In today’s world of inexpensive and highly capable microcontrollers, thoughts of signal processing with analog equipment may sometimes be pushed to the wayside. Perhaps that's for good reason, since for a few dollars or less, you can have a microcontroller take care of many processing tasks, programmatically. Sometimes, however, the analog solution is still appropriate.
Here we’ll be talking about three related signal processing circuits that can be constructed with a diode, resistor, and capacitor.These three elements by themselves can process signals as either an AC to DC converter, peak detector, or envelope detector. Experimentation was done in the context of sound waves, but there are other important uses, such as AM radio frequency demodulation.
Building up from a raw audio signal to a rectifier, peak detector, and envelope detector.
Pulsing signal + diode = half wave rectifier
Controlled sine wave in blue; rectified signal in yellow
Rectifiers transform an AC power source into a DC output. One of the simplest ways to do so is to use a diode to allow current to pass in only one direction. This cuts off the bottom half of a sine wave, leaving a series of positive-only pulses with gaps in between. A capacitor can be used to smooth things out. When configured properly, such a circuit is known as a peak detector.
Half wave rectifier + capacitor = peak detector
Audio signal in blue; yellow is capacitive peak detector
If you’re interested in the maximum voltage of a variable rectified signal—such as from a sound wave—this can be stored with a capacitor. The capacitor is connected between the diode’s cathode and the signal’s ground, quickly charging up to nearly the maximum voltage that’s pulsed by the input. A scope or multimeter can be attached across the two capacitor’s leads, reading this maximum value forever… in theory.
In reality, no components are perfect, and capacitors discharge over time. Depending on the circuit setup, you’ll actually see the peak level decrease on its own if not recharged by a positive signal.
Peak detector + resistor = envelope detector
Peak detection follows waveform trends but will require proper adjustment and correlation to use properly.
The peak charge level decreases naturally over time, but if this discharge was controlled, you could have it ride the top of the waveform, so to speak. This is accomplished by adding a resistor to drain the capacitor at a set rate, roughly following the “envelope” of the waveform (as seen in the above image). This kind of circuit is known as an envelope detector.
To produce the above waveform, the capacitor used was 10µF with a 10k resistor. These values were chosen via the RC time constant tau (τ), which is equal to the time (in seconds) needed for a resistor capacitor circuit to discharge to ~36.8% of its original value, expressed as:
τ = RC
R is in ohms, and C is capacitance in Farads. I was already using a 10µF (or .00001F) capacitor for the peak detector, and a .1 second discharge seemed like a good place to start. Therefore, the R value worked out to τ/C, or .1/.00001 = 10,000 ohms. As seen in the image above, this demonstrates the basic concept, but some tweaking of values, perhaps using a potentiometer or trimmer, would likely make the correlation much better.
Breadboard experimental setup at the peak detector phase. Both left and right signal outputs were used simultaneously during some experiments.
Whether you need to modify a sound signal, demodulate radio waves, or perhaps have some novel application for robotics or signal processing, don’t forget the humble capacitor/diode/resistor circuit. Such a setup could be a great way to clean up a signal before sending it on to your microcontroller, or perhaps even substitute for digital electronics altogether, depending upon your application.
For digital signal processing and analysis, you might consider an all-in-one Digilent Analog Discovery 2 setup, as outlined here, and here with a Raspberry Pi 4!
