Measuring the Frequency Accuracy and Stability of WWV and WWVH
Michael A. Lombardi
Radio station WWV, famous for the "at the tone ...." announcements broadcast at the top of each minute, is known to shortwave listeners and radio amateurs worldwide as a trusted source of accurate time. However, you might not know that the original purpose of WWV was not to broadcast time, but instead to provide standard frequency signals. The standard frequency service began on March 6, 1923, with signals then broadcast in the LF and MF bands. This was when the radio industry was brand new, and when having an accurate frequency reference was absolutely essential to the fast-growing number of stations, all of which needed to stay "on frequency" to avoid interfering with each other. A century later, the standard frequency signals broadcast in the HF bands remain essential, not just to radio broadcasters, but also to calibration laboratories, space weather researchers, and radio amateurs. This article examines the frequency accuracy and stability of the WWV and WWVH signals, both as transmitted and as received.