Text: A receiver for electromagnetic (radio) waves employing the heterodyne principle. The illustration Fig. 1 (see card id 47031) shows the original form of heterodyne receiver, wherein the local oscillations were produced by means of a direct current arc gap and associated circuits, and combined through a coupling medium, with the oscillations in the antenna circuit. Here the heavy lines represent a conventional crystal receiver circuit. L1 and L2 are respectively primary and secondary of a vario-coupler, C is a variable tuning condenser, D the crystal detector and T the head phones. If the dotted line is assumed as a connection to the ground, this will constitute a complete receiver. For the heterodyne purpose, however, a special coil L3 is employed in the ground circuit. Coupled to this is the secondary L4. C1 is another tuning condenser and A is an arc gap fed by 110 volts direct current. In operation the circuit represented by the light lines acts as a generator of undamped oscillations, these oscillations being supplied to the antenna circuit and the frequeney varied by means of the condenser C1 and the coil L4. If the receiving tuner is tuned to a certain wave-length corresponding to incoming signals, and the generator system tuned to a different frequency, the differenee between the two frequencies will be a beat frequency which will be present in the detector circuit. This new series of oscillations will be rectified by the detector D and will be audible in the head phones, T. The action of this system is shown by the series of curves given in Fig. 2. Here the incoming oscillations (signals) from the antenna are shown in graph 1 at the top of the illustration. These oscillations are of continuous amplitude or undamped. Graph 2 shows the local oscillations. If we assume that the distance from A to B represents the time, it will be apparent that there are more oscillations per unit of time in the case of the incoming signals -

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Source: 190