The PM 6667 and PM 6668 are microcomputer based frequency counters, spanning a frequency range of 10 Hz ... 120 MHz (PM 6667) and 10 Hz ... 1 GHz (PM 6668). The use of the microcomputer allows a new approach in frequency measurements, that eliminates the traditional ±1 cycle error. By making a multiple period measurement and computing the reciprocal value, these counters perform high resolution frequency measurements on low frequency signals. Another microcomputer feature in these counters is the automatic range selection. The measuring result is always displayed with maximum resolution without overflow and with proper indication of Hz, kHz, MHz and decimal point. There is choice between two measurement rates; NORMAL with 7-digits resolution every second or FAST with 6 or 7- digits resolution every 200 ms. The fast mode is used for measuring changing frequencies as with tuning. The following options are available: a more stable time base version with TCXO (/02 version), a rechargeable battery unit PM 9601 that can be mounted inside the counter, an impact resistant (ABS) protective carrying case PM 9602 and a 19" rack/panel mount adapter PM 9603. The 7-digit liquid crystal (LCD) display contains also the unit and decimal-point indicators. After you switch on the counter, a self test is executed. Should an error be detected, it is shown on the display by a diagnostic code.