Bandpass Filters > Technical Overview

BANDPASS FILTER

Terrestrial broadcasting of the television signal, both ATV (Analog TV) and DTV (Digital TV: DVB-T, DVB-H, ATSC), requires a high degree of linearity by the output amplifiers; the more an amplifier is pushed in power, the more it generates products from distortion within and outside the transmitted channel.
While in band distortion only affects the quality of the signal itself, the out of band emissions can disturb other broadcast channels or other services. For any television standard, there are rules for the various world countries, regulating the maximum out of band emissions. The linearity requirement is in contrast with the pursuit for maximum performance; class A high linearity amplifiers would have unacceptable disadvantages due to dimensions, size and low performance; for the current technical state of art class AB solid state final amplifiers are used, and the distortions are limited with sophisticated pre-correction techniques.
This way, the quality of the transmitted signal can be effectively controlled even in heavily output operating conditions; however, in these conditions the out of band emissions would be unacceptable. The addition of a selective output filter allows the amplifiers to be driven at maximum levels while still keeping the out of band emission within the required values.
The effectiveness of the transmitter is thus enhanced by the filter specification, the lower the losses and distortions, the more selective the filter is.