DAB: Class B Amplifiers and Transmitter Efficiency

With the BBC anxious to cut costs and reduce its CO2 emissions, a long-term project has been set up between the BBC and Arqiva (the transmission providers) to improve energy efficiency of the broadcast radio transmitters. AM (on LF and MF), FM and DAB are all within scope of the project. Work on the MF transmitters (Radio 5 and National) has already reduced the electricity bill from 12.7 to 10.6 GWh per annum.

The next question is whether we can achieve worthwhile energy savings on the DAB network. At first glance, the prospects do not look good: the DAB signal might have low average power, but the peak power is some 10 times as great. That is because the transmitted signal is noise-like in character. If the transmitter is made to handle the large peak powers, it is likely to be working at well below maximum efficiency for most of the time.

Although the latest DAB transmission equipment uses techniques such as Doherty amplification to achieve good efficiency, much of the existing equipment in service possesses straightforward Class B power amplifiers. Although these perform well compared with Class A amplifiers, the combination of high peak signal powers and fixed power supply voltage still implies poor efficiency.

In the longer term, the need for greater efficiency is likely to result in the replacement of the current Class B-based equipment. However, it is worth asking whether better efficiency is achievable in the meantime. Two possibilities arise: first, where a transmitter is not required to deliver its full rated power, the power supply voltage could be reduced without affecting signal quality; secondly, the supply voltage could be reduced if the consequent loss of signal quality has negligible impact on reception.

This report examines these possibilities. Even if the decision is not to modify the existing equipment, the work could still remain relevant, and should serve as a useful reference for any future reports on transmitter performance. This is particularly true for studies into the relationship between transmitted signal quality and the impact on reception. The statistics of the DAB signal also remain of interest, as they are not dependent on the technology.

This paper is authored by BBC Research & Development's Ranulph Poole, Adrian Wolmarans with Phil Kesby and Phil Brown, our partners from Arqiva.

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