A super-regenerative receiver is a type of regenerative circuit containing an oscillator which is automatically switched between an oscillating and a non-oscillating condition. Oscillations are initiated by introducing a voltage (quench voltage) in a feedback loop of the oscillator at a switching frequency known as the "quench" frequency. During operation of the receiver, the net voltage in the feedback loop of the oscillator increases near the positive peaks of each cycle of the quench voltage, i.e., when the quench voltage is added to the supply voltage, thereby causing oscillations to build up in the circuit. During its negative half-cycle, the quench voltage will lower the net voltage supplied to the oscillator to the point where any started oscillations die out. Signal voltages that are to be detected are also connected in the feedback loop of the oscillator to thereby initiate oscillations upon application of a signal voltage.
In the absence of an applied signal, the oscillations that build up during each positive cycle of the quench voltage start with an initial amplitude determined by the noise voltages in the input circuit, e.g., thermal agitation noise in the tuned input circuit, and reach a final value corresponding to the equilibrium value for the oscillator. These oscillations then die out as the quench voltage becomes small and then goes negative or too low to provide oscillating conditions. If an rf carrier signal is superimposed upon the system and is larger in magnitude than the noise voltage, the initial amplitude, as the oscillations start to build up, corresponds to the amplitude of the superimposed rf carrier signal. The oscillations, therefore, reach equilibrium more quickly during application of the rf carrier signal because of the larger initial amplitude. The rf carrier signal is amplitude modulated with a desired information signal which is recovered at the output of the receiver, e.g., with a low pass filter which rejects the rf carrier and the quench signal.
Traditionally, super-regenerative receivers were LC controlled. Such receivers use inductors and capacitors in the oscillator feedback circuit, thereby causing the oscillator to oscillate at the LC circuit's resonant frequency. Therefore, the operating frequency at which the receiver oscillates is directly related to the inductive and capacitive values respectively of the inductors and capacitors in the LC circuit. However, such LC circuits cause the receiver operating frequency to be unstable over time and to drift from its desired operating frequency due to changes in the values of the inductors and capacitors with age. In addition, LC circuits are temperature sensitive, which causes the receiver operating frequency to change with changes in temperature.
Generally, it is desirable for a super-regenerative receiver to have a relatively narrow frequency response, with the center operating frequency of the receiver (center frequency of reception) being selected to provide the desired reception band, i.e., the particular range of frequencies that the receiver will be sensitive to receiving. However, super-regenerative receivers having LC-controlled oscillations have relatively wide reception bands and are therefore influenced by stray signals and noise.
To overcome the above problems associated with using an LC feedback circuit in a super-regenerative receiver, surface acoustic wave (SAW) devices have been used to replace the LC devices in the receiver feedback circuit. For example, U.S. Pat. No. 4,749,964 to Ash discloses a super-regenerative detector having a SAW device in the feedback circuit of the detector. The SAW device utilized in the '964 patent is a single phase, unidirectional transducer with quarter wavelength electrodes in spacing allowing operation of the detector at very high frequencies. Such a SAW device provides a feedback circuit which has low loss, is temperature stable, and provides the necessary phase shift in the feedback circuit to cause oscillations of the detector.
In both a super-regenerative receiver having a LC feedback circuit and the SAW delay line receiver, the center frequency of reception is adjusted by means of a variable inductor. The variable inductor includes an air wound coil with a movable metal slug in the center. The slug is repositioned within the coil to provide a slight change in the actual inductance value, thereby changing the receiver center operating frequency. This adjustment normally requires a person in the manufacturing environment to perform. In low volume applications, such a manual adjustment may be tolerable. However, in highly competitive high volume markets, such as the automotive industry, this technique is far too costly.