Communication Systems/Envelope Filters

What is an Envelope Filter?
The envelope detector is a simple analog circuit that can be used to find the peaks in a quickly-changing waveform. Envelope detectors are used in a variety of devices, specifically because passing a sinusoid through an envelope detector will suppress the sinusoid.

Circuit Diagram
In essence, an envelope filter has the following diagram:

o--+--+--o +     |      |      +        \     (c) vin   /R     |     vout \     | -      |      |      - o--+--+--o

Where (c) represents a capacitor, and R is a resistor. Under zero input voltage (vin = 0), the capacitor carries no charge, and the resistor carries no current. When vin is increased, the capacitor stores charge until it reaches capacity, and then the capacitor becomes an open circuit. At this point, all current in the circuit is flowing through the resistor, R. As voltage decreases, the capacitor begins to discharge it's stored energy, slowing down the state change in the circuit from high voltage to low voltage.

Positive Voltages
By inserting a diode at the beginning of this circuit, we can negate the effect of a sinusoid, dipping into negative voltage, and forcing the capacitor to discharge faster:

diode o-->|--+--+--o +     |      |      +        \     (c) vin   /R     |     vout \     | -      |      |      - o--+--+--o

Purpose of Envelope Filters
Envelope filters help to find the outer bound of a signal that is changing in amplitude.