The XCU-303 Deluxe Crystal Calibrator Unit is designed expressly for use with your NC-303 receiver. In addition to providing crystal controlled marker frequencies every 100 kilocycles over the entire tuning range of your NC-303, the Deluxe Calibrator provides for direct calibration of the 100th harmonic of the 100-kilocycle frequency against WWV (10 mc). The calibrator consists of an electron-coupled 100-kilocycle crystal oscillator, and a 10-megacycle mixer. The mixer converts the beat between WWV and the 100th harmonic of the oscillator to 7.07 megacycles. The frequency is marked in red on the 40 meter dial scale of your NC-303.

INSTALLATION: The XCU-303 Deluxe Crystal Calibrator is installed in the NC-303 receiver by plugging the unit into the Crystal Calibrator Socket, X-1, on top of the chassis. A short antenna (10 to 30 feet long, depending upon geographical location) should be connected to the Antenna binding post on top of the XCU-303 Calibrator.

Note: The XCU-303 should not be used on 6, 2, or 1 1/4 meters because the calibrator does not supply an output through the converters.

Source: The National NC-303 Instruction Manual

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