07 May 2014

Envelope Tracking Amplifier Testing

Matthias Weilhammer and Johan Nilsson of Rohde & Schwarz look at efficient ways of testing envelope tracking RF amplifiers.

A major portion of the amplifier power in smartphones dissipates as heat. Envelope tracking, ET technology significantly improves amplifier efficiency as it allows the amplifier supply voltage to track the envelope of the RF signal.

The R&S SMW200A vector signal generator combined with the R&S FSW signal and spectrum analyzer now make comprehensive amplifier testing possible.

The video below gives and overview of the issues and solutions:


Improved amplifier efficiency with envelope tracking technology

Low battery warnings are all too familiar to smartphone users. In an effort to make these warnings less pervasive, manufacturers have long been working on a way to minimize power consumption. Beyond the processor and the display, a significant percentage of the energy is typically consumed by the amplifiers in the transmit path, which is why research is intense in this area. Amplifier efficiency is significantly improved with the envelope tracking technology.

Amplifiers, typically, are most efficient when they are operating in a range close to their maximum output power. ET takes advantage of that fact. Instead of receiving a constant supply voltage, the amplifier is fed a voltage from a DC modulator. This voltage is then controlled in such a way that it tracks the envelope of the RF signal. As a result, the amplifier operates at high efficiency most of the time, i. e. close to or in saturation.

Comparison of a conventional amplifier against an amplifier with envelope tracking technology.

Fig. 1: Comparison of a conventional amplifier against an amplifier with envelope tracking technology.

Additional requirements for amplifier testing including envelope tracking

In the case of envelope tracking, the amplifier can no longer be considered in isolation. Instead, its characteristics must always be studied in conjunction with the DC modulator.

Initially, a highly precise synchronization of the modulated supply voltage with the RF input signal (PIN) must be performed on the amplifier. In the case of an LTE signal (with a bandwidth of 20 MHz ), deviations in the nanosecond range would cause a significant increase in the modulation error vector magnitude, EVM.

Instead of a strictly linear depiction of the input power on the supply voltage, the envelope signal can be optimized with respect to efficiency or linearity by using shaping functions. The linearity optimization causes a constant amplification over the amplifier's entire power range. Optimization for efficiency changes the gain over the power range (PIN) because the amplifier is always operated in compression, and thus in the nonlinear range. The resulting distortions at the amplifier output are often compensated for by using pre-distortion.

When characterizing an amplifier, it is important also to analyze the power added efficiency (PAE) in addition to standard tests such as output power, adjacent channel leakage ratio (ACLR) or EVM. The PAE describes how efficiently an amplifier converts the received DC power into RF power using the following formula:

Envelope tracking formula

This test displays the efficiency versus time. It requires synchronized recording of the input and output power and also acquisition of the current and voltage trace for the DC modulator.

All-in-one solution

Rohde & Schwarz is now offering a customized all-in-one solution to meet the increasing requirements of chipset manufacturers to test power amplifiers with envelope tracking. This solution covers signal generation and analysis and is based on the high-end R&S SMW200A vector signal generator and the R&S FSW signal and spectrum analyzer.

Complete test setup for envelope tracking with the R&S SMW200A vector signal generator and the R&S FSW signal and spectrum analyzer

Fig. 2: Complete test setup for envelope tracking with the R&S SMW200A vector signal generator and the R&S FSW signal and spectrum analyzer.

RF and envelope signal from a single instrument

Equipped with the new R&S SMW-K540 envelope tracking option, the vector signal generator easily generates the RF signal and the corresponding envelope signal. Since the vector signal generator calculates the envelope signal from the baseband signals in real-time, any communications standard, including LTE or WLAN, as well as custom RF waveforms can be used on the generator.

As both signals come from a single instrument, users do not need to worry about synchronization. They can delay the RF signal and the envelope signal relative to each other in real-time by ±500 ns with 1 ps resolution. This ensures perfect synchronization between the modulated supply voltage and the RF signal. Due to the outstanding generator performance characteristics, the R&S SMW200A delivers an extremely low-noise envelope signal at the analog I/Q output.

A selection of flexible shaping functions allows users to optimize the envelope in real-time. In addition to a look-up table (LUT), the R&S SMW200A also provides polynomial functions or de-troughing. This gives users complete flexibility for optimizing the DC modulator and amplifier.

Shaping functions in the envelope tracking option

Fig. 3: Shaping functions in the envelope tracking option.

All amplifiers are tested over various power input levels. The auto envelope voltage adaptation mode speeds up the setup and testing time. The user can enter key parameters such as input power (Pin), supply voltage (Vcc) or DC modulator gain in a clear user interface. Automatic recalculation of the envelope in realtime based on these values makes it possible to perform testing over the amplifier's entire power range. This unique functionality significantly reduces testing time and eliminates time-consuming manual recalculation of the envelope at various power input levels.

User interface of the envelope tracking option for detailed entry of key parameters

Fig. 4: User interface of the envelope tracking option for detailed entry of key parameters.

Digital pre-distortion, DPD in real-time

Since amplifiers, when using the envelope tracking technology, usually operate in the nonlinear range, distortion is experienced at the output. To counteract such deterioration in the RF performance, pre-distortion is often employed.

The new R&S SMW-K541 digital pre-distortion option permits loading of an AM/AM and AM/PM table with the requisite correction values. These delta values are calculated in real-time using the baseband signal. In conjunction with envelope tracking, the configuration can be set so that the pre-distortion is applied only to the RF signal, or that the envelope signal is calculated based on the pre-distorted signal.

User interface of the digital pre-distortion option

Fig. 5: User interface of the digital pre-distortion option.

This option also makes it possible to import pre-distortion tables. The distortion can alternatively be determined using the FSW amplifier characterization software application. The associated coefficients for pre-distortion are then transmitted automatically to the vector signal generator.

Synchronous analysis of envelope signal and RF signal

Equipped with the R&S FSW-B71 hardware option, the R&S FSW signal and spectrum analyzer can offer envelope tracking tests. The hardware option adds analog I/Q baseband inputs of 40 MHz bandwidth each (an additional option is additionally required for a bandwidth of 80 MHz). The modulator output signal is accessed at one of the inputs. At the same time, the signal and spectrum analyzer receives the amplifier output signal via its RF input. The two signals can then be synchronized, displayed and analyzed. An oscilloscope is no longer required.

The current power added efficiency (PAE) can be calculated when adding a small resistor in the circuit between the DC modulator and the power amplifier.

Power amplifiers are typically characterized by their ability to perform EVM and ACLR measurements. The high-performance R&S FSW signal and spectrum analyzer is ideal for performing such tasks: a residual EVM for a 10 MHz LTE signal is only –48 dB and the WCDMA ACLR dynamic range is 88 dB. The influence of the test instrument on the result is thus negligible.

Measuring the noise in the output signal is often tricky for power amplifiers operated with envelope tracking and requires a spectrum analyzer with an excellent high dynamic range. The R&S FSW and its extremely low displayed average noise level (DANL) of typ. –159 dBm (1 Hz) at 2 GHz without using a preamplifier (when using a preamplifier it is up to 13 dB better) is more suitable than any other instrument for this task.

Thanks to efficient signal processing in the R&S FSW, standard-compliant measurements are exceptionally fast. For example, a 20 MHz LTE uplink signal is characterized in fewer than 50 ms.

All tests performed on one user interface

Testing amplifiers including envelope tracking is especially easy if the setup is controlled by the FSW amplifier characterization software for Windows. It controls both the R&S SMW200A and the R&S FSW, including their ET-related functions, automates the calculation of the AM/AM and AM/PM conversion for the DUT and then transmits relevant data to the generator.

The information is then used for a real-time digital pre-distortion. Since the software always recognizes the output signal of the generator, it can perform EVM calculations in parallel to the AM/AM or AM/PM conversion and other analysis tasks significantly faster than with an unknown signal (Fig. 6).

easurement of the AM/AM and AM/PM conversion with envelope tracking

Fig. 6: Measurement of the AM/AM and AM/PM conversion as well as EVM, peak and average power of an LTE UL signal using the FSW amplifier characterization software application.

Summary

The simple test setup consisting of only two instruments allows a quick startup without additional calibration effort. The testing time is greatly reduced by generating the envelope in realtime and by offering a customized analysis software. Equipped with the envelope tracking and digital predistortion options, the R&S SMW200A vector signal generator combined with the R&S FSW signal and spectrum analyzer featuring synchronous RF and baseband inputs, is unique on the market: this solution allows efficient amplifier testing including envelope tracking.

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About the author

Johan Nilsson, (pictured) is a Product Manager, Test & Measurement Division, Rohde & Schwarz, Munich. He earned a Degree of Master of Science in Engineering from Chalmers University of Technology in Gothenburg, Sweden, in 1994. Between 1994 and 1995 he worked as a design engineer for Seatronics AB developing under water sensors. Between 1995 and 2001 he worked for Ericsson with RF design. Since 2001 he is working as a product manager for spectrum analyzers for Rohde & Schwarz in Munich.

Matthias Weilhammer is a Product Manager, Test & Measurement Division, Rohde & Schwarz, Munich. He joined Rohde & Schwarz in Munich as product manager in 2012. In this role he is responsible for the signal generator product portfolio. Previously, he spent 5 years working for Agilent as an application engineer with focus on mobile communication. Before that he worked 4 years in R&D at Kathrein where he was involved in hardware development for projects such as remote radio heads. Matthias Weilhammer graduated from University of Applied Sciences in Munich and holds a diploma degree in electrical engineering and a Master degree in Business Administration and Engineering.

Rohde & Schwarz is an independent group of companies specializing in electronics. It is a leading supplier of solutions in the fields of test and measurement, broadcasting, radio monitoring and radio-location, as well as secure communications. Established almost 80 years ago, Rohde & Schwarz has a global presence and a dedicated service network in over 70 countries.

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