Radio Broadcast (May 1928-Apr 1929)

JULY, 1928 HOW TO BUILD A BEAT-FREQUENCY OSCILLATOR 157 0.002 mfd Shield 4 5 SCHEMATIC DIAGRAM + A +90 +135 FIG. 2 Many times we have promised Radio Broadcast's readers an article on how to make an audio oscillator. This is the circuit — at last! It uses parts which are easily obtainable or home constructed 4 \ " *4<1 iJ-U 1 £u 2 "■ -4« 2 -4. 1 %*<l % 1 -2k" FIG. 3 The dimensions of Mr. Lampkin's oscillator panel are given here condenser, C3, and the variable tuning condenser, Ci, are across only the grid coil, which allows the rotor and end plates of the variable to be grounded. The plate voltage is series-fed to the tubes. The oscillator (VTi in Fig. 2) has the tuned circuit opened at the filament end and brought to a binding post on the panel (No. 1 in Fig. 2) in order that the external coupling coil can be connected for the dummy transmitter use as outlined above. In normal work as a beatfrequency oscillator, terminals 1 and 2 are shorted. From the same oscillator is brought out a lead (No. 3 on Fig. 2) to a binding post where an external vernier condenser of about 70 mmfd. may be connected. The radio-frequencies from the two oscillators are combined through the 18-turn pickup coils and impressed on the detector. Resistance-coupling is used into the final amplifier tube because of its excellent response on all frequencies. The tubes used are ux-20ia's for the oscillators, a UX-200A detector and a ux1 12 a amplifier. The panel drilling, coil dimensions, and other constructional details are indicated in Fig. 1 and 3. An idea of the baseboard layout can be gained from the photographs. The oscillators are put at opposite ends of the panel, and the detector and amplifier occupy the smaller, 4^" wide, compartment. The copper shielding is cut to the given sizes and the panel sheet drilled so that it is held to the panel by the apparatus. The other sections are soldered in place. The top and back sections of the shielding are used only for the miniature-transmitter function, when they are held in place by screws to the side flanges. As the filament voltages are not critical, they are all controlled by a single ten-ohm rheostat, Rj. Other binding posts on the panel are for battery-supply input, and for the audiofrequency output from the plate of the amplifier tube. The voltage output characteristic for the finished oscillator is given in Fig. 4. It was taken for the normal operating voltages of 90 volts B supply to the oscillators and detector, 135 volts B and 9 volts C-bias on the amplifier, and filament voltage at 5. The audio-frequency output was measured with a vacuum-tube voltmeter across a 10,000-ohm resistance in the output. The B and C voltages for the amplifier tube are chosen so that at no point does it overload. The change in amplifier plate current from zero beat to full output is negligible. The output is, as may be seen, constant to within ten per cent, over the entire range from 30 to 10,000 cycles. The values of resistance R3, given in Fig. 2 for the coupling unit, is rather important, and should be adhered to if nearly constant output is desired. For instance, when the grid leak was changed from .25 to 1 megohm, and other conditions unaltered, the output voltage at 300 cycles was 18.2 and at 10,000 cycles 10.5 — a variation of over 75 per cent. LAYOUT OF THE APPARATUS THE layout of the instrument calls for a Cardwell type 192E, 0.0005-mfd. maximum, taper-plate condenser for tuning each oscillator. In operation of the instrument, one condenser is set at or near zero dial reading, and the other turned to give zero beat; in other words, the second condenser is used for trimming. Then from these settings the first condenser is varied to give the beat frequencies. The beat, or audiofrequency calibration for settings of the 192E condenser is plotted in Fig. 5. This beat curve starts from zero beat at 5 on the dial. At the low end of the spectrum the calibration is rather too steep to permit of accurate setting at these frequencies. A Pilot straight-line-frequency con BEHIND THE PANEL The two oscillators are at the left and right of the main panel. The detector and audio tubes are mounted in the center. The shielding panels are cut and flanged from copper plate