t + B_{2}\sin(\omega_{m2}t))) = \sum_{n=-\infty}^{\infty} \sum_{k=-\infty}^{\infty} J_{n}(B_{1}) J_{k}(n B_{2}) \sin(\omega_{c} + n \omega_{m1} + k \omega_{m2})t \\
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<img class="indented" src="pix/fmeq57.png" alt="cascade FM">

<p>
Unfortunately, FM and PM can produce energy 
at 0Hz (when, for example, the carrier frequency equals the modulating frequency), and in FM that 0Hz component becomes a constant offset in the 
phase increment (the "instantaneous frequency") of the outer or lowermost
carrier. 
O