Virtuoso Spectre Circuit Simulator RF Analysis User Guide

Virtuoso Spectre Circuit Simulator RF Analysis User Guide
Creating and Using Receiver K-Models
K-model would suggest that the model’s noise varied with input power. It does not.
Furthermore, noise analysis is usually of interest in bit error rate simulations. Such
simulations are usually long and the desired signal is weak. The linear K-model suffices
for weak desired signals and runs faster than the non-linear K-model.
Limitations and requirements resulting from non-linear K-model assumptions
The K-model requires two basic assumptions:
■ Small-signal components of the base-band input can be wide-band, but large
components must fall within the receiver’s bandwidth.
The signal of interest is usually within the receiver’s bandwidth, and interferers usually lie
outside this bandwidth. However, if the receiver has a band selection filter immediately
after the antenna, and that filter can be modeled apart from the K-model, this first
assumption is not a severe restriction. This is true because the filter makes the interferer
small by the time it reaches the K-model.
■ Non-linear behavior depends only on the input radius. Non-linear PM/PM and PM/AM
effects are negligible.
If the input signal is
rf(t)=a(t)cos(wct)-b(t)sin(wct)
the base-band signal is
a(t)+jb(t)
a trajectory in the complex plane. This is the rectangular representation.
The polar representation is in terms of radius and angle. The radius is
Sqrt[a(t)a(t)+b(t)b(t)]
The angle is
ArcTan[b(t)/a(t)]
Requirements resulting from the Alta SPW environment
Both K-models are part of Alta SPW, the Cierto Signal Processing Worksystem. Alta
SPW is a DSP design tool. This environment creates the following two consequences
you must remember:
■ Model extraction requires base-band input and output ports.
Because it is a base-band tool, Alta SPW operates on the information impressed on the
RF carrier rather than the instantaneous value of the RF signal. Each of two carrier
phases, sine and cosine, carry information. Because Alta SPW represents base-band
June 2007 322 Product Version 6.2