As wireless mobile devices grow in capability and complexity, the associated growth in power demand is driving new approaches to battery utilization and power efficiency. One of the single largest power consumers in a wireless handset is the RF Power Amplifier (PA) and as such, improved efficiency techniques like Envelope Tracking (ET) and Digital Pre-Distortion (DPD) are being increasingly utilized. The key implication for test engineers — whether in design, characterization, or manufacturing test — is that testing these devices with this additional capability can potentially drive up both test cost and overall test time. This article discusses various approaches to maximizing test equipment utilization and reducing test times for such component RF PAs and front-end modules.

The Problem

The demand for higher test speed spans from design validation to production test. As RF PAs support multiple modes, frequency ranges, and modulation formats, there is more to test during the validation phase. Thousands of tests are not uncommon. During RF PA production test, manufacturers have to deal with a number of critical issues; namely, speed, repeatability, cost, maintainability, and upgradability. Their biggest stress, however, comes from trying to balance speed and repeatability.

Typically, as test speed increases, repeatability decreases. Manufacturers must constantly struggle to balance these issues, while also keeping an eye on cost and maintainability. Addressing the speed challenge is further complicated by the fact that PAs are being manufactured in increasingly higher volumes to meet the demand for more and more wireless mobile devices, and have grown even more complex. Techniques like DPD and envelope tracking are often employed to help linearize the PA and increase its power efficiency, but these techniques only add to the testing that’s necessary during production, further slowing down the process. With PA manufacturers looking to reduce overall test times from 1.5 seconds to 500 ms or less, these slow-downs are simply no longer acceptable.

The Solution

Figure 1. System-level block diagram for a multi-DUT test. The RF PA power servo loop is a key requirement in PA testing and must be performed at each test condition.
The key to addressing the challenges now facing PA validation and manufacturing teams lies in finding a way to increase test speed while maintaining repeatability. Luckily, a number of test system techniques are now available to manufacturers for just such a task.

The first technique involves speeding up the PA power servo loop (Figure 1). A power servo loop is essentially a “test and adjust” process. The engineer sets the RF input power level to the Device-Under-Test (DUT), then checks the RF output of the DUT. If the RF output level is not within the required specification, the engineer changes the RF input level and checks again. This loop is continued until the correct output power level is achieved. Then, and only then, can the engineer start making measurements on the DUT. Getting this process done fast and allowing the engineer to quickly move on to making measurements is a key way to speed the overall RF PA test time.

Figure 2. Using the power servo loop approach in the PXI VSG, amplitude changes can be achieved in less than 200 μs.
Since power servos are a non-deterministic process, list mode cannot be used to determine the power level difference from step one to step two. Instead, it must be determined in real time. And, because PAs are typically not operated in the linear region of the amplifier, a 3-dB change in input power, for example, will not equate to a 3-dB change in output power. This is where baseband tuning methods like that available with a PXI vector signal generator (VSG) come in, offering a way to speed up the tuning process and, therefore, the test process itself.

The recommended PXI VSG approach for the power servo loop is to set the RF power level to the maximum level required from the source, then use the baseband power level to adjust the power level to the required input level. This is an iterative process that is performed until the output power reaches the required level for testing. The method is fast and accurate, enabling power servos to converge very quickly. In fact, with this baseband tuning technology, amplitude changes of up to 20 dB can be achieved in less than 200 μs (Figure 2). It can also be used for frequency offsets within the bandwidth of the generator, making it especially useful for measuring multiple channels within a band.