The circuit's sensitivity is better than 1 µV, and it operates over a wide range of VHF (Fig 1). You can build this small receiver for about $20. It draws less than 10 mA from its two 9V batteries.
Using modern discrete JFETs and ICs, you can build a low-cost super-regenerative receiver that has µV-level sensitivity at VHF and above. Such receivers also boast very low parts counts and low power consumption. Super-regenerative detectors are practical for wideband FM reception as well as for AM. They even can somewhat detect high-amplitude, narrowband, FM signals.
The circuit's sensitivity is better than 1 µV, and it operates over a wide range of VHF (Fig 1). You can build this small receiver for about $20. It draws less than 10 mA from its two 9V batteries.
The circuit comprises an RF stage, a super-regenerative detector, and an audio amplifier. The RF stage, Q1 and associated components, prevents the receiver from radiating its own signal back out the antenna and helps to eliminate dead spots in the receiver's tuning range. These dead spots arise from antenna-resonance effects.
The capacitively coupled output of the RF stage connects to a tap on coil L1. You have to determine the tap's exact location by experimenting; the higher up the tap is on the coil, the greater the sensitivity, but also the greater the loading on the detector.
The super-regenerative detector, Q2, is essentially a modulated oscillator whose input is coupled to the antenna via the RF-isolation stage. This section alternates between oscillating and being periodically quenched.
The periodic quenching resets the oscillator, allowing the input signal to build up again to very high levels. Small noise sources require greater periods of time to build to a self-sustaining free oscillation than do higher amplitude radio signals. Therefore, given the limited time between quench intervals, the signal sources get amplified to a much greater extent before they are quenched out than do any noise sources. Note that the oscillations must fully decay before being allowed to start up again. Even a small amount of residual signal initiates a premature free oscillation—ending all useful amplification for that period.
In this circuit, the RC time constant of R1 and C1 is long enough that capacitor C1 cannot discharge fast enough (at the signal frequency rate) through R1. This lack of discharging builds up a variable dc voltage across R1 and C1. This voltage periodically quenches the main oscillator. The 5-kOhms regeneration control sets the detector's operational voltage and also functions as the quench-frequency adjustment.
A 2-pF capacitor provides RF feedback between the FET's source and drain, causing it to oscillate. You may need to increase or decrease this capacitor's value by 1 pF or so to compensate for variations in your circuit's layout. A 15-µH RF choke keeps the FET's source above ground at RF.
The output of the detector drives a low-noise, op-amp, output stage. A simple lowpass filter removes the quench voltage from the detector's audio output.
Because super-regenerative detectors handle VHF, stray circuit capacitances or multiple ground paths can prevent the detector from oscillating. You absolutely must locate the super regenerative detector's tuning coil away from other objects—particularly chassis ground, the bottom and sides of the equipment enclosure (if it is metal), and any shielding.
EDN BBS /DI_SIG #1348