FM radio deviations

If you take a close look at some FM radio signals you find a lot of special characteristics. With standard PC equipment and free SpecLab software you can start measurements showing the plain signal and the modulated spectrum. An SDR like the Perseus receiver makes things easier.

The exact center frequency

Only in quiet audio gaps you can find a carrier on the center frequency. Typical deviations are up to 100 Hz. That's about the same percentage which you find on MW. But some range up to 20 KHz even with professional transmitter operators. Use the MHold function (maximum hold). If you know the deviation it is also possible to detect and identify inferior signals below a strong signal. Calibration of the receiver is an issue of course.
A few examples from Denmark (DNK) and Germany (D) in 2009:
91.00014D N-Joy Flensburg
91.50014D N-Joy Süderlügum
91.7997DNK Globus
93.80014DNK Globus Skaerbaek
94.5002D N-Joy Kiel
94.9003D N-Joy Heide
95.5003DNK P1 Vejle
99.0999DNK R.Globus Guld Tinglev
100.50017DNK 24syv Fyn
100.6994DNK R.Syddanmark
100.913DNK 24syv Vejle

The 19 KHz pilot tone

Now let's take a look at the demodulated spectrum. I've already written much about the exact frequency of the 19 KHz tone. You can draw conclusions from it about transmitter clusters. It's also interesting to look at the modulation level. It varies from -17 to -21 dB. If you have a strong signal you will also find that there is some unintended modulation around the 19 KHz, however it's rarely over -60 dB.

RDS modulation grade

There are two RDS carriers around 57 KHz with peaks of -40 dB. A few stations boost up their RDS with -34 dB.

additional carriers

some stations have additional data channels in the spectrum. In Germany digital signals at 62 KHz are very common.

The audio bandwidth

According to standards there must be a hicut filter at 15 KHz. Most transmitters use a slightly wider range up to 16 KHz. In some cases it's even 17 KHz getting closer to the essential 19 KHz carrier. back to