The modulator was somewhat unconventional, consisting of a single tube which could be made to produce sine wave or pulse modulation. For sine wave modulation, a conventional Hartley circuit was used with the primary of the modulation transformer acting as the tuning inductor in the resonant circuit. For pulse modulation, the transformer was connected to form an oscillating circuit with an abnormally large amount of feedback and a natural frequency of about 7 kc, determined by stray capacitances. The grid resistance was so chosen that only one cycle of 7 kc oscillation occurred before the grid blocked. The first half cycle, which was positive, plate-pulsed the r-f oscillator. Although a long pulse was produced by this modulator, it was found to be adequate for many field testing purposes.
For maximum utility in its intended application, the instrument was de signed to operate at all of the supply voltages and frequencies then in use by the Allied military services: 80, 115, and 230 volts, at frequencies between 50 and 2600 cycles. Provision was also made for operation from external batteries. A selector switch made the proper connections for the various types of power sources, and the type of power required was indicated on a drum attached to the switch which was visible through an opening in the panel.
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