Projection engineering (Jan 1932-Mar 1933)

JANUARY, 1932 Page 9 microvolts per meter). The variation in sensitivity over the broadcast band (550 to 1500 kilocycles) is 5 db., which is more than adequate for either local or distant reception. Fidelity of reproduction is one of the distinctive features of the receiver. Its overall audio characteristic shows a band from 25 to 7,000 cycles with only a 4 db. variation. A band width adjustment consisting of a low-pass filter, incorporated in the detector output circuit, will reduce the band width as shown in Fig. 3. This adjustment will permit operation through abnormal interference or noise. The automatic gain control circuit will hold the output level constant within 6 db. over a range of input signal of approximately 60 db. Fig. 4 shows the characteristics of the automatic gain control circuits indicating that an increase in the strength of the incoming signal from 100-100,000 microvolts (60 db.), will increase the audio output only 6 db. The automatic gain control circuit operates essentially by changing the sensitivity of the receiver in accordance with the variations in the intensity of the received carrier. A sensitivity switch is incorporated in the receiver to prevent the overloading of the gain control system from a signal of greater than 150,000 microvolts in the antenna. This switch permits a 34 db. reduction in signal input voltage to the first r-f. tube. A visual tuning meter permits a more accurate means of tuning than by aural methods. Description of Circuits Input : The antenna input circuit consists of resistances Rl, R2, and R3, which are so arranged as to provide for a constant input impedance of about 1400 ohms irrespective of the tuning of the receiver. The 10-A receivers Fig. 5, used in the Waldorf-Astoria are energized from a low impedance transmission line and the coupling between this line and the first filter is effected by inductance LI and the transformer L2. The primary of L2 is in series with LI while the secondary forms part of the first tuned circuit. The constants of these coils provide a substantially constant voltage step-up ratio of 7-1. The first tuned circuit is formed by condenser CI, the secondary of the transformer L2, and L3 and C9. The second tuned circuit consists of condenser C2, the inductance coil L4, and L3 and C9. Thus, L3 and C9 are common to both tuned circuits of the first band-pass filter or preselector. It is this combination of mutual inductance and capacity for the common circuit of the filter, that effects constant band width throughout the broadcast range. Interstage Coupling The interstage coupling between the first and second radio-frequency amplifier tubes and between the second and third tube, is a similar band-pass filter except that the connection to the plate circuits of each tube is made by means of a three-circuit transformer. This transformer is designed to give substantially constant gain throughout the tuning range of the receiver. The third radio frequency amplifier works into an input transformer, the secondary of which connects to the grids of the detector and automatic gain control tubes which are connected in parallel. Detector Circuit The three-electrode tube used as a detector is operated at a moderately high level. The automatic gain control circuit maintains a constant level to the input of the detector which is of the plate demodulator type, thus affording a large undistorted output. The output transformer and output circuit network transforms the detector output impedance to 200 ohms thereby permitting direct connection to the audio amplifiers used in p-a. and radio distribution systems. Output Circuit The potentiometer PI, connected directly to the secondary of the output transformer affords a continuous vari i»o r WTJV GROUND SIDE 00+I.2UIL5 R20 2000 U T To25MF 0.25 UF ui L" CI8 0.25 MF =i=C22 -> 0.25MF< Rl8 ,100,000 <-< Fig. 5. Schematic diagram of No. 10-A radio receiver.