Originally posted Feb 18, 2015
You can “turn it up to eleven” as long as you don’t leave it there
When I first heard the term “envelope tracking” I thought of the classic investigative/surveillance technique called “mail cover” in which law enforcement gets the postal service to compile information from the outside of envelopes. The practice was in the news a while back due to its use with digital communications.
Learning a little more, I quickly realized that it has nothing to do with the mail but, like the mail, has precedent that reaches back many years. “Riding the gain” or “gain riding” is a manual process that has been used for decades in audio recording and other applications where excessive dynamic range is a problem. Its use predates vinyl records, though I first encountered it in my previous life as a radio announcer, broadcasting live events.
When I was riding the gain, it was a manual process of twisting a knob, trying to reduce input dynamic range to something a small-town AM transmitter could handle. I was part of a crude feedback system, prone to delay and overshoot, as I’m sure my listeners would attest.
These days, envelope tracking is another example of how digital processing is used to solve analog problems. In this case it’s the conflict between amplifier efficiency and the wide variations in the RF envelope of digital modulation. If the power supply of an RF amplifier can be dynamically adjusted according to the power needed by modulation, it can—at every instant—be operating at its most efficient point.
In envelope tracking an RF power amplifier is constantly adjusted to track the envelope of the modulated input signal. The amplifier operates at higher efficiency and lower temperature, using less battery power and potentially creating less adjacent-channel interference.
Power efficiency has always been a major driver in mobile communications and its importance continues to grow. Batteries are limited by the size and weight of the handsets users are willing to carry and, yet again, Moore’s Law points the way to improvement. Available DSP now has the high speed and low power consumption to calculate RF envelope power on the fly. The envelope value is fed to a power supply with sufficient bandwidth or response time to adjust its drive of the RF power amplifier accordingly.
An envelope tracking power amplifier (ETPA) is dynamically controlled for optimum efficiency by tracking the required RF envelope power. The tracking is based on envelope calculations from the I/Q signal, modified by a shaping table.
This all seems fairly straightforward but, of course, is anything but. The calculation and response times are very short, and a high degree of time alignment is required. Power supplies must be extremely responsive and still very efficient. All of the DSP must itself be power efficient, to avoid compromising the fundamental power benefit.
Envelope tracking is a downstream solution to power amplifier efficiency, joining previous upstream techniques such as crest factor reduction and macro-scale approaches such as digital predistortion. To a great extent, all rely on sophisticated algorithms implemented in fast DSP.
That’s where Keysight’s design and test tools come in. You can find a collection of application notes and other information at www.keysight.com/find/ET.
With envelope tracking you can now turn your amplifiers up to eleven when you need to, and still have a battery that lasts all day.