Unit Oscillator For The To Mc Range

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Experimenter Index 4

• THE TYPE 1211-A Unit Oscillator, latest addition to the rapidly growing line of General Radio Unit Oscillators', extends the frequency range covered by the versatile units downward by a factor of 100. This new oscillator has a frequency span of 0.5-50 Mc, which is covered in two logarithmic ranges. The output power is well over one

»Eduard Karplus, <'V-H-F and U-H-F Unit Oscillators," General Radio Experimenter, Vol. 24, No. 12, May, 1950.

A. G. Bousquet,' 'A New Unit Oscill ator — 50 to 250 Mc," General Radio Experimenter, Vol. 27, No. S, January, 1953.

Figure 1. View of the Type 1211-A Unit Oscillator.

»Eduard Karplus, <'V-H-F and U-H-F Unit Oscillators," General Radio Experimenter, Vol. 24, No. 12, May, 1950.

A. G. Bousquet,' 'A New Unit Oscill ator — 50 to 250 Mc," General Radio Experimenter, Vol. 27, No. S, January, 1953.

Figure 1. View of the Type 1211-A Unit Oscillator.

watt over the 0.5-to-5 Mc range and is at least 200 milliwatts over the 5-to-50 Mc range. The frequency is indicated directly on a six-inch dial, and approximate increments of frequency expressed in percentage are given on a 3^-inch slow-motion-drive dial.

To avoid the necessity of frequent range switching, the tuning ranges have been made as wide as possible. The span of each of the two ranges in the Type 1211-A Oscillator is 10 to 1. This wide frequency range is obtained by varying simultaneously the capacitance and the inductance as the frequency dial is turned. A frequency change of about 5 to 1 is due to the variable capacitor, and the remaining 2 to 1 frequency change results from inductance variation.

Electrical Circuit

The Type 1211-A Unit Oscillator uses a Hartley circuit with a Type 5763 oscillator tube. This tube type was selected because it can handle all the power provided by the Type 1203-A Unit Power Supply. The output circuit is coupled inductively to the oscillator tuned circuit and includes a voltage divider as output control.

An audio oscillator can be connected to terminals in series with the plate supply for direct amplitude modulation of the oscillator. A convenient audio source is the Type 1214-A Unit Oscil lator which yields about 25 per cent modulation at 400 or 1000 cycles. The envelope distortion at this modulation level is around two to four per cent, depending on the carrier frequency.

Since modulation is accomplished directly in the oscillator circuit, some unwanted frequency modulation is unavoidable. Amplitude modulation practically free of frequency modulation can be obtained at carrier frequencies above 10 Mc by using the Type 1000-P6 Crystal Diode Modulator.

Construction

The oscillator circuit is assembled on an aluminum casting with filtering components mounted within the casting. A spun-aluminum cylindrical cover completes the shielding much more effectively than would a conventional rectangular dust cover. The output control and the coaxial output connector are at the rear of the cover; the entire assembly is mounted on an L-shaped panel and base.

The tuning capacitor and the switch contacts are the same as those used in the Type 1001-A Signal Generator and Type 1330-A Bridge Oscillator.

The 0.5- to 5-Mc and the 5- to 50-Mc oscillator coils are arranged in a plane perpendicular to the tuning capacitor shaft, and range selection is obtained by switching in the appropriate coil by means of a rocker arm on the panel. The

Figure 2. Elementary Schematic Diagram of the Type 1211-A Unit Oscillator.

Figure 2. Elementary Schematic Diagram of the Type 1211-A Unit Oscillator.

frequency ranges are engraved at the arm extremities, and the main frequency dial shields one end of the arm so that only the range selected is indicated.

The core assembly, which helps to produce the wide frequency span, is concentric with the coils and is mounted to turn with the capacitor shaft. This assembly consists of a dust core and an aluminum core, both of a sickle shape, to produce a smooth transition from full iron-dust core for maximum inductance through a minimum of core to a full aluminum core for minimum inductance. The cores and the tuning capacitor plates are shaped to yield an approximate logarithmic response of frequency with angular rotation over the 10-to-l frequency span. (See Figure 3.) The iron-dust core increases the circuit Q, while the aluminum core reduces it.

Features

The major features of this oscillator are the compact unit design, the 10-to-l frequency range for each switch position, and the approximately logarithmic frequency response of the main dial with a smaller auxiliary dial indicating frequency increments of 0.2% per division. The 874-type coaxial output system and the effective shielding add appreciably to the usefulness of the instrument. All power leads are carefully filtered, and the dial shaft is enclosed within a

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