To maximize power efficiency, RF PAs must have fast turn-on and turn-off switching times. We get the highest DC power efficiency when the PA turns on with minimal delays, but short delays can induce transient effects on the RF signal, degrading the modulation quality. Why? This transient response can affect the preamble at the start of the packet and cause an imperfect channel estimate.
Here in this demo, I’d like to show you how you can take dynamic EVM measurements of wideband power amplifiers using MIPI RFFE commands.
Here on the bench setup, we have a front-end module for a multiband cellular handset, which uses the MIPI RFFE protocol. For our instrumentation, we have a source measure unit supplying DC power to the PA under test, a digital instrument to send MIPI commands to the PA and turn it on and off dynamically, and a vector signal transceiver for RF measurements. The vector signal generator applies a 5G New Radio TDD signal to the PA under test, and the output of the PA goes to the vector signal analyzer.
Using the RFIC Test software application, we configure the instruments, the measurement parameters, the waveform, and the type of DUT triggering control for dynamic EVM measurements. We select here dynamic EVM, we select the PXIe-6570 digital instrument as the source generating the MIPI commands. The application lets us select the timing directly from the characteristics of the RF burst, that is, from the waveform itself, or we can define the ON/OFF times to understand how the EVM changes based on how fast we enable and disable the PA. So, let’s configure this duration to be 100 ns.
This PA uses MIPI RFFE commands to turn on and off. We already created specific scripts to do that using the Semiconductor Device Control module in InstrumentStudio. Here from the RFIC application, we can invoke and run those scripts to execute the dynamic EVM measurements that we need. So, we point here to the configuration file that has those scripts, which we saw earlier in this video, and we select the scripts to turn the DUT on, off, and to enable dynamic EVM. We also select our trigger source, and we’re ready to run.
Let’s turn on the PA… Now it’s biased. And we click the run button, and we have completed our first dynamic EVM measurement of this amplifier with a 500 µs long 5G NR TDD waveform.
Let’s look at the oscilloscope trace. Here we observe the timing and triggering of the MIPI RFFE commands and the start of the RF burst.
The digital predistortion capabilities of the RFIC Test Software also work with these bursty signals. Let’s turn on digital predistortion and observe how well we can linearize this PA and measure the dynamic EVM at the same time.
In summary, with just a few settings, the RFIC Test Software and this modular PXI PA validation bench can synchronize digital control and RF measurements to produce dynamic EVM results, without writing a single line of code.