Radio today (Apr-Dec 1939)

Record Details:

Something wrong or inaccurate about this page? Let us Know!

Thanks for helping us continually improve the quality of the Lantern search engine for all of our users! We have millions of scanned pages, so user reports are incredibly helpful for us to identify places where we can improve and update the metadata.

Please describe the issue below, and click "Submit" to send your comments to our team! If you'd prefer, you can also send us an email to mhdl@commarts.wisc.edu with your comments.




We use Optical Character Recognition (OCR) during our scanning and processing workflow to make the content of each page searchable. You can view the automatically generated text below as well as copy and paste individual pieces of text to quote in your own work.

Text recognition is never 100% accurate. Many parts of the scanned page may not be reflected in the OCR text output, including: images, page layout, certain fonts or handwriting.

Fig. 1 — Three common types of oscillators used in superhets. A, Ticklerfeedback; B, electron coupled; C, Hartley. Oscillator voltage taken from grid or cathode circuit. Characteristics, tests, and new features in first detectors The heart of the superheterodyne receiver is the oscillator, and mixer stage or stages. Here the signal frequency is combined with a locally generated frequency to produce the intermediate signal that still retains the original amplitude modulation, or intelligence. An analogous oscillator-mixer circuit could be conceived as two alternators, one representing the signal, and the other the local set oscillator. By varying the output voltage of the signal alternator, amplitude modulation would be effected. Conversely, varying the speed of the signal alternator and holding the output voltage constant would result in frequency modulation. When the frequencies of the signal and oscillator differ by some fixed frequency, a resultant wave that bears the same modulation characteristics as the signal will be present in the circuit. Its frequency will be the difference between the signal and oscillator frequencies. The oscillator signal voltage is nearly a sine wave and having a constant frequency difference with the carrier signal, a beat or intermediate frequency is produced. SEVERAL TYPES OF OSCILLATORS USED The oscillators commonly used employ several different fundamental types of circuits, all producing much the same results. Three types are shown in Fig. 1. The most commonly used type of separate oscillator is the familiar tickler-feedback circuit. A portion of the AC plate voltage is coupled back to the grid circuit through the transformer action of the two coils, and thereby maintains oscillation, since the grid circuit losses are supplied from a part of the plate power. The electron-coupled oscillator is featured by a high-degree of frequency stability. The screen-grid tube makes possible the use of the electron-coupled principle, since the screen electrode may be used as a plate in a typical triode oscillator circuit, such as the Hartley shown. The load in the plate circuit may be a resonant coil and condenser combination (not used in receiver oscillator circuits), or the plate may be grounded for rf through a bypass capacitor, thus putting it in par allel with the screen for rf currents. The frequency stability arises from the negligible effect of plate-voltage variation for a screen-grid tube. The voltage feedback from the plate to grid circuits is through the mutual coupling of the portion of the tank circuit between grid and cathode, and plate and cathode. The usual ratio of turns for the cathode-plate portion of the coil to the total coil is one to three. This gives a sufficient voltage feedback to maintain oscillation. For oscillator coils in the higher frequency ranges, the turn ratio may be slightly lower, 1:2.5, to compensate for the additional losses in the grid circuit at high frequencies. OSCILLATORS SIMILAR TO CLASS C AMPLIFIERS The Hartley oscillator is found in many forms, with the usual circuit variation being in the manner in which the plate voltage is fed to the tube. The electron-coupled circuit is a type of Hartley oscillator, and the two work much the same in respect to coupling of plate energy back to the grid. Self-excited oscillators, regardless of the type of connection, are Class C amplifiers with provision for feedback of a portion of the plate power to supply the grid losses and maintain the oscil Class C amplifiers (and oscillators) have a characteristic, pulse-like plate current wave, and they are distinguished by the amount of grid-bias with which they operate. The average grid-bias for an oscillator is 1.5 to 2 times the cut-off value for the tube. Since the grid must be driven positive in order to draw current and develop its own bias, a large value of exciting rf grid-voltage is necessary. For the best efficiency this rf grid voltage peak should approach the minimum value of rf plate voltage. As far as servicing oscillators Is concerned, these ideal curves and fundamentals are only a means of explaining what occurs when certain components of the circuit are not operating properly. COMPONENTS DETERMINE PERFORMANCE The grid condenser Cg serves two purposes. First, it offers a low impedance path to the rf currents and, second, it maintains a uniform dc bias on the grid by charging on the positive grid cycles and discharging through the leak on the negative rf cycles. The capacity is not critical and 100 mmfd. is a Fig. 2 — The conversion conductance of a tube is a measure of its gain. Conversion for 6L7G with — 6 volts on grid No. 1 is nearly uniform with oscillator voltage greater than 12 volts. Noise to signal ratio is much lower at high oscillator output voltages. 46 RADIO TODAY