious that the proposed second-order polynomial of eqn.(5) accurately represents the function Q(@).The average relative-root-mean- square (RRMS) errors are listed in Table I.

RESULTS
By combining eqns (4, 5) the unit-step response due to input v(t) u(t) is thus 1 1 V,,(s) s 1 + ,@ + or V,, where 2b ),/3'2 and w 1/3'2.Since w < b2, then the inverse Laplace transform of eqn ( 6) is obtained as v,,( where 6 and A are the roots of the equation q + 2bO+ wg=0 (2) depicts the unit-step responses for some of the nonuniform distributed RC structures under consideration.From Fig.
(2) it is obvi6us that the results obtained using the approximation of eqn (5) are in excellent agreement with the exact responses published previously ,5.
From eqns (4, 5) it is obvious that the approximation presented here to represent the transfer function of the open-circuit nonuniform distributed RC-structures un- der consideration correspond to the equivalent circuit shown in Fig. (3).In contrast with previously proposed models2'5,6, which require a relatively large number of passive elements, the equivalent circuit proposed here consists only of three passive elements and can be used for modelling nonuniform RC-structures with taper- geometries involving exponential, hyperbolic sine-squared, hyperbolic cosine- squared, and square functions.

CONCLUSIONS
A second-order polynomial representation has been proposed for the transfer func- tions of open-circuit nonuniform RC-structures involving exponential, hyperbolic sine squared, hyperbolic cosine squared, and square taper-geometries.Based on this representation a lumped network with three passive elements only can be used for modelling the performance of the open-circuit transfer characteristics of these Vo(t 0.5 0.5 "n Exact, [1] Equivalent dominant pole and excess phase [1] Four-pole approximation [1] FIGURE 2 Unit step response of open-circuit uniform and exponential taper-geometries, t, t/rocod nonuniform RC-structures.This model can be used for predicting the time-domain response of these structures.The proposed model can be easily implemented for computer-aided analysis and design of circuits and systems comprising exponential, hyperbolic sine squared, hyperbolic cosine squared, and square function nonuni- form RC-structures. Ro rod, Co 71 cod, Le Cod Re 71 and 72 are given by Table I.

FIGURE 3
FIGURE3 Proposed model for nonuniform RC structures involving exponential, hyperbolic sine squared, hyperbolic cosine squared, and square taper-geometries.