Envelope currents method with extended dynamic range for the simulation of nonlinear communication circuits

Abstract

Abstmct-A novel approach t o analyze the effects of nonlinear distortion in communication circuits is reported based on a recursive algorithm. I n each iteration a time-varying linear circuit excited by appropriate nonlinear currents obtained from an expansion of the nonlinearity is analyzed. This procedure has been applied to a common-source HEMT amplifier excited with a single tone and results show a very fast convergence together with a high dynamic range. The technique was used in the evaluation of the spectral regrowth and constellation distortion of a QPSK signal with raised cosine and root raised cosine conforming pulses p m duced by the amplifier at 5.8 GHz I . INTRODUCTIONThe design of wireless communications systems requires precise circuit level simulation t o predict nonlinear characteristics like adjacent channel power rejection ( ACPR)? constellation distortion and noise power ratio (NPR). The large nuniber of frequency components to be considered in these systems requires excessive resoiirces and very long simulation Limes using techniques like harmonic balance [I] or time domain techniques 121. A more efficient algorithm has been proposed in [3] extending the Nonlinear Currents method (NC) to the Envelope Currents method (EC). However, since the NC method has poor convergence properties and is an unmanageable technique when the order n of the nonlinearity increases, the method in I31 is limited to nonlinear systems excited by weak input signals. This restriction prevents the application of this technique to circuits operating with signal levels for which the study of nonlinear distortion is more important, i.e. near compression point. In this paper, we propose a new method of nodal analysis instead of the NC method. The basic idea behind this rnetlrod is to express the output of the circuit as a sun1 of incremental voltages, each one of them is the solution of a timevarying linear circuit excited by a nonlinear current. The time dependence of this changeless linear circuit a,nd its exciting noillinear curreiits are updated at cacfi step of the iterative process. This technique exhibits faster THEORETICAL D~SCUSSIONFor the sake of clarity, let consider a single node nonlinear circuit driven by the excitation current I, + is(t). The integrodifferential equation can be symholically expressed aswith w ( t ) being the node voltage, and I ( v ) , c(v)du/dt are the currents across a nonlinear conductance and a nonlinear capacitance, respectively. L[.] represents a linear operator.In the present approach w ( t ) is given 5y 1) = " 0 + 21, + . . . + + sw, = c7, + sun( 2 ) The DC voltage W O is calculated considering that only I, is present in (1). Expanding I(") and c ( v ) in Taylor series about ug and using (2) with n = 1, equation (1) yields + c(u0)-"1 = Zb (3) at dl for the first incremental voltage V I , so that it is computed in the same form as the linear term in the standard NC method. Thus, the first residnal voltage Swl satisfies the equation dv du 1LIbVl] + I(v) + c(n);ii. = q u o ) +g(vo)Vl...