Figure 12-2 is the block diagram of asynchronous ASK detector. This structure is a typical asynchronous ASK detector. When the ASK signal pass through the rectifier, we can obtain the positive half wave signal. After that the signal will pass through a low-pass filter and obtain an envelop detection. Then get rid of the DC signal, the digital signal will be recurred.
Figure 12-3 is the circuit diagram of asynchronous ASK detector, which R1, R2 and μA741 comprise an inverting amplifier to amplify the input signal. Then D1 is the rectifying diode to make the modulation signal passes through D1 half wave rectifier. R3 and C1 comprise a low-pass filter. μA741, VR1 , D2, R4 and C2 comprise a comparator, therefore, the output terminal can demodulate the digital demodulated signal.