Radio Broadcast (May 1928-Apr 1929)

TWO SCREEN-GRID TUBES ARE USED IN THE INTERMEDIATE AMPLIFIER OF THIS SUPER-HETERODYNE UNIT The unit has been devised for use with an existing short-wave receiver (such as that described in the previous article), converting it into a remarkably efficient short-wave super-heterodyne, especially suited for code signals A Superheterodyne for Short* Wave Code Signals By Lloyd T. Goldsmith THE standard receiver used for short-wave code reception usually consists of a detector (such as that described in the preceding two pages) and one stage of audio-frequency amplification, although sometimes a second audio stage is used in an effort to secure more volume. In the latter case, unless a selective amplifier is used, the noise level is amplified in about the same proportion as the received signal, which is undesirable, as the real aim is to amplify the signal and not the noise. This difficulty suggests that the signal be amplified at a radio frequency rather than at an audio frequency in an effort to amplify it more than the noise background. Radio-frequency amplification at very high radio frequencies has not been found very satisfactory but if the signal be changed to a radio frequency of the order of 20 to 100 kilocycles, amplification is not only satisfactory for the signal gain alone but from the point of view of increasing the signalto-noise ratio. This is what is done in the superheterodyne. In addition, if the radio-frequency amplifier stages are sharply tuned, the selectivity of the receiving apparatus as a whole is very materially increased. With the increase in the number of transmitting stations operating on the shorter waves, the need is becoming greater for a receiver giving all possible practical selectivity. In an attempt to realize these requirements the super-heterodyne described in this article was built. Super-heterodyne receivers using 201 -a type tubes with impedance-coupled and tuned transformer-coupled intermediate-frequency amplifiers built and tested at the Massachusetts Institute of Technology have been found very much worth while in the reception of long-distance short-wave telegraph signals. Upon obtaining the new screen-grid tubes, a super-heterodyne was constructed at the Institute using these tubes as the intermediate-frequency amplifiers. The construction of this receiver is described here. The arrangement, as will be seen by referring to Fig. [, consists of two stages of intermediatefrequency amplification using screen-grid tubes, a beat frequency oscillator, a detector, and one stage of audio amplification, using 201-A tubes. The unit as a whole is intended to follow any short-wave receiver, using the latter's detector tube as an autodyne frequency converter, and changing the signal frequency to 50 kilocycles. The super-heterodyne unit is particularly adaptable to the short-wave circuit described on the previous two pages. The first intermediate-frequency stage has a tuned transformer input circuit. The transformer, T, has an air core and is a spool made of f" diameter bakelite having a winding space f" wide by f " deep. The secondary winding consists of 1000 turns of No. 28 d.c.c. wire. Over this are placed a few layers of paper to prevent possible grounding of the two windings. Over the paper the primary is placed, consisting of 250 turns of the same wire. The secondary can be tuned to an intermediate frequency of 50 kilocycles by means of a 500-mmfd. variable condenser or to 30 kilocycles if an additional fixed capacity of 1000 mmfd. is shunted across the variable condenser. With a given plate voltage the voltage amplification of the screen-grid tube is directly proportional to the effective impedance which can be built up in its plate circuit. (See "Characteristics of shielded-grid Pliotrons," by A. W. Hull and N. H. Williams, and " Measurements of HighFrequency Amplification with Shielded-Grid Pliotrons," by A. W. Hull. Both of these papers appeared in the Physical Review, April, 1926, Vol. 27.) This fact should be kept in mind when choosing values of inductance and capacitance to be used as a coupling impedance. In this case the coils, L, are No. 85 Samson choke coils which can be tuned from 20 to 50 kilocycles by a 500mmfd. variable condenser. Honeycomb coils were tried as coupling impedances and slightly greater amplification was obtained, but because of the ease of mounting and compactness of the Samson choke coils, they were used in the final receiver. These chokes are helical wound and although their direct-current resistance is higher than that of a honeycomb coil having the same inductance, their effective resistance at radio frequencies is much less. The variable condensers are provided with insulating shafts to keep the radio-frequency circuits as far away from the panel as possible and all except the first are mounted on the shields with insulating pillars. The interstage coupling condensers, d, are of 2000-mmfd. capacity and the grid leak, Ri, is of 100,000 ohms. The Bbattery voltage is 135 volts and the screen-grid voltage is 67.5 volts. The filament voltage is reduced to 5 volts by means of a 2-ohm master rheostat in the positive A-battery lead and the filament voltage of the screen-grid tube is further reduced to 3.3 volts by 1 5-ohm fixed resistances, R, placed in each of their negative filament leads. The grid returns are brought to the battery sides of these resistances giving a grid bias, equal to the drop in the resistance R, of approximately 1.7 volts negative. For grid detection in the second detector the grid condenser is of 2000-mmfd. capacity and the leak can be from 4 to 8 megohms where the plate voltage is 45 volts. Provision is made for a C battery if plate detection is used. A small amount of regeneration or resistance neutralization is introduced into the plate coupling impedance of the second intermediatefrequency stage to secure increased amplification in that stage. The effective shunt impedance of the parallel circuit, consisting of the plate coil and condenser, is limited by the resistance of the circuit. The introduction of regeneration reduces this effective resistance, giving a greater effective impedance in the plate circuit, with 15