Application Note: The Crest Factor In DVB-T (OFDM) Transmitter Systems And Its Influence On The Dimensioning Of Power Components

By Rohde & Schwarz GmbH & Co. KG

Are there really power peaks that are 20 dB above the average value? Yes, there are. Ever since the introduction of digital transmission technology, it has been necessary to deal with such orders of magnitude. RF power components must be suitably dimensioned to handle the expected voltage peaks and avoid breaking down. If we can determine the crest factor, i.e. the ratio of the peak value to the average or RMS value, with sufficient accuracy (even though it can only be assessed statistically), then we will have solved the question of dimensioning power components. This Application Note is intended to help with this problem by providing some useful insights including basic formulas, some statistical background and a practical look at the limiting factors when it comes to dealing with real transmitter systems.

The content of this Application Note is divided into two parts. Part 1 (Chapter 2) deals with the determination of the crest factor and its consequences for the dimensioning of transmitter components. Part 2 (Chapter 3) provides additional background material for readers who want to go further. Based on the example of a DVB-T signal in 2K mode, we then take a precise look at OFDM signals ranging from generation to the modulated RF signal.