Radio Broadcast (May 1929-Apr 1930)

RADIO BROADCAST .X" the detector is biased by its own plate current. In calculating overloading of this type the peak a.c. voltages on the grid and plate must be considered in addition to the changes in d.c. voltage. The grid voltage goes positive from the dynamic d.c. operating point by the amount of the amplitude of the modulated carrier voltage. At the same time the instantaneous plate voltage may swing below the dynamic d.c. point by the amount of the audio peak output voltage. To avoid overload distortion the minimum instantaneous plate voltage should always be greater than the maximum instantaneous grid voltage. When a screen-grid tube is used the minimum instantaneous plate voltage should always be about ten volts above the screen-grid voltage to avoid distortion. This type of overloading may be avoided easily by a suitable choice of operating voltages. The desire to realize the full advantage of the high-output detector has, in some instances, resulted in operation too near the overload point. While it is entirely practical to eliminate the first a.f. stage, the use of a low-gain a.f. stage permits operation well below the overload point. Since it is difficult to design an a.f. transformer that will give high-quality reproduction with a biased detector, impedance coupling is preferable. The first a.f. stage may be coupled through a relatively inexpensive yet high-quality, one-to-oneratio a.f. transformer to a single or pushpull power stage. In this way the overload capacity of the detector and a.f. system may be increased greatly, especially with the push-pull power stage. Plate Vs. Bias Voltages Plate voltages and corresponding bias voltages satisfactory for the type c-327 tube as a bias detector are: ■ — i~ I =3 1*— tso h.ShuntedN v | loo ti.iiriunr.ee; "v > J by 0.25 megohm • , Plate Voltage 45.0 90.0 135 . 0 180.0 250.0 (max.) Bias Voltage —5.0 —10.0 —15.0 —20.0 —30.0* *The 250-volt maximum is recommended only when, with normal maximum signal, the d.c. plate current does not exceed 5 raA. High-Output Figs. 3 and 4 show detector sensitivity curves for types cx-322 and c-324. Bias detection is recommended as most satis 50 100 ~ 200 500 1000 2000 FREQUENCY (CYCLES) Fig. 8 factory with either tube. The screen-grid types combine the advantages of sensitivity and high output. Radio-frequency inputs of a few volts can be applied without overloading. Outputs large enough to operate a cx-345 directly from the detector are readily obtained. To show the actual performance of the detector stage the signal was applied to a c-324 r.f. stage with the tuned secondary of the r.f. transformer coupled to the de 10.000 C-327 Bias Detection RJ Mnput-A.F.Outp Curve-Eg-Er A 45 -5 ut •Poir — B 90 -10 — C 180 -20 ts at which Ig st£ Modulation=0.22 rts — PU te Load =200,000 mpedan ohms ce 2 4 6 8 10 12 RADIO-FREQUENCY INPUT IVOLTS R.M.SJ Fig. 10 RADIO-FREQUENCY INPUT (VOLTS R.M.S.) Fig. 9 tector, as in Fig. 5. The detector plate load impedance was 200,000 ohms, (power factor 0.96; d.c. resistance 3000 ohms.) In Fig. 6 the sensitivity with the c-327 grid-leak detector is shown by curve a. Curve b shows the sensitivity with the c327 bias detector, and curve c with the c-324 bias detector. All three curves were obtained with the same signal at the input to the r.f. stage. It is evident that both the sensitivity and selectivity is decidedly superior with the c-324 tube. The difference in selectivity is shown best in Fig. 7. The operating conditions are the same as for Fig. 6, except that the r.f. input was increased until the same a.f. output was obtained at resonance. If a horizontal line is drawn through these curves at 0.5 of the maximum ordinate, the frequency width between intercepts on this line is a measure of the selectivity. The width of curve a for the c-327 grid-leak detector is 57 kc. For curve b, the c-327 bias detector, the width is 22. kc, and for curve c, the c-324 bias detector, the width is 12.5 kc. The load in Figs. 6 and 7 consisted of five Amertran 854 type choke coils in series, and shunt by an 0.25-megohm resistor and an 0.0001-mfd condenser. The fidelity characteristics are shown in Fig. 8. The a.f. output in millivolts is plotted against the audio frequency for a constant r.f. signal with constant modulation. Curve a is for the c-327 grid-leak detector using a onemegohm grid leak, a 0.00025mfd. grid condenser, and the primary of an Amertran transformer (first-stage type) for the plate load. The plate-cathode r.f. bypass was a 0.0005-mfd. condenser. Curve b is for the c-327 as a bias detector in the same circuit but with the grid leak and condenser removed and an appropriate bias voltage on the grid. A greater loss of low frequencies occurs with the bias than with the grid-leak detector. The output from the c-324 bias detector is shown by curve c. For this curve the plate load was approximately 150 henries shunted by a 0.25-megohm resistor. The plate-cathode r.f. bypass was 0.0001-mfd. The characteristic is uniform except for the large loss below 200 cycles. The use of a 500-henry choke shunted by a 0.25megohm resistor is recommended as a satisfactory load for the c-324 bias detector. A choke employing a series of pie windings to reduce distributed capacity is preferable. One that does not saturate with 1.5 to 2.0 milliamperes is adequate. Input-Output Characteristics In Fig. 9 the a.f. output voltage is plotted against r.f. input voltage for the c-327 tube operating as a high-output bias detector. The modulation was 22 per cent, and the plate load was 200,000 ohms. Fig. 10 shows the input-output characteristics of several tubes operating as highoutput bias detectors. Curves a, b, and c are for the c-324 tube with screen-grid potentials of 25 volts, 45 volts, and 75 volts, and bias potentials of -2.5 volts, -4.5 volts, -7.5 volts, respectively. The plate supply was 180 volts, the load impedance 200,000 ohms, and the modulation 22 per cent. Curve d shows the output from the c-324 self biased by its plate current through a 25,000-ohm resistor. Curve e shows the output from a type cx-322 tube at 180 volts on the plate, 45 volts on the screen-grid, and -9.0 volts of grid bias. Curve f is for the c-327 at 180 volts on the plate and -20.0 volts grid bias. The points at which grid current starts to flow (for 22 per cent, modulation) are marked. Fig. 11 shows the output obtainable (Concluded on page 364) B 0.160 1 1 1 RF.Input A.F.Output Small Signal Detection Curve Tube EB Ec. Ec, A C-327 .«cid&t-~oo&cf B C-324 »180 *45 -4.5 C C-324 '180 *75 7.5 D C-327 • 45 6 V nfd. / r. Modula 1SU lion =0.22 zu /s A. — E ) 0 04 0 08 0. 2 0 6 0 20 0 24 RADIO-FREOUENCY INPUT (VOLTS R M.S.) Fig. 11 352 • • OCTOBER 192 9 •