# MODELING AND SIMULATION FOR RF SYSTEM DESIGN

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116 Chapter
6
Tc is the current simulation time. Tn is the next time point where a digital
event can be found in the digital event queue.
The analog solver starts to simulate from Tc to Tn. Remember that
between event time points the signals do not change their values. That
means that the analog solver does not have to wait for new results from
the digital solver.
If Q'ABOVE(E) is used and Q crosses E then an event is placed into the
event queue. Tn is set to Tn', that is the time point where the crossing
occurs.
At Tn digital signals and analog quantities are determined within a so-
called delta-cycle until they are in accordance.
After the delta-cycle is stable the analog solver can again continue until
the new next event time point.
Digital time values are of type TIME. Analog time values are of type
REAL. The following expressions allow converting TIME values to REAL
values and vice versa:
REAL (TIME'POS (time_of_type_time)) * 1.0E-15 => time of type REAL
INTEGER (time_of_type_real * 1.0E15) * 1 fs => time if type TIME
6.6
Analysis Domains
In order to describe and to analyze physical systems the underlying
domains have to be considered. The most common domains are operating
point analysis, time domain analysis, and frequency domain analysis.
Other domains are used for special analyses (for example noise
simulation) or for a special kind of systems (for example periodic steady
state analysis for RF circuits as described in Chapter 3 "Simulation Tools in
System Design"). In the following we will describe the analysis domains that
are supported by VHDL-AMS. We will pay special attention to the
frequency domain analyses.
6.6.1
Supported domains
VHDL-AMS supports different kinds of analyses. The DOMAIN signal
is updated by the simulation engine, which provides a hint of which kind of
analysis is used. The following kinds of analyses are possible
Determination of operating point
Quiescent domain analysis is used to determine the operating point. The
DOMAIN signal is set to QUIESCENT_DOMAIN. The time NOW

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Summary :

## 116 Chapter 6 Tc is the current simulation time. Digital time values are of type TIME. time if type TIME 6.6 Analysis Domains In order to describe and to analyze physical systems the underlying domains have to be considered. The most common domains are operating point analysis, time domain analysis, and frequency domain analysis.

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