Frequency Shift Keying

As mentioned above, the digital data that the telemetry system generates is converted to an analog signal using Frequency Shift Keying (FSK). FSK is a special type of modulation where the digital signals (``0" & ``1") changes the frequency of the pseudo carrier to one of two frequencies, usually denoted as MARK and SPACE respectively.

If $T$ is the duration of a bit, then the bandwidth (BW) occupied by the FSK signal is:

$$\bigl[\nu(\rm MARK)+{1\over T}\bigr] - \bigl[\nu(\rm SPACE)... ...\over T}\bigr] = \nu(\rm MARK) - \nu(\rm SPACE) + {2 \over T}.$$ (24.4.2)

For example, in the Forward Link, $\nu(\rm mark) = 19$ MHz, $\nu(\rm space) = 17$ MHz and $t = 4$ microseconds (i.e. corresponding to a data rate of 250 kbps). Therefore, the bandwidth of the FSK signal in the forward link is

$$\Delta \nu = (19-17) + {2\over {4\times 10^{-6}}} = 2.5~{\rm MHz.}$$ (24.4.3)