Radio Broadcast (May 1929-Apr 1930)

Record Details:

Something wrong or inaccurate about this page? Let us Know!

Thanks for helping us continually improve the quality of the Lantern search engine for all of our users! We have millions of scanned pages, so user reports are incredibly helpful for us to identify places where we can improve and update the metadata.

Please describe the issue below, and click "Submit" to send your comments to our team! If you'd prefer, you can also send us an email to mhdl@commarts.wisc.edu with your comments.




We use Optical Character Recognition (OCR) during our scanning and processing workflow to make the content of each page searchable. You can view the automatically generated text below as well as copy and paste individual pieces of text to quote in your own work.

Text recognition is never 100% accurate. Many parts of the scanned page may not be reflected in the OCR text output, including: images, page layout, certain fonts or handwriting.

To illustrate the use of these relations the detector characteristics of a Cunningham type c-327 tube will be calculated. A grid current-grid voltage curve of a typical c-327 tube is shown in Fig. 1. Reading the grid current at intervals of 0.025 volt along this curve, and dividing the current change into the voltage change per interval gives a good approximation to the value of the grid resistance (rg) at the mid-point of the interval. This data is plotted in Fig. 2. Two scales from zero to one and from zero to ten megohms have been used to extend the accurately readable range. A similar procedure is applied to the rg curve. Increments of grid resistance are divided by increments of grid voltage and these data are plotted as the rg' curve shown in the insert graph of Fig. 2. At any given operating point the factor for the internally generated ^ j j^-^voltage of rectification can be evaluated readily from these curves. For convenience this factor is plotted against grid-bias voltage in Fig. 3. The operating point can be determined by drawing a line with slope equal to the grid-leak resistance and intersecting the 35 40 45 50 PLATE VOLTAGE Eb Fig. 5 Ec axis at a voltage equal to the biasing potential. On filament-type tubes the positive filament or A terminal gives a bias of plus 5 or 6 volts. The internal contact potential of the c-327-type tube causes sufficient grid current flow with the connection returned to the cathode. The intersection of the grid-leak line and the grid current curve determines the operating point. In Fig. 1 these points have been located for grid leaks from 0.5 to 5.0 megohms. Referring to the corresponding points in Fig. 2 the grid resistance is found to range from 67,000 to 450,000 ohms. This accounts for the broad tuning in the stage feeding a grid-leak detector. In a later section, it will be shown that the smaller size grid leak is advantageous in reducing the loss of the high audio frequencies in the grid condenser and in maintaining good detection with large signals. Assuming a grid-leak of one megohm, a load resistance of 50,000 ohms, and 20 per cent, modulation, the detected audio voltage can be calculated from equation (6). The results are shown below. The last column shows data obtained by actual measurement with a 675-kc. carrier modulated 20 per cent, with 60 cycles. The measured data is about 15 per cent, low due to a loss of radio-frequency voltage in the leads from the signal generator and in the grid condenser. I'sually the measured and the calculated data are in close 0.9 0.8 0.7 0.6 0.5 0.4 0.3 "Grid Leak.'Detector.Audio Freq.Output ~ vs Radio Freq.Carrier Input . C-327 Tube No.2 . .Ef =2.5 Volts / Eb-45 Volts — ti RADIO FREQUENCY CARRIER-R.M.S.VOLTS Fig. 4 agreement for small signal voltages. An increasing error is evident with larger signals. A.F. Modulation = 0.20 at 60 cycles Rg: = 1.0-megohm grid leak Rp = 50,000-ohm plate load Ep =+45 volts Carrier Ec rg A.F. Measured Volts Volts Ohms Volts a.f. r.ni.s. d.u. r.m.s. r.m.s. Volts ( ed+v'T) r.m.s. 0.020 —0.88 127000 0.0034 0.040 —0.88 127000 0.0134 0.0115 0.060 —0.89 138000 0.0290 0.0255 0.080 —0.89 138000 0.0516 0.0445 0.100 — 0.90 150000 0.077 0.067 0.140 —0.93 188000 0.138 0.113 0.170 —0.94 202000 0.202 0.150 0.200 —0.95 218000 0.285 0.175 The data above is plotted on logarithmic coordinate paper in Fig. 4. The dotted line has a slope of 2, representing a true square-law detector. The measured data follows the square law until an amplitude of nearly 0.100 volt is reached when the measured output increases at a lower rate than the square law. Since equation (6) is the first term of an infinite series resulting from an expansion of a function in the region of a point by Taylor's Theorem, it is only a first order approximation and accurate only for amplitudes over which the slope-derivatives remain constant. If the current characteristic is parabolic over the range of operation the second derivative is a constant and rectification would be proportional to Plate Rectification Factor (Total)= [<M j$') + <fc #'>] Vs ' Plate Voltage Per Cent Mu Detection vs Plate Voltage 40 35 2 o 32 g _J 28g the square of the signal voltage. In addition to the amplitude limitation on the characteristic of Fig. 3 the d.c. component of the rectification causes an increase in the bias voltage developed across the grid leak. Also a small decrease in mu and increase in plate resistance occur. The plate detection which is approximately in phase opposition to the grid detection may become large enough to cause a small reduction of the detected voltage. All of these factors contribute to the reduction of the detected audio output voltage when the signal is large. Plate Circuit Detection Inserting a bias of — 4.5 volts between grid leak and cathode would bring the grid circuit to a point of operation well beyond grid current cut-off. Signal amplitudes having a modulated peak from 3.0 to 3.5 volts could be applied without causing any flow of grid current. Assuming 45 volts of plate potential, the plate circuit operating point will be well down on the curved part of the characteristic. Taking increments of current and voltage the plate resistance (rp) curve 0.20 0.15 0.10 0.05 0;50 0.100 0.150 0.200 0.250 0.300 R.F.CARRIER-PEAK VOLTS Fig. 6 may be obtained. The mu curve may be read on a Miller bridge. This curve can be obtained very accurately by taking the slopes of plate voltage-grid voltage curves plotted for constant plate current as the parameter. In the usual range of operation these curves are straight fines for many tube types. When this relation holds the mu is a function of plate current only. This relation does not hold with the type c-327 tube though the method is useful for indicating the mu variations. The curves of Fig. 5 for rp and [j. were obtained by taking increments on the curves. The factors for the plate circuit rectification due to the plate resistance variation and to the mu variation are plotted in Fig. 5. Assuming a load resistance of 50,000 ohms, an effective plate voltage of 45 volts, — 1.5 volts grid bias, and 20 per cent, modulation, the detected audio voltage is calculated from equation (4). The results for several signal amplitudes added to the term ^-p^ of equation 12 < are shown in Table I. The term ^5 30 35 40 45 50 55 ' 60 PLATE VOLTAGE Eb Fig. 7 (4). The contribution due to mu variation is about 16.7 per cent, of the total detection. The detector output for a signal of 0.2 volts r.m.s. is only 6.1 per cent, the output at this voltage when grid detection (.Continued on page 2^2) 2 4 0 • • AUGUST 1929 •