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REGENERATIVE DETECTOR

 

A simple, one-transistor REGENERATIVE DETECTOR circuit that uses the heterodyning principlefor cw operation is shown in figure 3-4. The circuit can be made to oscillate by increasing the amount ofenergy fed back to the tank circuit from the collector-output circuit (by physically moving tickler coil L2closer to L1 using the regeneration control). This feedback overcomes losses in the base-input circuit andcauses self-oscillations which are controlled by tuning capacitor C1. The received signal from the antennaand the oscillating frequency are both present at the base of transistor Q1. These two frequencies areheterodyned by the nonlinearity of the transistor. The resulting beat frequencies are then rectified by theemitter-base junction and produce a beat note which is amplified in the collector-output circuit. The afcurrents in the collector circuit actuate the phones. The REGENERATIVE DETECTOR (figure 3-4)produces its own oscillations, heterodynes them with an incoming signal, and rectifies or detects them.

 

Figure 3-4.—Regenerative detector.

 

The regenerative detector is used to receive short-wave code signals because it is easy to adjust andhas high sensitivity and good selectivity. At high frequencies, the amount of signal detuning necessary toproduce an audio-beat note is a small percentage of the signal frequency and causes no trouble. The use ofthe regenerative detector for low-frequency code reception, however, is usually avoided. At lowfrequencies the detuning required to produce the proper audio-beat frequency is a considerable percentageof the signal frequency. Although this type detector may be used for AM signals, it has high distortion

and is not often used.

Q-7.What simple, one-transistor detector circuit uses the heterodyne principle?

Q-8.What three functions does the transistor in a regenerative detector serve?