Nov 282010
 

One of the most challenging elements of FM broadcasting is accurately setting up modulation levels. Schedule 1 h of the GURL specifies

(h) Unwanted emissions shall not exceed the limits as follows;

Unwanted Emission            Frequency Range                    Spectrum Analyser
Limits – in the                         from carrier                              Resolution  Bandwidth
resolution bandwidth

-25 dBW                                   128 to 240 kHz                          Not to exceed 10 kHz*
-35 dBW                                   240 to 600 kHz                         Not to exceed 100 kHz*
-56 dBW                                   > 600 kHz                                    100 kHz

* The spectrum analyser shall be set for 5 minute maximum hold.

But what does all this mean? I can guarantee most LPFM broadcasters just plug everything in and adjust the volume to sound similar to another station. The danger of this approach is that many stations use specialised and expensive processing. High volumes are obtained by reducing the dynamic range. Trying to match the volume of a commercial radio station will inevitably lead you to breaching the GURL. With a PIRA75 modualtion monitor you can set your modualtion levels correctly. It will also assist with the ability to deal with interference complaints. This will be available to members.

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