Assume, \(\omega_{c}\) be the carrier wave frequency
Signal wave frequency=\(\omega_{s}\)
Signal received, \(V = V _{1} \cos \left(\omega_{c}+\omega_{s}\right) t\)
Instantaneous voltage of the carrier wave, \(V_{in}=V_{c} \cos \omega_{c} t\)
\(V \cdot V _{ in }= V _{1} \cos \left(\omega_{c}+\omega_{s}\right) t \cdot\left( V _{ c } \cos \omega_{c} t \right)\)
\(= V _{1} V _{ c }\left[\cos \left(\omega_{c}+\omega_{s}\right) t \cdot \cos \omega_{c} t \right]\)
\(=\frac{V_{1} V_{c}}{2}\left[\cos \left(\omega_{c}+\omega_{s}\right) t+\omega_{c} t+\cos \left(\omega_{c}+\omega_{s}\right) t-\omega_{c} t\right]\)
Only the high frequency signals are allowed to pass through the low pass filter. The low frequency signal \(\omega_{s}\) is obstructed by it.
Therefore, at the receiving station, we can record the modulating signal,
\(\frac{V_{1} V_{c}}{2}\left[\cos \left(2 \omega_{c}+\omega_{s}\right) t+\cos \omega_{s} t\right]\) which is the signal frequency.