11 Feb 2015

Production Calibration of Envelope Tracking Amplifiers

Jeremy Hendy, VP Marketing, Nujira looks at how to overcome the challenges of setting up and calibrating envelope tracking amplifiers in mass production

Despite the benefits of improved efficiency, better RF performance and lower temperatures of Envelope Tracking, ET for handsets, production line calibration of envelope tracking enabled RF front ends represents a major headache for OEMs. With some ET implementations, calibration time of the transmitter has ballooned to well over one minute.

For OEMs this is a significant barrier to envelope tracking adoption. Calibration of the transmit path needs to take as little time as possible - ideally just a few seconds per device - particularly on high volume production lines. This gap between desired and actual calibration time is unacceptable.

One problem is that envelope tracking, by its very nature, introduces additional complexity into the RF signal path in the form of new analogue paths that need to be calibrated at production. By fundamentally changing the behaviour of the RF front end, ET enables significant performance gains, but the new signal paths can represent a significant increase in the calibration complexity.

Overcoming these issues requires specialist tools and innovative new approaches to the calibration challenge. This article will detail the issues facing chipset vendors, OEMs and test equipment manufacturers, and explain the techniques that are being developed to speed the production calibration process and minimise design validation time with envelope tracking.

Current TX calibration methods

In the majority of cases production calibration remains a fairly basic exercise - using a simple measurement of transmit power from the antenna output.

Without envelope tracking, this simple calibration set up is all that is generally required. The main issue that needs to be resolved is calculating the gain corrections between the digital baseband and the antenna connector, to ensure correct operation of the transmit power control loop. The D/A converters, transmit filters, variable gain amplifiers, PAs, filters and switches all introduce variable gains, which must be calibrated over the wide TX power control range used by the handset in operation. Once these analogue variations are calibrated and stored in non-volatile memory, simple digital corrections can be performed in the handset to compensate.

On the production line, measuring and correcting these offsets needs to be as fast as possible – particularly since many of the measurements must be repeated for each LTE frequency band, and even at different frequencies across the band. Manufacturers don’t want calibration techniques that require complex interactive loops involving lots of external test equipment - it simply takes too long for repeated communications between the external test equipment and the device under test. Traditional TX calibration has involved one or two spot measurements of absolute output power from an external power meter, with the rest of the calibration performed on-chip via a calibrated measurement receiver in the modem chipset, minimising the time taken to communicate with external equipment.

An on-chip measurement receiver can capture the TX output signal from an RF coupler, which may be implemented either in the PA, or from the combined outputs to the antennas. By enabling most of the data captures and calculations to be performed without external equipment, the measurement receiver is a valuable tool to speed production calibration. For devices without dedicated measurement receivers, it may be possible to re-use the RX path circuits to capture the TX output during test.

However, the capabilities of these on-chip measurement receivers varies wildly amongst chipset vendors. While some vendors provide measurement receivers capable of high bandwidth, high dynamic range complex IQ measurements, others provide slow power control measurements with limited precision, and some vendors simply don’t have any form of measurement receiver on-chip.

For OEMs this causes problems, as it makes it very difficult to establish standardised calibration processes for all of their production lines.

Calibration challenge of envelope tracking

Regardless of these issues, calibration without ET is still a relatively straightforward task, even if it must be repeated across multiple frequency bands. However, envelope tracking introduces some new TX calibration challenges for the RF front end.

The fundamental principle remains the same - calculating and correcting gain errors and offsets within the system. However, with ET there are now three independent components that can have analogue gain and/or offset errors - the RF chain up to the PA input, the PA itself, and the new analogue paths in the envelope tracking chain – namely the ET path DAC, filter, and ET IC (see figure 1). Proper calibration of the TX path requires you to correct for all three.

Signal Paths in Envelope Tracking System

Figure 1: Signal paths in an envelope tracking system

The additional complexity of ET, whereby the PA is operated as a 3-port component (input, supply and output), also adds to the calibration challenge. It means that when you turn the system on for calibration it is difficult to establish which element of the system is responsible for the gain or offset errors – it could be either the ET path or the RF path causing the error.

Finally, timing alignment also becomes more of a critical issue to allow for part-to-part variations in the RF and envelope tracking signal paths. Analogue delays can be introduced in DACs, in the ET IC, and in particular in the filters in both the RF and ET paths. Calibrating the relative delay between the ET and RF paths is also now a task that must be conducted at production.

Trying to perform these calibration tasks on the test bench in the lab is one thing, but on the production line these additional calibration requirements can turn into a major challenge. A simple power measurement from the antenna output cannot tell you where in the more complicated ET system a problem is occurring. If you cannot determine the sources of errors, then you cannot correct for them.

On-chip solution

It is possible to use the on-chip measurement receiver more intelligently, in order to do the more complex calibration required by envelope tracking. However, to do this requires a relatively high performance measurement receiver capable of capturing high bandwidth, high precision TX samples, and performing iterative measurements.

If you have these capabilities in your chipset, then there are certainly some significant benefits to this approach. Not only does it minimise the requirements for external test equipment, it also ensures that the chipset vendors can keep much closer control over RF performance.

This approach also opens up the possibility for additional calibration to be performed in the field at a later date, or even continuously during operation. Where a high performance measurement receiver is available, Nujira has developed a comprehensive suite of calibration algorithms that take only a few milliseconds to execute, allowing them to be used as part of a self-calibration routine on the production line, or when the handset is in operation.

The alternative solution

However, not every chipset vendor has a high performance on-chip measurement receiver available. Although many of Nujira’s algorithms for production calibration can also be used with lower performance receivers – with a corresponding increase in execution time – a solution still needs to be found for systems with no measurement receiver at all.

In the absence of a measurement receiver, the only alternative solution is to perform the calibration using external production test equipment. Of course, if you are going to go down the external test equipment route you need to be smart about the test procedure. Time is very much of the essence when it comes to calibration. With communication overhead dominating the measurement procedure, it’s important to avoid iterative loops with external test equipment. It needs to be as efficient a process as possible.

With external equipment, each measurement has a significant “cost” in terms of the time taken, so it is important to use the minimum amount of data in order to reach an accurate conclusion - not only in terms of the speed of operation, but also in terms of minimising the communication bandwidth between external test equipment and the device under test.

Nujira, using its own calibration algorithms, has shown how it is possible to perform highly efficient production line calibration using standard RF test equipment, typically the existing RF signal analyser. By running Nujira’s software on the signal analyser, the algorithms can “untangle” the separate sources of envelope tracking errors from a single I/Q data capture, providing rapid feedback of the correction parameters to the low-level firmware in the device under test. Nujira’s production test software can analyse and correct for all of the common sources of ET error, including ET/RF path timing alignment, ET path gain and offset errors, and RF/ET path gain mismatch (see figure 2).

Output of envelope tracking test

Figure 2: Output from Envelope Tracking test using Nujira’s software

This enables chipset vendors, OEMs and test equipment manufacturers to perform a unique one-pass process using external test equipment. This approach extracts and separates out all individual errors from a single RF capture, eliminating the need for time-consuming iterative measurements and minimising the communication overhead.

This solution significantly speeds the production calibration process, and minimises design validation time. Compared to current envelope tracking calibrations which can take a minute or more, this approach brings total calibration time for an ET phone back down to a few seconds.

ET - re-evaluating critical processes

Envelope tracking offers significant performance benefits for mobile handsets, but production calibration is another example of how the adoption of ET in the RF front end is impacting other aspects of phone design. The additional system complexity of ET is creating opportunities for many other smaller innovations, solving new issues and ensuring that disruption to existing production techniques and supply chains is minimised.

It is possible to compare envelope tracking to the introduction of fuel injection in the automotive industry – and not just in terms of the efficiency gains that come through precise control of the “fuel” delivery. The additional system complexity of fuel injection initially lengthened the development cycle, but the technology quickly became universally adopted, because the ultimate benefits were too significant to ignore. So it is with ET, where the benefits in energy efficiency, RF performance, and thermal dissipation are too hard to ignore.

Introducing ET with minimum disruption requires some intelligence to be applied to processes that have perhaps been taken for granted by the industry for a long time. Production calibration is among the most important of these processes that need to be re-evaluated.

As this article has shown, it does not necessarily require a huge leap in thinking to resolve these issues, but key to delivering the solutions required by OEMs and chipset vendors is taking a broad system-level approach to the problem. With more than 20 ET-enabled phones now on the market, accounting for around 125 million unit shipments of ET chips in 2014, vendors are slowly but surely figuring out the problems and delivering those solutions. Our approach to production calibration is just another piece of the puzzle, and part of our philosophy for making ET Easy.

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About the author

Jeremy Hendy is VP of Sales and Marketing at Nujira and he brings the company considerable experience of semiconductor sales and marketing across multiple technologies for wireless communication and digital video. Previous positions include Marketing Director of wireless USB start-up Artimi, VP Marketing for Aspex Semiconductor, and Strategic Technology Director of Cadence’s Wireless and Multimedia business unit. He started his career with Texas Instruments, and holds a first class honours degree in Electronic Engineering from the University of Liverpool.

Nujira Ltd is the world leader in Envelope Tracking technology and solutions for powering energy efficient 4G cellular terminals, base stations and digital broadcast transmitters. Since the Company’s formation in 2002, Nujira has developed the most extensive and complete patent portfolio around Envelope Tracking, with over 200 patents filed or granted worldwide. Nujira’s corporate headquarters is located in Cambridge, UK. The Company currently has design centres in Cambridge and Bath, England and in Edinburgh, Scotland, and sales locations in North America, Germany, Japan and China.

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