www.anritsu.com | 29FM Measurements with a Spectrum AnalyzerThe spectrum analyzer is a very useful tool for measuring Δf and m and for making fast and accurate adjustments of FM transmitters. It is also frequently used for calibrating frequency deviation meters.A signal generator or transmitter is adjusted to a precise frequency deviation with the aid of a spectrum analyzer using one of the carrier zeros and selecting the appropriate modulating frequency. In Figure 34, a modulation frequency of 1 kHz and a modulation index of 2.405 (first carrier null) necessitate a carrier peak frequency deviation of exactly 2.405 kHz. Since we can accurately set the modulation frequency using the spectrum analyzer or, if need be, a frequency counter and since the modulation index is also known accurately, the frequency deviation thus generated will be equally accurate.Table 2 gives the modulation frequencies and common values of deviation for the various orders of carrier zeros.Order of Carrier ZeroMod IndexCommonly Used Values of FM Peak Deviation7.5 kHz10 kHz15 kHz25 kHz30 kHz50 kHz75 kHz100 kHz150 kHz12.4053.124.166.2510.4212.5020.8331.2541.6762.5025.521.361.182.724.535.439.0813.5918.1227.1738.650.871.161.732.893.475.788.6711.5617.34411.790.660.851.272.122.544.246.368.4812.72514.930.500.671.001.672.013.355.026.7010.05618.070.420.550.831.881.662.774.155.538.30Table 2Figure 34