Signal Processing
Peak extraction from a modulated carrier wave occurs through this circuit configuration to isolate the low frequency information contained within the amplitude variations of the input. The envelope detector extracts the modulating baseband signal by using a diode to rectify the carrier and a parallel resistor capacitor network to smooth the result. Precision depends on the time constant relative to the highest frequency of the modulating signal.
If the time constant remains too long, the circuit fails to track rapid decreases in amplitude.
Operational Dynamics
The diode passes the positive portion of the input voltage and prevents current from returning from the capacitor during negative cycles. Charge accumulates across the capacitor during each peak of the radio frequency wave. Discharge occurs through the parallel resistor at a rate dictated by the product of the resistance and capacitance values.
This decay must remain faster than the rate of change of the modulating signal to prevent signal distortion.
Performance Constraints
Diode non-linearity introduces small errors in the tracked output when the input signal level stays near the threshold voltage of the semiconductor. Thermal drift of the diode forward voltage drop alters the calibration accuracy of the detected output over time. Leakage current through the capacitor dielectric creates an offset error in the recovered voltage.
Engineers mitigate these effects by using active circuits with operational amplifiers to compensate for the diode voltage drop.
Measurement Verification
Gain errors emerge during testing when the source impedance of the signal generator interacts with the input impedance of the detector. Calibration of the device requires a sweep of amplitude modulated waves across the rated frequency bandwidth to plot the tracking accuracy against known reference values. Output linearity suffers if the load impedance draws excessive current and prematurely drains the storage capacitor.
Total harmonic distortion in the recovered signal acts as the standard metric for verifying the fidelity of the demodulation process.
