Radio Broadcast (May 1928-Apr 1929)

FOR THE EXPERIMENTALLY INCLINED A beat-frequency oscillator of the type shown here is one of the most valuable pieces of testing equipment that the home experimenter can have. This one is built entirely of standard parts, with the exception of the home-made coils, which any radio fan can make for himself. It employs two oscillating tubes, a detector and a stage of audio to raise the detector output to the desired level. By G. F. LAMPKIN COMBINING the factors of simplicity, portability, and flexibility, a beatfrequency oscillator is a most useful piece of apparatus to have in the laboratory. It utilizes, as may be known, two radio-frequency oscillators, one of which has its frequency varied so as to yield in the audio range beat frequencies with the other oscillator. A detector takes out the audio components, and amplification as desired may be used to bring up the level of their power. A range of audio frequencies of reasonable purity from 30 to 10,000 cycles may be had with adjustment of but one dial; an output of approximately 15 volts r.m.s. can be obtained using only four tubes; and finally, the output over the above mentioned range may be held constant within ten per cent. Such an oscillator was constructed by the writer for use in thesis work at the University of Cincinnati; and constructional data may be helpful to others who have use for a similar instrument. The selection of the radio frequency at which the oscillators should work is made by compromise. If the frequency carrier is low, it becomes difficult to eliminate that frequency in the output of the oscillator. The resistance-coupling between detector and amplifier is capable of passing too low a carrier frequency, and the ordinary vacuum tube voltmeter is likewise capable of responding to such a frequency, so that even at zero beat between the radio-frequency oscillators, the output meter shows a deflection. Increasing the carrier frequency permits more perfect discrimination between it and the audio frequencies. On the other hand, at high carrier frequencies, the inherent coupling between the oscillators causes "pulling." This means that, as the frequency of one oscillator approaches that of the other, a point is found where the two "pull" into synchronism, and the beat note disappears. The point at which the oscillators pull into step may be as high as two or three thousand cycles — -the closer the coupling the higher is this frequency. The carrier-frequency determined for the particular oscillators was 175 kc. At this frequency, the magnitude of carrier present in the output was less than one-tenth of a volt, the A MOST convenient type of audio-frequency *■ oscillator that anyone can make is what is known as a "heat to frequency" oscillator. Two radio frequencies heat together to produce an audio frequency note. This note is amplified and may he used to test loud speakers, audio-frequency amplifiers, or for bridge measurements of capacities and inductances. All frequencies from ^ero to 10,000 are obtainable from a single dial. Such an oscillator is described in this article by Mr. Lampkin who is Baldwin Fellow in Electrical Engineering at the University of Cincinnati. The apparatus is such as may be found in any experimenter's laboratory: the only special part is the coil — which anyone can make. — The Editor. lowest calibration of the vacuum-tube voltmeter that was used. Only partial shielding is necessary between the two oscillators to reduce the coupling to a point where the beat note can be lowered to 1 5 cycles without the occurrence of pulling. Complete shielding was provided for, however, with the idea in mind that the carrier 55 Turns •40 U .J ,;L 5"— FIG. I Two coils of this type are used for the two oscillators. They should be made exactly alike in dimensions. The wire used in all these coils is No. 24 sec 156 frequency of the oscillators may be made to cover the broadcast band — by means of another coil, of course. At any particular radio frequency setting for the one oscillator, the frequency of the other is varied to either side by vernier control to yield in effect a modulated radio frequency, so that the instrument becomes a miniature broadcasting station for receiver testing. To this end, provision is made for leading out the modulated radio-frequency currents from one oscillator by means of an external coupling coil to be connected to posts 1 and 3 on Fig. 2. Provision is made also for exterior vernier control. Plug-in coils in the oscillators make it possible to cover other frequency ranges. A FOUR-TUBE UNIT IS BEST A N ATTEMPT was made to use one of the oscillators also in the role of detector. The beat-frequency oscillator thus used one radiofrequency oscillator, one oscillator-detector, and one amplifier. The output voltage obtained from such an instrument was comparatively low, and varied in the ratio of four to one over the 100 to 10,000 cycle range. When the circuit was reconstructed to use a separate detector, the resulting output of the device averaged three and a half times larger than before, and was approximately constant over the same frequency range as above. In point of both magnitude of output and constancy of output the inclusion of the fourth tube is well worth while. Still more tubes may be included as audio-frequency amplifiers if it is desired to raise the level of the output. However, the 1 5-volt output that can be obtained from four tubes is enough to give a fair output when comparing and testing loud speakers, and it easily fulfills the requirements when the user is measuring amplifiers, transformers, and similar work. The schematic for the beat-frequency oscillator is presented in Fig. 2. The two radio-frequency oscillators are made as alike as possible so that changes in operating conditions will affect both of them simultaneously. A tickler-feed-back^ oscillating circuit is used; a 0.002-mfd. fixed