The miracle circuit
Brad Thompson, Contributing Technical Editor- October 1, 2002
Suppose someone offered you a circuit that demodulates microvolt-level AM, FM, and pulse-modulated signals from darn near DC to daylight. The circuit also will "capture" the stronger of two incoming signals within its bandwidth. Best of all, you can build the circuit around a single active device, and it consumes microwatts.
Interested? Such a design has existed since before the Great Depression. Major Edwin H. Armstrong, the genius who invented the regenerative detector, the superheterodyne, and the FM radio, described the superregenerative detector in 1922.
Here's how to build a superregenerative circuit: Use any active device that oscillates at the frequency of interest. Apply a periodic "quench" voltage or current that stops the oscillator and gradually allows the oscillations to restart.
As the oscillator attempts to restart, the circuit's feedback teeters on the edge of signal-frequency oscillation. Internal noise within the oscillator will tip the balance and randomly initiate oscillation. An incoming signal will start the process earlier in the restart phase and alter the oscillator's current drain. You recover the incoming signal's modulation by filtering the oscillator's average current drain.
In a superregen circuit designed around a single active device, every parameter affects every other parameter. Two nonlinear and interacting oscillators amount to an analytical headache that will probably break any circuit-simulation package.
But there's no free lunch. Unless you carefully shield a superregen circuit and equip it with an isolation amplifier, it can radiate broadband noise through its antenna. Also, the superregen's tendency to capture the strongest signal limits its applications in crowded portions of the spectrum.
Superregen circuits saw service before and during World War II in communications and IFF (Identification, friend or foe) radar receivers. After World War II, the first publicly licensed two-way radio systems used superregen receivers. Also, NMR (nuclear-magnetic resonance) instruments featured superregen detectors.
But the superregen must be obsolete today, right? Not at all—open the case of a garage-door opener or a remote-control automobile-lock receiver, and chances are, there's a superregen receiver inside. Modern superregen receivers—built around MMIC front-end amplifiers and SAW (surface-acoustic wave) filters—minimize radiation and offer narrow bandwidth with good adjacent-channel immunity.
Where does the superregen fit into modern test instrumentation? Probably as a measurement target and not as an instrument component, but you might have an application awaiting a small, cheap, and sensitive circuit. After all, tough economic times call for ingenuity and thrift.
Contact Brad Thompson at firstname.lastname@example.org