Agilent Technologies Load Pull DesignGuide

Advanced Design System 2011.01 - ADS Load Pull DesignGuide
Advanced Design System 2011.01
Feburary 2011
ADS Load Pull DesignGuide
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
Load Pull DesignGuide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
One Tone Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
One Tone, Constant Available Source Power Load Pull . . . . . . . . . . . . . . . . .
One Tone, Constant Available Source Power Load Pull with Swept Parameter
One Tone, Constant Power Delivered 2nd Harmonic Load Pull . . . . . . . . . . .
One Tone, Constant Power Delivered Load Pull . . . . . . . . . . . . . . . . . . . . . .
One Tone, Constant Power Delivered Load Pull with Swept Parameter . . . . .
One Tone, Constant Power Delivered Source Pull . . . . . . . . . . . . . . . . . . . .
One Tone, Swept Available Source Power Load Pull . . . . . . . . . . . . . . . . . .
Two Tone Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Two Tone, Constant Available Source Power Load Pull . . . . . . . . . . . . . . . .
Two Tone, Constant Power Delivered Load Pull . . . . . . . . . . . . . . . . . . . . . .
Two Tone, Constant Power Delivered Load Pull with Swept Parameter . . . . .
Two Tone, Swept Available Source Power Load Pull . . . . . . . . . . . . . . . . . .
WCDMA Signal Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
Load Pull DesignGuide
Load Pull simulation is frequently used by power amplifier designers to determine, which
load impedance to present to a device or amplifier in order to achieve a particular power
delivered, power-added efficiency, intermodulation distortion level, adjacent-channel
leakage ratio, and other specifications.
The Load Pull DesignGuide has simulation setups for working with measured load pull data
files and with nonlinear device models. The first two selections, that begin with Load Pull
Measured Data (Maury) - Check Contours…, enable you to specify a region of the
Smith Chart and verify that the measured data file will generate the contours you expect.
These are useful for determining approximately the optimal load impedance. The third
selection, Load Pull Measured Data (Maury) – Matching Network Optimization…,
shows an example of optimizing an impedance matching network. The first three
selections each utilize the DataBasedLoadPull component. This component simulates the
S-parameters of the network connected to it, and uses S11 as an index into the measured
load pull data file to read out (possibly with interpolation and extrapolation) the
corresponding measured data.
The Load Pull Measurement Data Import Utility (Focus or Maury) uses the Load
Pull Utility, which is unchanged from earlier ADS releases. This is necessary because the
DataBasedLoadPull component does not yet work with Focus data files.
The remaining selections in this Load Pull DesignGuide are all for running various different
types of load and source pulls on nonlinear device or amplifier models.
From a Schematic window, Select DesignGuide > Load Pull where you can see different
options in the Load Pull dialog box.
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
When you click on one of the options in the Load Pull dialog box, for example, OneTone,
Constant Available Source Power Load Pull, a simulation schematic and the corresponding
data display file are copied into your working workspace directory. You modify the
schematic by deleting the sample device, inserting your device, editing the parameters on
the load pull instrument to set voltages as needed, and specifying the circular region of
the Smith Chart for the load reflection coefficients. Then you run the simulation and view
the results in the corresponding data display file. Following are the three load pull
simulations:
One Tone Simulation (dgldpull)
Two Tone Simulation (dgldpull)
WCDMA Signal Simulation (dgldpull)
The constant power delivered simulations are achieved using an optimization, and include
one-tone, two-tone, and WCDMA input signals.
One Tone Simulation
Contents
One Tone, Constant Available Source Power Load Pull (dgldpull)
One Tone, Swept Available Source Power Load Pull (dgldpull)
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
One Tone,
(dgldpull)
One Tone,
One Tone,
One Tone,
One Tone,
One Tone,
Constant Available Source Power Load Pull with Swept Parameter
Constant
Constant
Constant
Constant
Constant
Available Source Power Source Pull (dgldpull)
Power Delivered Load Pull (dgldpull)
Power Delivered Load Pull with Swept Parameter (dgldpull)
Power Delivered 2nd Harmonic Load Pull (dgldpull)
Power Delivered Source Pull (dgldpull)
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
One Tone, Constant Available Source
Power Load Pull
This setup simulates the device or amplifier with a constant available source power while
the load reflection coefficient is swept in a circular region of the Smith Chart. This
simulation is the fastest, simplest one. It just runs a load pull simulation at a single
available source power level. This is useful for seeing the power, gain, and PAE contours of
a particular device at a single input power level. It does not provide information about
how far into compression the device is. However, you can quickly change the settings in
the schematic and see if you are getting the performance you are expecting.
You need to specify multiple things while using this schematic, such as:
1. Replace the device with your device or amplifier.
2. Set the bias voltages V_Bias1 and V_Bias2 or modify the bias network in the
Load_Pull_Instrument1 subcircuit, as needed. However, the DC power consumption is
computed in an equation within the subcircuit, assuming current probe Is_1 is
connected to supply voltage node Vs_1 and current probe Is_2 is connected to supply
voltage node Vs_2. If you delete any of these or re-name them, you must update the
Pdc equation so the DC power consumption is computed correctly.
3. Specify the available source power Pavs_dBm and input frequency RF_Freq, source
impedances at the fundamental Z_Source_Fund_, and 2nd harmonic Z_Source_2nd
frequencies.
4. There are three main ways to specify the region of the Smith Chart that will be
sampled by the load pull:
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
1. Set the reference impedance Z0 to 50, and set Specify_Load_Center_S=1. In
this case, the center of the circle of simulated reflection coefficients (and
harmonics) will be set by the S_Load_Center_* parameters, which are reflection
coefficients.
2. Set the reference impedance Z0 to 50, and set Specify_Load_Center_S=0. In
this case, the center of the circle of simulated reflection coefficients (and
harmonics) will be set by the Z_Load_Center_* parameters, which are
impedances.
3. Set the reference impedance Z0 to the complex conjugate of the impedance at
the center of the circle of reflection coefficients you want to simulate. In this
case, you could set Specify_Load_Center_S=1 and use the S_Load_Center_*
parameters. Remember that in this case, a reflection coefficient of 0
corresponds to setting the load impedance to the complex conjugate of the
reference impedance.
5. Specify the radius of the circle of the reflection coefficients S_Load_Radius and the
number of points Num_Points.
If the device or amplifier is potentially unstable and the circle of reflection coefficients that
you specify includes the unstable region, the simulation may run into convergence
problems. This would be due to the device wanting to oscillate. A solution to this problem
is to add stabilizing components at the input, output, or in parallel with the device. You
may want to use a simulation setup for this purpose, DesignGuide > Amplifier > SParameter Simulations > Feedback Network Optimization to Attain Stability.
Another solution is to specify the circle of reflection coefficients such that the unstable
region is avoided.
You can select DesignGuide > Load Pull > Reflection Coefficient Utility to see a data
display with a graphical tool to help you see the circle that corresponds to particular
values of the s11_rho and s11_center variables.
Run the simulation just as you would any other. When it finishes, open the
HB1Tone_LoadPull data display.
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
This shows the contours of power delivered, PAE, and gain. You may want to change the
step sizes between and the number of contour lines. The boxes on the left show that you
get slightly different performance results depending on whether you choose the load to
maximize power delivered or PAE. The lower Smith Chart shows how the input reflection
coefficients vary as a function of the load reflection coefficients.
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
One Tone, Constant Available Source
Power Load Pull with Swept Parameter
The One Tone, Constant Available Source Power Loadpull with Swept Parameter
simulation allows you to see how the contours and performances vary with some arbitrary
swept parameter (such as a bias voltage and the input frequency). However, the available
source power is held constant.
This setup adds the sweep of an arbitrary parameter to the simplest constant available
source power load pull simulation. You have to assign the parameter SweepVar to some
component or variable on the schematic. In this case it is assigned to the input signal
frequency RF_Freq.
The above figure specifies the limits of the swept variable, SweepVar, and assigning it to
RF_Freq.
One Tone, Constant Available Source Power Source Pull
This setup simulates the device or amplifier with a constant available source power while
the load reflection coefficient is swept in a circular region of the Smith Chart. This
simulation is the fastest, simplest one. It just runs a load pull simulation at a single
available source power level. This is useful for seeing the power, gain, and PAE contours of
a particular device at a single input power level. It does not provide information about
how far into compression the device is. However, you can quickly change the settings in
the schematic and see if you are getting the performance you are expecting.
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
Because the optimal source impedance is usually close to the edge of the Smith Chart, in
this case we have set the reference impedance for the source Z0_Src to a value that is
near the complex conjugate of what we expect will be the optimum.
The left Smith Chart below shows much better resolution of the contour lines because its
center corresponds to 5+j*10 Ohms.
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
One Tone, Constant Power Delivered
2nd Harmonic Load Pull
The One Tone, Constant Power Delivered 2nd Harmonic Loadpull
(HB1Tone_2ndHarmLoadPull_ConstPdel) simulation runs a load pull in which the load
at the fundamental frequency is held constant and the load at the 2nd harmonic is allowed
to vary. As above, the available source power is optimized until the desired power is
delivered to the load. The second harmonic may only have a significant effect on the
performances of devices being driven well into compression. This simulation is identical to
other load pull simulations, except that the load impedance at the second harmonic
frequency is swept.
This schematic is identical to the corresponding one for a load pull at the fundamental
except that now the Swept_Harmonic_Num variable is set to 2 instead of 1. (In fact, you
could just use the schematic set up for the fundamental sweep, instead of this one.) Since
we are sweeping the impedance at the second harmonic frequency, the load at the
fundamental frequency will be fixed at S_Load_Center_Fund or Z_Load_Center_Fund. The
further into compression the device is being driven or the more non-linearly it is
operating, the larger the effect of the second harmonic impedance.
The variation in performance due to the 2nd harmonic impedance may be small.
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
One Tone, Constant Power Delivered
Load Pull
The One Tone, Constant Power Delivered Loadpull
(HB1Tone_LoadPull_ConstPdel) simulation optimizes the available source power level
until the desired power is delivered to each load reflection coefficient. You would use this if
your device or amplifier needs to deliver a particular power level and you want to choose
the optimum load considering other performances (such as gain, gain compression, PAE,
and bias current.)
This setup sweeps the load reflection coefficient in a circular region of the Smith Chart and
optimizes the source power level for each load reflection coefficient until the desired power
is delivered to the load. The data display shows contours of constant PAE, bias current,
gain, and gain compression. The input reflection coefficient is also shown for a particular
load that you specify. These data allow you to pick the optimal load that produces the best
PAE, gain, gain compression, or bias current, or make trade-offs amongst these
specifications.
You have to make the same types of edits to this schematic as with the others described
above . Also, you have to specify the minimum and maximum allowed values of the
available source power, Pavs_dBm_min and Pavs_dBm_max, respectively.
During the optimization, the available source power is adjusted within these limits until
the power that you want is delivered to the load. Depending on how high a power you
want delivered to the load and the gain of the device, you may have to adjust the
Pavs_dBm_max limit.
In this example, the power delivered (based on Pdel_dBm_goal_min and
17
Advanced Design System 2011.01 - ADS Load Pull DesignGuide
Pdel_dBm_goal_max) is to be between 25 and 25.1 dBm. With this value and a
Pavs_dBm_max value of 20 dBm, it is specified that the lowest transducer power gain
accepted is 5 dB.
You may also specify different load impedances or reflection coefficients at the harmonic
frequencies and (for the source) at the fundamental and harmonic frequencies.
To launch the simulation, click the Optimize icon
. If the simulation is started by hitting
the F7 key or by selecting Simulate > Simulate, then an optimization is not executed
and the simulation results are not displayed correctly in the data display.
After running the optimization, the HB1Tone_LoadPull_ConstPdel data display shows
the results.
To see the contours effectively, you may need to change the CurrentStep, PAE_step,
Gain_step, and GainCompStep variables. These set the step sizes between the contours.
The upper Smith Chart shows contours of constant gain and gain compression. The lower
left Smith Chart shows contours of constant bias current and power-added efficiency
(PAE), as well as the simulated load reflection coefficients and the corresponding input
reflection coefficients.
In the green boxes on the left side are data that correspond to a particular optimal
condition such as minimum bias current, maximum PAE, or minimum gain compression.
However, you have to make sure that the desired power delivered was actually achieved.
For some load impedances close to the edge of the Smith Chart this may be difficult.
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
You also have the option of selecting any of the simulated load reflection coefficients with
marker m1. The corresponding data appears in a separate box.
Data within the red box corresponds to the reflection coefficient selected by marker M1.
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
One Tone, Constant Power Delivered
Load Pull with Swept Parameter
The One Tone, Constant Power Delivered Loadpull with Swept Parameter
(HB1Tone_LoadPull_ConstPdel_Sweep) simulation adds the sweep of an arbitrary
parameter. This sort of simulation is very useful if you want to see how the optimal load
impedance, load pull contours, and the device performances vary versus some arbitrary
parameter. For example, how does the optimal load vary with frequency and how do the
power added efficiency and other parameters change with the bias voltage. If you have
included some stabilization network around the device, how does the performance vary as
you change one of the parameters.
This simulation setup and data display are nearly identical to the version without the
parameter sweep.
You have to assign the swept variable SweepVar to some parameter on the schematic as
well as set its sweep limits. In this case, the gate bias voltage V_Bias1 will be swept from
1.5 to 2.25 Volts. After running the optimization, open the
HB1Tone_LoadPull_ConstPdel_Sweep data display and make sure the default dataset
name is set to the name of the dataset the simulation just created.
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
All the data is now indexed to the swept parameter value, which you select by moving the
SweepIndx marker. There is a change in the power-added efficiency as you change the
bias voltage.
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
The above figure displays the data with swept variable (gate bias) set to 2.25 Volts.
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
The above figure displays the data with swept variable (gate bias) set to 1.5 Volts.
When the GainComp values are negative, it means gain expansion is occurring. This could
be because the device is biased off or at a low bias current and that this bias point shifts
when a large enough signal is applied.
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
One Tone, Constant Power Delivered
Source Pull
The One Tone, Constant Power Delivered Source Pull
(HB1Tone_SourcePull_ConstPdel) simulation is most useful for determining the
optimal source impedance to present to a device. The source impedance should mostly
affect the gain and gain compression. This setup sweeps the source reflection coefficient
in a circular region of the Smith Chart and optimizes the source power level for each
source reflection coefficient until the desired power is delivered to the load. The data
display shows contours of constant PAE, gain, and gain compression. This allows you to
pick the optimal source that produces the best PAE, gain, or gain compression, or make
trade-offs amongst these specifications.
The lower left SmallSignal and Sweep3 parameter sweeps are only used to obtain output
powers when the device is being driven with a small signal. These output powers are used
as references in the gain compression computations.
Relative to the other constant power delivered simulation schematics above, the only
difference is that you have to specify the constant load impedance at the fundamental
frequency, instead of the source impedance. You may also specify different load and
source impedances at the harmonic frequencies. Typically, you would first run a load pull
simulation to determine the fundamental load impedance to use here.
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
To launch the simulation, click the Optimize
icon. If the simulation is started by hitting
the F7 key or by selecting Simulate > Simulate, then an optimization is not executed
and the simulation results are not displayed correctly in the data display.
After running the optimization, the HB1Tone_SourcePull_ConstPdel data display shows
the results.
To see the contours effectively, you may need to change the PAE_step, Gain_step, and
GainCompStep variables. These set the step sizes between the contours. As stated
above, if you have modified the bias network, you will have to edit the Pdc equation on
the Equations page.
Also, the bias supply current calculations only include the current in the probe Is_high. If
you change the name of the current probe, you will need to edit the BiasCurrent equation
on the Equations page.
The upper Smith Chart shows contours of constant gain and gain compression. The lower
left Smith Chart shows contours of constant power-added efficiency (PAE) which may not
vary much, as well as the simulated source reflection coefficients and the corresponding
input reflection coefficients, which may be just a single point since they should not depend
on the source impedance. The lower right Smith Chart shows the same data on a Smith
Chart with a different reference impedance.
In the boxes on the left side are data that correspond to a particular optimal condition
such as maximum gain, maximum PAE, or minimum gain compression. However, you
have to make sure that the desired power delivered was actually achieved.
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
The source that corresponds to the maximum gain is very nearly satisfying the conditions
for oscillation at the input (Z_In_at_MaxGain+Zsource_at_MaxGain =0, approximately),
so you would want to avoid setting the source impedance to this value or you would want
to add some sort of stabilization network around the device.
You also have the option of selecting any of the simulated source reflection coefficients
with marker m1. The corresponding data appear in a separate box.
The above figure shows the performance data corresponding to the source impedance
selected by marker 1. This enables you to see potential trade-offs. As you move away
from the maximum gain source impedance, the gain drops rapidly.
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
One Tone, Swept Available Source
Power Load Pull
The One Tone, Swept Available Source Power Load Pull setup adds a sweep of the
available source power for each value of the load reflection coefficient. It enables you to
see how the contours change as the device is driven from small signal to large signal into
compression. The schematic has an additional parameter sweep for the available source
power variable Pavs_dBm.
In addition to the various settings done in the HB1Tone_LoadPull schematic, you also
have to specify the sweep limits for the Pavs (Power available from the source) variable.
Here we have two ranges, one with a coarser step (assumed to be the linear region) and
one with a finer step (in the compressed region.)
After running the simulation, open the HB1Tone_LoadPull_PSweep data display and
make sure the default dataset name is set to the name of the dataset generated by the
simulation.
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
The data display shows the transducer power gain, gain compression, power delivered,
and PAE contours for a particular value of the available source power that you select using
SwpIndx marker. The gain and gain compression curves that correspond to several
optimal load points (for minimum gain compression, maximum PAE, and maximum power
delivered) are also shown.
Two Tone Simulation
Contents
Two
Two
Two
Two
Tone,
Tone,
Tone,
Tone,
Constant Available Source Power Load Pull (dgldpull)
Swept Available Source Power Load Pull (dgldpull)
Constant Power Delivered Load Pull (dgldpull)
Constant Power Delivered Load Pull with Swept Parameter (dgldpull)
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
Two Tone, Constant Available Source
Power Load Pull
This simulation setup and data display is identical to the "One Tone" version, except that
now two tones are supplied instead of one. A two tone test signal stresses the device
more because of its much higher peak-to-average ratio. The data display from this
simulation shows the same information as shown in the one-tone version and also
includes intermodulation distortion.
Here also you have to replace the sample device with yours and adjust the bias voltages
as needed. In addition to all the other variables that you must specify as before, you need
to specify the following two variables:
1. Frequency spacing between the two tones F_Spacing.
2. Maximum order of intermodulation distortion tones to be included in the simulation
Max_IMD_Order.
The simulation results include similar information as shown above, with the addition
of intermodulation distortion.
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
Two Tone, Constant Power Delivered
Load Pull
This simulation setup and data display is identical to the "One Tone" version, except that
now two tones are supplied instead of one. A two tone test signal stresses the device
more because of its much higher peak-to-average ratio. The data display from this
simulation shows the same information as shown in the one-tone version and also
includes intermodulation distortion.
A different device is used here.
Here also you have to replace the sample device with yours and adjust the bias voltages
as needed. In addition to all the other variables that you must specify as before, you need
to specify the following two variables:
1. Frequency spacing between the two tones F_Spacing.
2. Maximum order of intermodulation distortion tones to be included in the simulation
Max_IMD_Order.
The simulation results include similar information as shown above, with the addition of
intermodulation distortion.
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
There is a clear trade-off between PAE and distortion. For this bias point, if you want
maximum PAE, you suffer a lot of gain compression and intermodulation distortion.
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
Tolerating a lower PAE allows much lower gain compression and intermodulation
distortion.
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
Two Tone, Constant Power Delivered
Load Pull with Swept Parameter
This simulation setup and data display are identical to the "One Tone" version, except that
now two tones are supplied instead of one. A two tone test signal stresses the device
more because of its much higher peak-to-average ratio. The data display from this
simulation shows the same information as shown in the one-tone version and also
includes intermodulation distortion.
A different device is used here.
Here also you have to replace the sample device with yours and adjust the bias voltages
as needed. You have to assign the swept variable SweepVar to some parameter on the
schematic. In addition to all the other variables that you must specify as before, you need
to specify the following two variables:
1. Frequency spacing between the two tones F_Spacing.
2. Maximum order of intermodulation distortion tones to be included in the simulation
Max_IMD_Order.
The simulation results include similar information as shown above, with the addition of
intermodulation distortion.
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
There is a clear trade-off between PAE and distortion. For this bias point, if you want
maximum PAE, you suffer a lot of gain compression and intermodulation distortion.
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
Two Tone, Swept Available Source
Power Load Pull
This simulation setup and data display is identical to the "One Tone" version, except that
now two tones are supplied instead of one. A two tone test signal stresses the device
more because of its much higher peak-to-average ratio. The data display from this
simulation shows the same information as shown in the one-tone version and also
includes intermodulation distortion.
Here also you have to replace the sample device with yours and adjust the bias voltages
as needed. In addition to all the other variables that you must specify as before, you need
to specify the following two variables:
1. Frequency spacing between the two tones F_Spacing.
2. Maximum order of intermodulation distortion tones to be included in the simulation
Max_IMD_Order.
The simulation results include similar information as shown above, with the addition of
intermodulation distortion.
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
WCDMA Signal Simulation
WCDMA uses a source with WCDMA modulation instead of one or two sinusoids. The
adjacent channel power ratios are computed in addition to other performances such as
gain, gain compression, and PAE. These simulations take longer to run, so you might want
to run at least one of the one- or two-tone simulations above first.
Select WCDMA Load Pull > Constant Power Delivered, Mag/Phase Load Pull to
copy WCDMA_LoadPullMagPh_ConstPdel schematic and corresponding data display into
your workspace.
This setup sweeps the load reflection coefficient in a fan-shaped region of the Smith Chart
and optimizes the source power level for each load reflection coefficient until the desired
power is delivered to the load. The source is a WCDMA signal read in from a dataset, and
its amplitude (and thus the available source power) is set by a variable, SFexp, and the
gain applied to this signal is 10**(SFexp). The WCDMA signal was generated by
connecting a Timed Sink to the output of the signal source in the ADS example
examples/WCDMA3G/WCDMA3G_PA_Test_wrk/WCDMA3G_PA_UE_ACLR schematic.
The load pull is performed twice.
1. In first case, SFexp is set to 0.01. This is assumed to make the input signal small
enough that the amplifier is operating linearly. The gain under this condition for each
load reflection coefficient is the reference used to compute the gain compression.
2. In second case, SFexp is optimized until the desired power is delivered to the load.
The data display shows contours of constant PAE, ACLR, bias current, gain, and gain
compression. The input reflection coefficient is also shown for a particular load that you
specify. This allows you to pick the optimal load that produces the best PAE, ACLR, gain
compression, or bias current, or make trade-offs among these specifications.
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
When using this schematic, there are a number of different things you need to specify,
and these are listed in a paragraph on the schematic. First, you would replace the device
with your device or amplifier. You have to set the bias voltages or modify the bias
network, as needed. However, the data display calculates the DC power consumption
assuming current probe Is_low is connected to supply voltage node Vs_low and current
probe Is_high is connected to supply voltage node Vs_high. If you delete any of these or
re-name them, you will have to modify the equations like
Is_highDC=mean(Is_high.i[0]) and the Pdc equation on the schematic.
You have to specify the range of phases and magnitudes of the reflection coefficients. The
total simulation time will increase linearly with the product of the numbers of phases and
magnitudes simulated. The tradeoff is that you should get better contour lines with more
points simulated.
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
If the device or amplifier is potentially unstable and the region of reflection coefficients
that you specify includes the unstable region, the simulation may run into convergence
problems. This would be due to the device wanting to oscillate. A solution to this problem
is to add stabilizing components at the input, output, or in parallel with the device. You
may want to use a simulation setup for this purpose, DesignGuide > Amplifier > SParameter Simulations > Feedback Network Optimization to Attain Stability.
Another solution is to specify the range of reflection coefficients such that the unstable
region is avoided.
You also have to specify the reference impedance, Z0, and the source center frequency,
RFfreq.
You have to specify the nominal and allowed range of the signal source gain scale factor
exponent, SFexp. It is necessary to adjust this gain to set the available source power,
because we are using a voltage source to generate the signal.
It is not obvious what the relationship is between this exponent and the available source
power. However, the WCDMA_SrcTest schematic in the same workspace allows you to
sweep this scale factor and calculate the corresponding available source power. In this
case, stepping SFexp from -1 to 1 increases the available source power from about -20
dBm to +20 dBm. This does vary with the source impedance you specify.
When setting the range of SFexp values, you want the highest SFexp value to correspond
to the maximum available source power you will accept. For example, if you want to
deliver 27 dBm to the load and you want the device to provide at least 7 dB of transducer
power gain, you would set the maximum value of SFexp to correspond to an available
source power of approximately 20 dBm. The optimization will run fastest if SFexp is
allowed to vary over a relatively large range, but with the nominal value close to the value
needed to give you the desired output power. You may get extremely high gain
compression values for some reflection coefficients near the edge of the Smith Chart.
Figure: Prior to starting on-screen editing
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
Figure: While performing on-screen editing
During the optimization, this variable is adjusted within the limits until the power that you
want is delivered to the load.
You specify the desired power to be delivered to the load in OptimGoal1.
In this case, we want the power delivered to be within 0.25 dB of 27 dBm. You may also
specify different load and source impedances at the harmonic frequencies and (for the
source) at the fundamental frequency.
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
There is a bank of raised cosine filters connected to the output node. These are used to
compute the upper and lower adjacent and second adjacent channel leakage ratios. If you
modify this setup to simulate a signal corresponding to a different standard (non-WCDMA,
for example) then you most likely will need to modify the filter parameters and the
channel frequency limits, which are specified as an offset from the carrier center
frequency, RFfreq.
As mentioned above the simulation time is directly proportional to the total number of
different load reflection coefficients. It also depends directly on the number of symbols
simulated at each load. When initially exploring the Smith Chart to find an approximate
optimal load, it might be useful to run the simulation with a relatively small number of
symbols. Later, after determining a smaller, optimal region of the Smith Chart, you might
want to increase the number of symbols to get more accurate results.
If using a data file as the source, setting the simulation time step tstep equal to the time
step in the data file is good, to minimize effects that may arise due to interpolation.
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
The Envelope analysis includes a SweepOffset. This simulates but does not keep the first
12 symbols in the simulation during which the input signal amplitude is ramping on. If you
want to include this turn-on ramp data in the post-processing computations, just set
SweepOffset=0.
Because the simulation includes an optimization, click Optimize icon to launch the
simulation. If instead you just launch the simulation by hitting the F7 key or selecting
Simulate > Simulate, an optimization will not be run and the data display will not
display the simulation results.
After running the optimization, this WCDMA_LoadPullMagPh_ConstPdel data display shows
the results.
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
To see the contours effectively, you may need to change the CurrentStep, PAE_step,
Gain_step, GainCompStep, and ACLR_step variables. These set the step sizes between the
contours. The bias supply current calculations only include the current in the probe
Is_high. If you change the name of the current probe, you will need to edit the
BiasCurrent equation on the Equations page.
The upper Smith Chart shows contours of constant gain and gain compression. The lower
left Smith Chart shows contours of constant bias current, power-added efficiency (PAE),
and lower adjacent channel ACLR, as well as the simulated load reflection coefficients and
the corresponding input reflection coefficients. The lower right Smith Chart shows the
same data on a Smith Chart with a different reference impedance.
In the red boxes on the left side are data that correspond to a particular optimal condition
such as minimum bias current, maximum PAE, minimum gain compression, or minimum
ACLR. However, you have to make sure that the desired power delivered was actually
achieved. In some cases with load reflection coefficients very close to the edge of the
Smith Chart, the desired power will not be achieved.
The results show there is a trade-off between power-added efficiency and distortion. You
can get slightly better PAE if you are willing to tolerate higher ACLR levels.
Note that the power delivered specification is not satisfied. You could re-run the
simulation, allowing SFexp to vary over a larger range, or you could move marker m1 to a
load near this one and see if the power delivered specification is satisfied. Moving marker
m1 allows you to select any of the simulated load reflection coefficients. The
corresponding data appears in a separate box, which allows you to see potential tradeoffs
as you move around the Smith Chart.
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Advanced Design System 2011.01 - ADS Load Pull DesignGuide
The gain compression is still excessive, however.
The ACLR is somewhat sensitive to the load impedance.
The WCDMA Load Pull > Constant Power Delivered, Circular Region Load Pull
menu pick copies into your workspace a schematic and data display nearly identical to the
WCDMA_LoadPullMagPh_ConstPdel ones above, except that it sweeps a circular
region of the Smith Chart instead of a fan-shaped region.
Time Taken by Simulation
For 25 different load reflection coefficients, numSymbols=128, and the optimization type
set to Gradient with MaxIters=5, this simulation required about 4 minutes. For 49
different load reflection coefficients, about 6.75 minutes were required.
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