AGILENT ADS SIMULATION EXAMPLE – Performing Basic Circuit
Envelope Simulation with Single Source (Using ADS2002C)
Objectives:
 To learn the basics of Circuit Envelope simulation using single source.
 To learn how to set the parameters in the Circuit Envelope simulation control.
Background:
 Circuit Envelope (CE) simulation can be considered as a type of time-varying
Harmonic Balance (HB) simulation. HB simulation is performed at each time step
specified by the parameter “Step” in the CE simulation control. Usually in basic CE
simulation there is a sinusoidal source that is modulated in the time domain. This is
shown in the simple schematic in Figure 1.
 There are a number of ways to choose the time step. For this example, we let the time
step to be 10% the rise/fall time of the envelope modulating the sinusoidal source.
n sec  1 nsec
t step  1010

The modulating envelope is usually called the baseband signal in telecommunication.
For more complicated baseband signal, we could use another method to determine the
minimum time step required.
t step 
1
N  BWbaseband
5  N  10
Figure 1 – Basic CE simulation setup.
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

In Figure 1, Freq[1] is the fundamental frequency when carrying out the HB analysis
at each time step. There is only one sinusoidal source in this example. If there is
another sinusoidal source then we have to include another parameter “Freq[2]”. The
Order[1] = 3 tells the simulator to consider up to 3rd harmonic for Freq[1] during HB
simulation. When more than one sinusoidal sources are present, we also have to set
the “MaxOrder” parameter, in the CE simulation control.
We carry out two simulations, one using Transient Simulation and the other using CE
simulation. This is aimed to illustrate that the two will give similar results if the
parameter “Step” in CE simulator control is properly set. The CE simulation has the
advantage of giving the envelope of the modulating signal too.
Result 1 – Activating the Transient Simulation
800
600
Vout, mV
400
200
0
-200
-400
-600
-800
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
time, nsec
Figure 2 – Voltage waveform Vout using Transient Simulation.
The data Vout stored in the result for Transient Simulation is an array of 1 dimension.
Thus it is plotted as shown in Figure 2 above.
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Result 2 – Activating the CE Simulator
The data Vout stored in the result for CE Simulation is a multi-dimensional array. Using
the built-in function “what”, we see that Vout is a fifth dimension array. It consist of 151
points in time domain, and 4 dimension in frequency domain. The 4 frequency domains
corresponds to d.c., fundamental, 1st harmonic, and 2nd harmonic.
what(Vout)
Dependency : [time,freq]
Num. Points : [151, 4 ]
Matrix Size : scalar
Type
: Complex
Figure 3 - Using the “what” function to understand the structure of data Vout.
To look at the time domain waveform of Vout, we use the “ts” function, this stands for
time series. As seen in Figure 3 that provided the parameter “step” is sufficiently small,
the time series waveform for Vout from CE simulation is almost identical from the
waveform obtained using Transient Simulation.
800
600
ts(Vout), mV
400
200
0
-200
-400
-600
-800
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
time, nsec
Figure 4 – The time domain waveform of Vout from CE simulation.
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In order to observe the envelope of Vout, we use indexing technique to extract the
envelope. Vout[::,1] means we are looking at the magnitude of the fundamental
components (“::” is the wildcard character in ADS, it means all). As seen in Figure 4 the
fundamental component amplitude increase and decrease with time, thus Vout[::,1] is as
shown in Figure 5.
0.7
0.6
mag(Vout[::,1])
0.5
0.4
0.3
0.2
0.1
0.0
0
20
40
60
80
100
120
140
160
time, nsec
Figure 5 – The envelope of the fundamental component.
The schematic of Figure 1 is a linear circuit, so it does not generate any harmonics and
d.c. component. We can look at the amplitude for 1st harmonic versus time, and it is zero
as shown in Figure 6. This also applies to the d.c. and 2nd harmonic components.
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1.0E-300
Vout[::,2]
5.0E-301
0.0
-5.0E-301
-1.0E-300
0
20
40
60
80
100
120
140
160
time, nsec
Figure 6 – Amplitude of 1st harmonic versus time.
Finally, we can also look at the amplitude of all frequency components at a certain time
step. Again we made use of the indexing technique. Figure 7 and Figure 8 show the
frequency components at time 30nsec and 60nsec respectively. Only the fundamental
component is present as this example is a linear circuit. At t = 60nsec, the fundamental
component is also zero, as evident from the time series plot in Figure 4.
0.8
0.7
Vout[30,::]
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
freq, GHz
Figure 7 – Frequency components at t = 30nsec.
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0.8
0.7
Vout[60,::]
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
freq, GHz
Figure 8 – Frequency component at t = 60nsec.
Reference
The online help of the ADS software provides a good overview of the Circuit Envelope
method (Version 2003 and later). For information on Harmonic Balance method, see the
associated document on basic Harmonic Balance simulation.
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May 2004