A unit for expressing frequency which is the number of times a wave-like radio signal changes from maximum positive to maximum negative charge per second. 1 Hz = 1 cycle per second. 1 kilohertz (kHz) = 1,000 Hz; 1 megahertz (MHz) = 1,000 kHz or 1,000,000 Hz; 1 gigahertz (GHz) = 1,000 MHz or 1 million kHz or 1 billion Hz. AMPS (analog) cellular phones in Canada and the US use the 800 MHz band. Digital phones use either the 800 MHz or 1900 MHz (or 1.9 GHz) frequencies. Specifically, CDMA and TDMA use either 800 or 1900 MHz; iDEN uses only 800 MHz; GSM uses only the 1900 MHz spectrum in North America. GSM uses 900, 1800, and/or 1900 MHz on other continents.