Projection engineering (Sept 1929-Nov 1930)

Projection Engineering, May, 1930 Page 9 1 in: 1 1 , ' 1 1 i : 1 . ' i , i . Fig. 5. One method of scanning. photo-sensitive material sensitive over the entire range of light wavelengths but with a marked peak toward the ultra-violet end of the spectrum. They are exceptionally quiet in operation. Contrary to first thought, the circuit into which the cell is operated has a marked effect upon the quality of the picture as the dynamic characteristic of the cell exhibits a hyperbolic curvature where impressed light flux is plotted against output. Tims SLZ e = 1 — LGZ L = Light flux S = Sensitivity (slope of the light flux/anode current curve ) G = Conductance Z = Load Impedance — the treatment employed by Metcalf, I.R.E. Vol. 17, No. 11. Therein is given the computations for harmonics present in the output. To avoid harmonic distortion the load impedance must be kept small, thus lowering the possible output. Capacity effects also require a low resistive impedance if cutoff of the high frequencies is to be avoided. The best d-c. potential should be ascertained by trial and will be substantially below that at which ionization occurs. Double Amplifiers The amplifiers used in feeding the transmitters are constructed in two units ; the photocell amplifiers which feed approximately one volt into a 500-ohm transmission line and which are mounted together with the photoelectric cell as a fixed portion of each scanning system, and a main amplifier which feeds the grid of a 1 k.w. modulator tube. The problems encountered in the design of these amplifiers were many. It was necessary, in order to attain the desired results, to obtain a gain frequency characteristic flat within twenty per cent, from 15 cycles to 50 kilocycles. A curve taken on one of these circuits is shown in Fig. 8. In the design of an a-f. amplifier for a frequency range of from 10 cycles to 50 kilocycles there are but two considerations which govern the frequency characteristic. First, the lowfrequency cutoff is determined by the Fig. 6. Scanning by means of intense "flying spot" light. point at which the voltage across the grid becomes appreciably attenuated due to the grid resistance and the reactance of the coupling condenser heing in series. The high-frequency cutoff is a function of the tube and stray capacitances and is amenable to calculation where they are known. It is necessary only to maintain the active circuit impedances low so that the strays will be negligibly high up to the required high-frequency point. In other words, the chosen circuit parameters must remain constant in value over the required bond. If these requirements are met it may be postulated that the da/dco or "envelope delay" will also not deviate throughout the amplifier circuit. To this end it is important that the active elements in the circuit remain resistive at all frequencies and that no attempt to equalize internally by minor resonance effects be made. Fig. 7. Photoelectric cells. Equalization of the transmission line and of the calculated discrimination due to the physical aspects of the scanning system is carried out in each line as is any equalization of phase displacement (da/do) necessary. It has been found necessary to measure accurately the phase distortion present and to hold the displacement linear to within a few microseconds at the higher frequencies. Figs. 9 and 10 show the distortion evident in the transmission of a simple geometric figure. This particular figure is one which has been used by many investigators and is ideal for observation of improvement of phase and frequency distortion. The measurements of phase distortion are made by the Nyquist method, familiar to most communication engineers and will not be described in detail. The transmission lines within the plant have not presented any major difficulties. Coupling to the lines is, in most cases, through a transformer designed for the purpose and employing a core of specially rolled 1 mil high permeability core material. The coils are universal wound in pies and are so disposed as to obtain almost perfect coupling. No d-c. is permitted to flow in the primaries and a curve