Radio Broadcast (May 1929-Apr 1930)

100 90 £ 80 o a. 5 60 O I 50 5 40 s ° 30 Z £ 20 a? 10 0 Effect of Stagg -Tuning on side I Cutting.Colonia ered 1 Staggered Tuning and/ 1 V \ Receiver Model 1 t \ 1 1 1 I 1 — \ \ / \ \ ✓ ✓ \ Au dioSide BandRa — -10K.C. -v \ v\ s ' Fig. 2 essential qualities of the audio circuits under consideration. The detector output is represented by a generator having a voltage tj.1 Egi, the plate resistance of the tubes are represented by RPi and RP2, respectively. R and C, together, form the coupling impedance between the detector and the first audio stage, and L is the inductance of the primary of the second stage E coupling-transformer. The relation will Ei be represented by oti and ==r~ by <*>. When the plate resistance of the detector is very high with respect to its load impedance, as is the case in this particular instance where RP'i is in the order of several million ohms, the expression for the amplification in the detector circuit may be approximated with sufficient accuracy by the equation: (i) Rpi and that for the amplification in the first stage by Rpi(RP22 +(^1?)' The combined amplification of both circuits is obviously the product of ai, and 02. Referring to equations (1) and (2), the total amplification may evidently be expressed by the equation RP1(RP22 + G)2L2)* (3) Within limits, the amplification will be constant at all frequencies in the lower portion of the audio scale when (a>2R2 + C2 Transposing, (RP22 + ay2L2) = k u2R2 kw2L2 = kRP22 — g-2 (4) (5) which condition, when satisfied, will give the uniformity of amplification that is so desirable. Solving equation (5) for R, L, and C, and simplifying: R L Rp-aC L = CRRp2 L C = RRp2 Fulfilling these requirements actually peaks the first audio amplifier at the lower frequencies to a degree which accurately compensates the decreasing impedance of the second-stage transformer primary at the low end of the scale. The principal limiting factor in this scheme is the possibility of overloading the grid of the first a.f. tube by reason of the high amplitudes attained at low frequencies with this arrangement. In the Model 32 receiver, this is provided for by the use of a second-stage coupling-transformer having a characteristic that requires only moderate compensation in the equalizer. £ 40 c -1 1 1 1 1 1 Overall Fidelity C jrve FREQUENCYC.P.S Fig, 4 o s 1 I 1 I I Sensitivity Vs. Frequenc Colonial Receiver Model 32 y 600 700 800 900 1000 1100 1200 1300 1400 1500 FREQUENCY K1L0CYLESI Fig. 5 Am rtificai Co on Vb onial Freqi ?eceiv ency o sr Moc f Sing el 32/ e R.F. Stage' 500 600 700 800 900 1000 1100 1200 1300 1400 1500 FREQUENCY-K.C, Fig. 6 Amplification Vs. Frequency ' of Antenna Coupling Device Colonial Radio Corporation Model 32 A. C. Fig. 3 500 600 700 800 900 1000 11001200130014001500 FREQUENCY IN KILOCYCLES Fig. 7 The fidelity characteristic — antenna to output circuit — is given in Fig. 4. Sensitivity of the Set The sensitivity, in terms of microvolts per meter, averages about 2| microvolts over the entire broadcast band. The sensitivity curve is given in Fig. 5. It will be observed that it is practically flat throughout the entire range. This was accomplished with a unique coupling scheme be Det< i ctor iii Characteristic UY 224 Autc al Receiver Model 32 A. C. 1 J 1 1 J mati Bia 3 15 r ercen t Mo Julati sn 0 123456789 1C RADIO INPUT PEAK VOLTS Fig. 8 tween stages in the r.f. amplifier. This coupling, as indicated in the circuit diagram, consists essentially of a tuned r.f. transformer having a 450-turn primary with a 5-mmfd. capacity connecting the high side of the primary with the high side of the secondary. The primary, with its distributed capacity and circuit capacity in shunt is resonant at some point below 500 kc. At 1500 kc. the arrangement functions essentially as tuned-impedance coupling, as the greater part of the current flows directly to the high side of the tuned secondary. Radio-frequency filters in the leads supplying voltages to the screen-grid tubes are, of course, essential, for one of the • most serious causes of feed-back is conduction through common leads. Filters comprised of resistances in series and condensers in parallel were found to serve admirably. Several such filters are used in each tuned stage. Each filter serves three purposes. (a) (b) (c) As an r.f. filter As a means for securing a desired d.c. voltage drop As an a.f. filter, to reduce the line frequency ripple in the d.c. applied to the 224's. This last effect is particularly important in the case of screen-grid tubes as they have a tendency to rectify readily. Any ripple applied to their elements, especially their control-grids, modulates the signals and results in more disturbance in the output than would occur with any other type of tube. It is necessary to keep the peak value of the ripple on the plate below 0.15 volts, that on the screen grid below 0.05 volts, and that on the control grid below 0.03 volts. Figs. 6 and 7, respectively, indicate the gain per stage in the r.f. amplifier and the gain in the antenna circuit. The r.f. transformers are constructed as follows. The secondary consists of a single layer wound with 102f turns of No. 30 enamelled wire. The primary, consisting of 450 turns of No. 36 d.s.c. enamelled wire, is wound on a wooden bobbin inside one end of the secondary. Cross-Talk This term is used to describe the conch\ tion that exists when the same station is (Concluded on page 362) 40 35 30 a SSU 25 PRE 20 o ZD 15 O c/> 10 5 oj bouna rressure vs. frequency Colonial Receiver Model 32 A. C, \ FREQUENCY C.P.S. Fig. 9 • OCTOBER 1929 • • 359