In a Binary FSK system, symbols 1 and 0 are distinguished from each other by transmitting one of two sinusoidal waves that differ in frequency by a fixed amount.
(or)
The frequency of the carrier signal shifted to two frequencies 
for symbols ‘1’ and ‘0’ transmission.
The equation for FSK signal is
\[ S_{FSK}(t)= \sqrt{\frac{2E_{b}}{T_{b}}} \ \ cos (2\pi f_{c_{i}}t),\ 0\leq t\leq T_{b} , \ i=1,2 \]
\[ S_{FSK}(t)= 0 elsewhere \]
i.e,
where
is generally a high frequency.
is a low frequency and vice-versa is also true.
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