Signal Guide

Service Description Selection FM

Here you will find information about the selected service, along with measurement examples.

FM Theory & Details
Broadcast
Typical Frequency Range:

87,5 – 108 MHz

General:

FM stereo radio

Modulation: FM

Combined modulation. The stereo sum signal + 19 kHz pilot, difference signal, 38 kHz aux carrier and 57 kHz RDS (radio data signal) are combined into the baseband. The baseband is frequency modulated with up to 75 kHz shift. Fully modulated stereo uses 256 kHz, mono 180 kHz bandwidth.

FM Measurement Examples
Spectrum
Overview of the VHF band 87.5 – 108 MHz. Frequency range captured simultaneously with one FFT, so there are no time gaps in each sweep. The device delivers a “snapshot” every 18 ms.
Spectrogram
Shows one VHF FM channel over a bandwidth of 300 kHz. The device delivers a “snapshot” every 126 ms. The time characteristic over a period of 30 s is displayed. Frequency range captured simultaneously with one FFT, so there are no time gaps in each sweep. Only the 19 and 38 kHz carriers and the 57 kHz RDS are present in the modulation gaps.
HiRes Spectrogram Full
Display of a complete VHF FM channel of bandwidth 256 kHz. Gapless time display over 781 ms. The carrier in the center as well as the 19 kHz pilot, 38 kHz aux carrier symmetrically placed, and the 57 kHz RDS can be clearly seen.
HiRes Spectrogram Zoom
Section of the HiRes Spectrogram Full. The displayed frequency range is still 256 kHz. The time axis can be shifted as with a magnifier.
Persistence Spectrum
Calculated from the same data as the HiRes Spectrogram Full. Shows the frequency of occurrence over a period of 781 ms. The continuous carriers are clearly visible in the modulation gaps.
Magnitude
Time domain display based on the same data as the HiRes Spectrogram Full. The display is without time gaps. The FM signal is shown as a continuous line. Longer recordings of up to 24 h can be made in Long-Time Scope mode.
Scope