Is a transmitter output filter essential?
Yes, you really can’t broadcast without it. Not even if you’re the only transmitter on a tower and not even when no other station is broadcasting in the vicinity of your transmitter.
What is the difference compared to FM broadcasting?
When several FM channels are broadcasting from the same site, intermodulation (‘mixing’) products are generated, which results in a new combined FM signal on other frequencies. Filters can overcome this problem.
Knowing that the DAB(+) transmitter sends out on 1.536 frequencies (independent carriers), there could be a lot of intermodulation products. However, these products are very weak and almost not existent. The reason is that the carriers are on orthogonal spaced frequencies. It means that the carriers have minimal interference with each other.
We will explain the principle of “orthogonal carriers” in deep in one of our future articles.
On the contrary, a DAB(+) filter has to suppress the massive harmonic remains generated by the harmonic distortion created in the transmitter power amplifier. It is an unknown phenomenon with FM transmitter power amplifiers unless you would over-excite the FM power amplifier. Typically the transmit power of an FM transmitter is continuous. Whether there is an audio signal or not, you will always measure the same power.
With DAB(+), things are different. The power will highly depend on the digital audio information that is transmitted. Phase modulated carriers can enforce each other phase or eliminate each other phase. They are all added together in time, and sometimes massive peaks will occur.
We can compare it with audio recording, where the recording device will have limits in recording volume. Peaks arrive unexpectedly, so you have to foresee some audio headroom to prevent the audio from being clipped.
The same happens with DAB power amplifiers. A power amplifier that can deliver 1.000 Watt peak power can not be fully utilized and is used most of the time for a 250 Watt average (RMS). It means it is used for 25% and provides a headroom of 75% for peaks.
However, even with this headroom of 75%, you are still not sure clipping will not happen. Therefore a filter is necessary at the output of the transmitter power amplifier.
- An FM transmitter will always mention the peak power in its specifications
- A DAB(+) transmitter always notes average (RMS) power.
A deeper technical explanation can be found here in our discussion of “DAB+ in more details“