Envelope Detector

An envelope detector will be used to recover the 30 MHz signal. Figure 13 below shows the schematic of the design.

Envelope Detector Circuit

Figure 13
Envelope Detector

The diode in the circuit rectifies the sinusoid, and the resistor - capacitor combination allow the output waveform to follow the envelope of the sinusoid. The following relationship is used to select the values of R and C:

     B is much less than 1/(2*pi*R*C) is much less than fc

     B is the bandwidth of the detected modulation waveform
     fc is the carrier frequency

The output waveform is fed to the analog to digital converter.

Envelope Detector Circuit Output

Figure 14
Time Domain Analysis of Envelope Detector

The envelope detector will essentially average the signal if the RC time constant is high enough. In this case, the signal will be held at its peak until the next pulse arises. The magnitide of the output of the envelope detector may be too small, so a baseband amplifier may be needed.