Load-Pull and Signal Management in Analog Devices

March 2, 2020 Cadence PCB Solutions

Load-pull and signal management for RF amplifiers in telecom

The electronics in this dish make copious use of RF power amplifiers

 

The telecom world is set to explode with new classes of RF amplifiers to support upcoming and future 5G rollouts. The same goes for IoT devices, many of which will likely use a plethora of wireless protocols to connect with the broader world. All of these products and many more will need power amplifier circuits. In fact, some financial analysts expect the power amplifier IC market to double within the next several years. One would expect more digital designers to start leading a double life as analog designers just to keep up with design demands.

As part of this double life, signal chain design methods will be critical for ensuring signal integrity in devices with power amplifiers. These power amplifiers require load-pull and signal management techniques to ensure power transfer while preventing distortion. The same goes for power amplifiers in other applications, the only difference is the frequency and signal level at which they operate. Here’s how load-pull analysis fits into the overall theme of signal management and how designers should think about working with load-pull in signal management.

Power Amplifiers and Signal Chain Design

With design for signal integrity in power amplifiers being so important, designers need to consider the nonlinear characteristics of amplifiers that are run near saturation. All amplifiers are slightly nonlinear, which leads to minor harmonic generation at low input signal levels. As the input signal level increases, so do any distortions created in the output signal. With analog signals, any generated harmonics are simple to handle as they are integer multiples of the input frequency.

As power amplifiers operate in successive stages, any noise or spurious harmonic content passing through an amplifier stage passes to the output. To recover the original signal, simply pass the output through a low-pass filter and you’ve recovered the signal. At the system level, signal management would be performed with a simple block diagram such as that shown below:

 

Block diagram for signal chain design with load-pull and signal management

Simplified block diagram for a signal chain with analog signals.

 

In addition to ensuring your signals are clean from noise and spurious harmonics, it’s important to ensure maximum power transfer along your signal chain. At a single harmonic, this simply means impedance matching at the desired harmonic while filtering unwanted harmonics, as mentioned above. With frequency modulated signals, it can be quite difficult to ensure complete impedance matching when the signal bandwidth is wide. This is quite important in RF devices, such as chirped FMCW radar or frequency-modulated signals sent to an antenna.

Load-Pull and Signal Management

Due to the nonlinear behavior of the output signal level as a function of the input signal level, the output impedance from a power amplifier is effectively a function of the input signal level. As a result, the optimal impedance matching may not occur when the amplifier output and load input impedances are complex conjugates of each other. By “optimal impedance matching,” we mean the impedance at which maximum power transfer occurs. Determining this analytically may not be possible, especially when working with broadband signals.

Instead, an iterative technique called load-pull analysis can be used to determine the optimal impedance matching. As the load has some fixed impedance, you need to determine the equivalent impedance of the load plus the matching network such that maximum power is transferred from the amplifier to the load. The general process for load-pull is as follows:

  1. Select an impedance (magnitude) that will be seen by the DUT. This is the equivalent impedance of your matching network plus the load.

  2. Vary the reactance and resistance while holding the magnitude constant. At each value, measure the power transferred to the load.

  3. Plot each set of reactance/resistance pairs on a Smith Chart. The contour on the chart with the largest power transfer is the optimally matched impedance.

  4. Design the impedance matching network such that the equivalent impedance of the network plus load matches the value you determined in Step 3.

 

Block diagram for load-pull and signal management

Schematic showing load-pull analysis in signal management.

 

With broadband analog signals, impedance matching can get difficult as the impedance needs to be optimally matched throughout the signal’s bandwidth. If you’ve done your homework, then your amplifier should have a flat gain spectrum throughout the signal bandwidth. The same could be said for the impedance spectrum for the load component. However, the matching network may not have a flat impedance spectrum, which causes the imperfect matching at some portion of the signal bandwidth. Similarly, the phase presented by the matching network may not have a flat spectrum, which distorts frequency modulated signals.

As part of impedance matching network design, you can tailor the impedance spectrum using parameter sweeps in a circuit simulation. This allows you to graphically compare the equivalent impedance of the matched load throughout the relevant bandwidth.

Digital and analog systems both benefit from a set of powerful load-pull and signal management features, and you can implement the design you need with the right PCB layout and design software. Allegro PCB Designer and Cadence’s full suite of design tools are ideal for creating a layout for a new device, simulating the device’s behavior, and preparing for manufacturing. No other product provides these capabilities in a single platform.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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