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Jan., 1946 PUSH-PULL FREQUENCY MODULATED CIRCUIT 51
the signal passes through a 3-position switch which selects the "flat compensator," the "orthacoustic," or the "no compensation" circuits. These positions can be used for different purposes as follows: "No compensation" is used for measurements with a vacuum tube voltmeter, for frequency response tests, and input versus output curves. "Flat" response can be used for monitoring on disks requiring that type of response; however, the important function of this position is for intermodulation tests. "Orthacoustic" can also be used for monitoring while the record is being cut, and is designed for that purpose.
To obtain consistent readings, it is important to have the oscillator tuned to the same frequency relative to the optimum point of operation of the discriminator circuit. To accomplish this, a meter is provided in the diode circuit. The oscillator circuit is tuned for maximum diode current and is then backed down, in the same direction each time, to 70 per cent of the peak current.
The use of this instrument and results of measurements are completely covered in the paper by H. E. Roys entitled "Experience with an FM Calibrator for Disk Recording Heads."5
ACKNOWLEDGMENT
The author wishes to acknowledge the helpful suggestions and advice of E. W. Kellogg, H. E. Roys, C. M. Sinnett, and A. C. Blaney in this work.
REFERENCES
1 BEERS, G. L., AND SINNETT, C. M. : "Some Recent Developments in Record Reproducing Systems," /. Soc. Mot. Pict. Eng., XL, 4 (Apr., 1943), p. 222.
2 TERMAN, F. E. : "Radio Engineering," McGraw-Hill (New York), 1937, p. 360; Dow, J. B.: "A Recent Development in Vacuum Tube Oscillator Circuits," Proc. I. R. £., 19 (Dec., 1931), p. 2095.
3 LANDON, V. D., AND MC!LWAIN, K.: "Fender's and Mcllwain's Electrical Engineers' Handbook (Electric Communication and Electronics)," John Wiley and Sons (New York), 3d Ed., p. 7.
4 FINK, D. G.: "Electronic's Engineering Manual," McGraw-Hill (New York), p. 240.
6 ROYS, H. E. : "Experience with an FM Calibrator for Disk Recording Heads," J. Soc. Mot. Pict. Eng., 44, 6 (June, 1945), p. 461.