LoadPullConstPdel

ADS load pull simulations for constant output power

Harmonic Balance 1-Tone simulation (HB1Tone_LoadPull_ConstPdel):

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. This allows you to pick the optimal load that produces the best PAE, gain, gain compression, or bias current, 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.

When using this schematic, there are a number of different things you need to specify. 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 Pdc equation on the corresponding data display so the DC power consumption is computed correctly.

You have to specify the center and radius of the circle of the reflection coefficients.

These are set by the s11_center and s11_rho variables (s11_rho is automatically reduced in order to keep the reflection coefficient <1.) 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 > S-Parameter 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 also have to specify the number of reflection coefficient points to be simulated, pts, and the reference impedance, Z0.

You have to specify the nominal and allowed range of the available source power, Pavs.

Prior to starting on-screen editing

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. The nominal value of Pavs does not matter that much, since it will only be used as the initial value for the first optimization. Depending on how high a power you want delivered to the load and the gain of the device, you may have to adjust the maximum allowed value of Pavs.

On the SmallSignal Parameter Sweep, you should make sure the value is small enough that the device behaves linearly.

The power delivered to the load with this “small signal” available source power is kept as the reference for the gain compression computation. You specify the desired power to be delivered to the load in OptimGoal1.

In this case, we want the power delivered to be between 25 and 25.1 dBm. With this value and a maximum Pavs value of 15 dBm, we are effectively specifying that the lowest transducer power gain we will accept is 10 dB.

You may also specify different load and source impedances at the harmonic frequencies and (for the source) at the fundamental frequency.

Because the simulation includes an optimization, you launch the simulation by clicking on the optimization icon (if using ADS 2009 Update 1 or later.) 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.

Optimization Icon

After running the optimization, this 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. 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 bias current and power-added efficiency (PAE), 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, or minimum gain compression. However, you have to make sure that the desired power delivered was actually achieved. For example, at the load or reflection coefficient that gave the minimum bias current, the power delivered was < 23 dBm. This minimum bias

current load is very close to 0 Ohms, and it is very difficult to deliver any power to it. The gain (transducer power gain) and gain compression with this load are unreasonable, also.

The load that corresponds to the maximum PAE is very close to the one that corresponds to the minimum bias current, but the power delivered meets the 25 dBm requirement and the gain and gain compression values are much more reasonable.

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.

This enables you to see potential trade-offs. For example, for this load impedance, the PAE is worse, but the amount of gain compression is much less. Also the input reflection coefficient now has a positive real part, which should aid stability.

Harmonic Balance 2-Tone simulation (HB2Tone_LoadPull_ConstPdel):

This simulation setup and data display is nearly identical to the one above, except that now two tones are supplied instead of one. A 2-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 above and also includes intermodulation distortion. (Note that a different device is used.)

There are two new variables you have to specify, the frequency spacing between the two tones fspacing, and the 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.

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.

How long does this 2 tone simulation take? With 64 different load reflection coefficients, the simulation required less than 12 seconds.

Tolerating a lower PAE allows much lower gain compression and intermodulation distortion.