How to test Nujira’s envelope tracking PAs
Gerard Wimpenny, chief technology officer at Cambridge-based power technology company Nujira describes the special challenges of testing envelope tracking power amplifiers for mobile phone systems
As the signal complexity of mobile communications standards has increased, so RF designers have had to re-think conventional power amplifier (PA) architectures.
With peak-to-average power ratios approaching 10 dB, the high spectral efficiency of signals like OFDM and SC-FDMA comes at the price of low energy efficiency in the final stage PA.
A traditional DC-DC converter approach is too power hungry for today’s smartphones and other mobile devices. Envelope tracking (ET) is now gaining broad acceptance as the de facto standard model for modern PA power supplies.
ET improves the energy efficiency of RF PAs by replacing the fixed DC supply with a dynamic supply voltage, which closely tracks the amplitude, or “envelope” of the transmitted RF signal.
However, the challenge for designers is that envelope tracking brings in new complexities to PA characterisation and design, as the PA is now a three port device, with the supply pin acting as a high bandwidth control input. It requires more sophisticated test and measurement techniques compared to traditional fixed supply PAs to properly characterise and optimise ET PA performance and achieve the full efficiency gains enabled by ET.
Overcoming test limitations under dynamic power modulation conditions raises several difficulties. Accurately measuring the efficiency of an envelope tracking PA under dynamic supply modulation is one of the most challenging tasks faced by developers of RF PAs.
It requires generation of precisely time-aligned envelope and RF signals; a high bandwidth, high current, low impedance power supply modulator; high bandwidth sensing of instantaneous current and voltage supplied to the PA; and measurement of the instantaneous RF power at input and output of the PA.
All of this data must be captured at more than 100 million samples/second, and then correlated to calculate the instantaneous efficiency of the RF PA.
The key issue is that by varying the PA supply voltage in synchronism with the envelope of the RF signal to improve efficiency means that the PA’s fundamental output characteristics (power, efficiency, gain, phase) now depend on two ‘control’ inputs – RF input power and supply voltage. The power supply bandwidth required is typically 1.5-3x the RF channel bandwidth, ie up to 60MHz for a 20MHz channel.
In a typical ET system, supply voltage is adjusted dynamically so as to track the RF envelope at high instantaneous power. Here, the PA operates with high efficiency in compression and its output characteristics are mostly determined by the instantaneous supply voltage.
Conversely, when the instantaneous RF power is low, the supply voltage is held substantially constant and the PA output characteristics are primarily determined by the instantaneous input power (linear region). There is a transition region between these two extremes in which both supply voltage and input power influence the output characteristics (see Figure 1).
Performance metrics for traditional power amplifiers are well established, with accompanying test and measurement methodologies. However introducing envelope tracking to PA systems makes characterisation more of a challenge with these two different control inputs being tracked and measured.
To further complicate the measurement process, ET applications generally employ a digital look-up-table to map instantaneous signal amplitude to supply voltage – this nonlinear “shaping table” is applied to each sample of the signal amplitude before D/A conversion, and is specific to each type of PA.
The ‘standalone’ performance of ET PAs cannot be measured unless the shaping table is first defined. This requires measurement of the PA’s fundamental characteristics (Pout, Efficiency, Gain, Phase…) over the full range of supply voltage and input power.
In principle, this characterization could be carried out using a continuous wave (CW) network analyser together with a variable DC supply. Unfortunately the results are typically poor as a result of thermal effects, ranging errors and drift in phase measurements. This solution is also far too slow to allow load-pull techniques to be used – the most common method globally for RF PA characterization.
An alternative approach is to use a pulse characterization using ATE controlled standard test equipment. This avoids the need for a high bandwidth, low-impedance supply, and is sufficiently fast for load-pull to be viable, but has the drawback that it is difficult to make accurate phase measurements.
This leaves us with the only viable approach – to use real waveforms and vary the shaping table so as to allow all combinations of input power and supply voltage to be measured. Although this requires a supply modulator, it is very fast, allows accurate phase information to be gathered, and can also be used to characterize memory effects.
In order to deliver this test capability Nujira has developed its own technique. Called the ET surface explorer, it is used in conjunction with the firm’s development system waveform generator to capture data across the entire 2-dimensional ET RF power/supply voltage “plane” in a single database.
Capture conditions are carefully designed to mimic true ET operation and provide accurate modeling of linearity and efficiency over a wide dynamic range.
ET surface explorer accelerates PA characterisation, provides PA and product designers greater insight into the performance of ET PAs, and enables them to maximise the linearity, efficiency and output power benefits of operating PAs in ET mode.
By replacing thousands of complex, repetitive and time-consuming lab measurements with a single measurement pass, whi ch typically takes less than two minutes to capture and process, ET surface explorer provides a massive productivity boost to RF designers.
Crucially ET surface explorer lets designers visualise how PAs behave under live ET supply modulation conditions, unlocking the optimum performance and efficiency characteristics of a given PA.
Postprocessing and offline analysis tools create 3D surfaces of gain, phase and efficiency, and allow rapid interactive simulation of the effects of different shaping tables, RF waveforms and power levels, giving designers far greater insight into ET PA performance. The tool can also automatically generate, model and export a wide variety of shaping tables, including ISOGAIN or Maximum Efficiency.
While ET does introduce new layers of complexity for RF engineers, with these tools at their fingertips this challenge can be easily overcome. Moreover it allows PA designers to fully harness the advantages of envelope tracking PAs – whether that is enhanced efficiency, increased output power, improved operation into mismatched loads, and insensitivity to temperature variations.