Transmitter power efficiency
Nov 1, 2009 12:00 PM, By Jeremy Hendy
Reduce your costs with envelope-tracking technology.
Figure 1. Power amplifier inefficiency can greatly
Select image to enlarge.
The worldwide introduction of terrestrial DTV and radio systems will lead to a significant number of new transmitters being installed in the coming years. With the recent rise in energy prices, and increasing concerns over environmental impact, it is important that these new transmitters are as energy-efficient as possible.
This article discusses the issues relating to the power efficiency of DTV transmitters and describes how the implementation of envelope-tracking technology can reduce the operational costs and environmental impact of new networks.
The impact of increasing energy costs
Transmission networks account for about 75 percent of broadcasters' energy costs. An average large transmitter transmits about 8kW per multiplex, but is only about 20-percent efficient, consuming approximately 40,000W of electricity, 24 hours a day, 365 days a year. This averages to about 12MW across the whole network for a typical EU country and an electricity bill of something like €8.2 million per year at today's prices.
Most (perhaps 55 precent to 60 percent) of the energy is used by the power amplifiers (also known as the pallet) in the RF transmitter itself. Poor power amplifier efficiency has a direct impact on the transmitter's cooling requirements. With a power amplifier efficiency of 20 percent, 80 percent of the electrical power supplied is wasted as heat, which necessitates careful cooling design to reduce equipment temperatures and ensure reliability.
Figure 2. Envelope-tracking technology delivers efficiency improvements over a wide-frequency spectrum.
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Although lower-power transmitters can use air cooling (either with or without air-conditioning), higher-power (or less efficient) transmitters require water cooling, incurring further cost and complication. Furthermore, as power density increases, a higher proportion of power is required for cooling.
Cooling is a major concern for broadcasters for a number of reasons. First, all forms of forced cooling involve moving parts in one form or another (fans, pumps, etc.) and other items, such as filters, that require regular maintenance. Second, mechanical noise can be a concern in many urban transmitter locations; some countries are already starting to pass legislation to control noise levels at such sites. Finally, the cooling plant increases the equipment required on-site and, hence, site rental costs.
Another impact of power amplifier inefficiency is on the on-site backup power supply; higher transmitter power consumption necessitates either larger battery packs or larger diesel backup facilities, both of which again push up equipment and site costs.
It can be seen, therefore, that power amplifier inefficiency has a significant impact not just on the design of the transmitter itself, but also on the cooling and backup facilities required on-site, impacting both initial costs (CAPEX) and also long-term operational costs (OPEX). The overall impact of power amplifier inefficiency on the total OPEX is shown in Figure 1 on page 8.
RF power amplifier design
Each DTV transmitter typically uses one exciter or modulator to drive a number of identical RF power amplifiers — perhaps 20 to 100 for a typical medium- to high-power transmitter in a corporate structure architecture with the outputs of all the amplifiers combined to produce the total output power. DTV systems use orthogonal frequency-division multiplexing (OFDM), broad channel bandwidths and complex modulation schemes to achieve high spectral efficiency. Unfortunately, the modulation accuracy, noise and spurious requirements effectively mandate the use of linear (Class AB) RF power amplifiers in the transmitter.
Figure 3. Envelope tracking reduces the voltage difference between the supply voltage and the signal envelope, dramatically reducing the energy dissipated as heat.
Select image to enlarge.
Linear power amplifiers can be very efficient when operating close to their peak power, and with careful design, this can be achieved over a relatively broad bandwidth. However, they are inefficient when handling real-world signals with varying amplitude components, such as OFDM signals.
OFDM signals are composed of a large number of individual components, the power of each varying with time. The resultant amplitude of the composite signal over time is, therefore, not constant but “peaky” in nature and can be characterized by its peak-to-average power ratio (PAPR) or crest factor. PAPR values for broadcast TV signals can, in theory, be quite large (greater than 35dB), but in practice, values of 11dB to 12dB (a crest factor of more than 10:1) are more commonly observed.
High PAPR signals make the design of power amplifiers difficult. The amplifier must be linear over a wide dynamic range to preserve modulation accuracy and spurious performance. It is possible to use a technique called crest factor reduction to allow the power amplifier to operate closer to peak power most of the time by limiting the peaks of the signal using DSP techniques; however, this needs to be done with care to minimize distortion and maintain adequate signal error vector magnitude. Typically, crest factor reduction will reduce the PAPR to about 8dB to 8.5dB.
Envelope tracking
Several techniques are available to improve power amplifier efficiency, including digital predistortion, Doherty amplifier linearization and high-accuracy tracking, a form of envelope tracking. The majority of these have found their first use in the cellular industry, where the problems of high network power consumption and environmental impact have already caused many network operators to demand significantly improved equipment efficiency from their suppliers. Most of these techniques are inherently narrowband with the consequence that seven or more power amplifiers and associated circuits are required to cover all the potential transmission frequencies.
By contrast, envelope tracking is effective over a wide bandwidth, so a single power amplifier can be used to cover the entire UHF band at high efficiency. (See Figure 2 on page 10.)
Envelope tracking is based on modulating the supply voltage of the final RF stage power transistor in sync with the transmitted RF power demanded at any given point. This ensures that the output device remains in its most efficient operating region (i.e. in saturation). The modulation of the supply voltage is performed by a power modulator device, which replaces the normal DC-DC converter providing the supply voltage.
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