The frequency standard 6689 is equipped with a highly stable rubidium oscillator. The frequency deviation due to aging is less than 0.001 ppm after 10 years, so that 6689 is suitable as a primary reference. The frequency standard was specially designed for use in test systems for telecommunications or in calibration laboratories, where particularly fast and economical calibration of several devices such as B. frequency counter and synthesizer is required. The standard output of 5 x 10 MHz and 1 x 5 MHz can optionally be expanded by five additional 10 MHz outputs.