Radio Broadcast (May 1928-Apr 1929)

" Time Amplifier o o o o CX-326 CX-326 CX-326 C-327 110 V. A.C. FIG. I Switching provides for the use of this circuit either as a super-heterodyne or as a single-circuit receiver A Flexible Super*Heterody By Dana Adams THE receiver described in this article has several unique points of interest which distinguish it from the commonplace. In the first case, it is a.c. operated, a feature which is becoming more and more popular. The most interesting feature of the receiver, however, is the fact that, by the mere flip of a switch, the degree of selectivity and sensitivity of the system may be augmented or decreased, depending upon the requirements of the operator and his geographical location with respect to the station he wishes to hear. The circuit, in its most sensitive form, is a super-heterodyne employing a three-stage intermediate-frequency amplifier. The switching, which controls the selectivity and sensitivity, provides for three circuit arrangements as follows: ist Position: The receiver is converted into a single-circuit receiver with one tuning control and one volume control, the output of the detector tube inputting directly to the audio amplifier. 2nd Position: Same as ist Position with the exception that a series condenser in the antenna circuit is switched in by means of a Yaxley No. 10 antenna switch, thus adding a further tuning control, but at the same time improving sensitivity. 3rd Position: The complete super-heterodyne receiver is thrown into operation, a third main tuning control being added. The antenna tuning condenser need not necessarily be switched into circuit in this case. The switch used for switching from single-circuit receiver to super-heterodyne receiver is a Yaxley No. 63 triple-pole switch. In Fig. 1 (the complete circuit diagram up to the output of a singlestage audio amplifier), various connections to this switch are indicated by number but are not grouped. The numbers for the switch terminals are determined by counting from right to left from a rear view of the switch. Fig. 1 shows a coil and the condenser in series with the antenna. Variation of this condenser tunes the antenna to any desired frequency in the same manner that the grid circuit of a tube is tuned, with a consequent increase in signal strength. At the same time, the strength of sig nals flowing in the antenna circuit at other than the resonant frequency is reduced, following the law of all series-tuned circuits. Laboratory measurements show this gain at the resonant frequency to be equal to that of one radiofrequency stage. Coupling to the detector circuit is obtained through a small variable coil, L2, which is a part of the Samson No. 3 1 coupler (which comprises L2, L3, and L4). This coupling coil, particularly when the antenna tuning is used, is generally set very near the minimum coupling point, with a consequent increase in selectivity. Having made the antenna circuit and the coupling method as efficient as possible we turn next to the first detector. This tube is regenerative with the sensitive grid leak and condenser method of detection and at this point the circuit departs from the ordinary. The Yaxley No. 63 triple-pole switch is used here to cut out the oscillator and intermediate stages of the receiver, at the same time transferring the first detector plate lead from the intermediate amplifier input to that of the audio amplifier. The result is a highly efficient regenerative receiver with an audio amplifier. The middle dial may be used alone in tuning-in the local programs with the antenna coupling acting as a volume control. If additional efficiency is required, a flip of the antenna switch makes the antenna tuning feature immediately available. A change of position of the three-pole switch returns the detector plate to its normal connection in the super-heterodyne circuit, at the same time lighting the oscillator and intermediateamplifier tubes, which gives us a two or threedial super-heterodyne receiver. Instead of the ordinary coupling coil method of . introducing the heterodyne frequency to the detector grid circuit this is done by placing the oscillator coil itself in inductive relation to the detector coil. This eliminates the losses of signal strength frequently caused by a tightly coupled coil which is, in most cases, at ground potential. The oscillator circuit is thus made entirely independent of the receiver except for its power supply. The oscillator circuit is the familiar modified Hartley circuit, grounded rotor plates preventing any hand-capacity while tuning. The beat note set up by the oscillator and first A FRONT VIEW OF THE COMPLETE RECEIVER 39