Radio Broadcast (May 1928-Apr 1929)

MAY, 1928 A SUPER-HETERODYNE FOR SHORTWAVE CODE SIGNALS 17 400 350 300 o 250 S 200 49 50 51 FREQUENCY-KILOCYCLES FIG. 2 Showing the voltage amplification for a single without regeneration and for different tubes and EXAMINING THE CURVES AN EXAMINATION of the amplification curves of Fig. 2 shows that at 50 kilocycles the maximum amplification obtained in one stage with the plate impedance used is 155 without regeneration and 370 with regeneration. By amplification is meant the ratio of the voltage appearing across the plate impedance to the voltage impressed on the grid. The curves also indicate the sharpness of tuning of a single stage. The amplification obtained in both stages of intermediate frequency is rather difficult to measure accurately because it is so great but is in the order of 25,000 with regeneration. The amplification obtained in the stages of intermediate frequency of other super-heterodynes built at Massachusetts Institute of Technology using UX-201-A tubes has been (see Fig. 2) in the order of 25 for one tuned amplifier stage and 16 for one stage of impedance-coupled amplification with high-mu tubes. (These data were presented in an unpublished report of Green Radio Research, Massachusetts Institute of Technology). A compromise had to be made between greater amplification secured by increased regeneration, and sharpness of tuning. In the present arrangement the amplifier is not well adapted to the reception of voice and music as the width of the band of frequencies passed in a single stage is not more than 1400 cycles. Too much stress cannot be laid upon the need of proper shielding when using the screen-grid tube. The grid leads from the tubes must be shielded from all the plate circuit apparatus of the same tube. The covers of the shields would be much better flanged to eliminate cracks after the cover is put on. The holes for the wires should be as few as possible and no larger than necessary All radiofrequency circuits should be kept within the shields, by the use of chokes and large bypass condensers in the plate circuits particularly. The amplification curves were obtained by the substitution method using a vacuumtube volt-meter. A 50kilocycle voltage from an oscillator is impressed by means of a calibrated resistance on the input of the single stage and a reading is taken on the vacuum-tube voltmeter connected in the plate circuit of the detector. Then the stage is cut out and a second voltage is impressed upon the input terminals of the detector of such a value as will give the same reading on the voltmeter. This input voltage will be larger than when the stage of amplification is used and if the current through the variable resistances is kept the same in both cases, the voltage impressed will be directly proportional to the corresponding values of resistance. The ratio of the value of resistance used without the stage of amplification to the value of resistance used with the stage of amplification gives the voltage amplification of the stage. All measurements are made with the oscillator tube removed from its socket and a resistance of 10,000 ohms is placed in series with the test leads from the calibrated resistance to represent the plate resistance of the first detector which is normally shunted across the primary of the input transformer. As has been mentioned, the amplifier is for code reception with any autodyne receiver and it may either be connected in place of the primary of the audio-frequency transformer of the receiver, or more simply by connecting its input terminals in series with the positive detector Bbattery lead of the receiver. The operation of the complete receiver is no more complicated than the operation of the r.f. stage with or forms of coupling usual two-control short-wave outfit. The first three dials of the amplifier are set at about the same values and a station tuned-in on the receiver while the oscillator dial is varied to give the desired note. The first three dials are readjusted carefully for maximum volume. Now the entire amplifier can be left untouched, all the tuning being done with the tuning and regeneration controls of the short-wave receiver. The regeneration switch on the amplifier can be used as a rough volume and selectivity control. If the receiver described by the author on pages 13 and 14 is to be operated with the screengrid super-heterodyne unit, the following statements hold. Because of the fact that the variable condenser across the secondary of the input transformer of the super-heterodyne unit is mounted directly on the aluminum shield, the ground post of the shields of the super-heterodyne cannot be directly grounded to the positive A battery but can be done so through a i-mfd. fixed condenser. This is necessary because the copper shield of the receiver is necessarily grounded to the positive side of the A battery while the first aluminum shield of the "super" has been connected to the negative side of the battery. When using the two sets together, then, the positive side of the A battery is grounded and the shields of the "super" are connected to the positive A binding post through the large condenser. The following parts were used in the construction of the super-heterodyne unit: List of Parts T — Special Input Transformer (See Text) Ti — Samson Audio Transformer, 6-1 Ratio L — Two Samson No. 85 Choke Coils Li — Five Samson No. 3 Choke Coils L2 — 25-Turn Tickler Coil (See Text) L.3 — Special Oscillator Coil (See Text) C — Four 500-mmfd. National, Variable Condensers Ci — Four 2000-mmfd. Sangamo Fixed Condensers C2 — Three 1000-Mmfd. Sangamo Fixed Condensers C3 — Five i-mfd. Dubilier Fixed Condensers R — Two 15-Ohm Carter Fixed Resistances Ri — 100,000-ohm Tobe Resistor with Mounting R2 — 4-Megohm Durham Grid Leak with Mounting R3 — 2-Ohm General Radio Rheostat Five General Radio Sockets Yaxley Filament Switch K— Yaxley S.P.D.T. Jack Switch Carter Midget Open-Circuit Jack Ten Eby Engraved Binding Posts Four 4" National Velvet Vernier Dials White Pine Baseboard 235" by 10" by \" Bakelite Panel 24" by 7" by Ty Aluminum Shields, Extension Shafts. Insulating Pillars, Angles, Wood Screws, Etc A PANEL VIEW OF THE SHORT-WAVE "SUPER UNIT Despite the number of dials on the panel, the receiver is no more difficult to tune than the ordinary two-control short-wave receiver. After the dials on the super-heterodyne unit have once been set, all the tuning is done by means of the controls of the short-wave receiver which precedes the "super"