INTRODUCTION The B & K-PRECISION Model 1820 80 MHz Universal Counter is a high quality, lightweight, autoranging Frequency counter It measures frequencies in the range of 5 Hz to 80 MHz. In addition, the Model 1820 is capable of measuring the period required for one cycle to occur, accumulated cycles and time. The functions of the Counter are selected by the four FUNCTION pushbutton switches on the front panel. When the ACCU (accumulate) or TIME function is selected, the display may be reset to ZERO by momentarily depressing the RESET pushbutton. The AUTO/1 SEC (100 period average) switch selects either a one-second preset gate interval or the “AUTO” range. When the switch is in the “AUTO” position (out) the gate interval or period average that allows maximum resolution without overranging is automatically determined and the appropriate frequency/ time unit indicator for either KHz/juS or MHz/mS is illuminated. When the AUTO/1 SEC pushbutton is in the 1 SEC (depressed) position, the display indicates frequency to the closest hertz or period to the nearest nanosecond even if the most significant digit (MSD) is beyond the display range. When the X1/X10 pushbutton is in the XI position (out), the input signal is applied to the Counter circuits without attenuation. However, when the switch is depressed, the input signal is routed through an X10 attenuator before being applied to the Counter circuitry. When a function switch is pressed the first time, it locks in the “in” position. When another function pushbutton is pressed, all other function pushbuttons return to the “out” position. The RESET pushbutton is a spring-loaded momentary contact switch that is independent of all other pushbuttons. The standard input impedance of one megohm in conjunction with a ten-to-one scope Probe makes this instrument ideal for use in applications where source loading must be kept to a minimum. The display consists of six seven-segment light emitting diode (LED) numerical display units and three LED’s to indicate KHz//xS, MHz/mS and OVER (overrange). The internal 10MHz time base is generated by a crystal controlled Oscillator However, it may be desirable to use an external time base for certain Counter applications. Consequently, the EXTERNAL TIME BASE INPUT connector has been included. This female BNC jack, on the rear panel, provides a convenient means of connecting the external time base to the Counter In addition, the TIME BASE SELECT switch, also on the rear panel, allows the user to select either the internal or external time base. The optional Temperature Compensated Crystal Oscillator (TCXO) is available for use as an internal time base with the Model 1820 The TCXO is a highly stable crystal-controlled Oscillator that produces a closely controlled time base frequency over a wide temperature range. When the TCXO is used with the Counter the TCXO replaces the original time base Generator in the 1820 The TCXO may be installed by the user. The exceptional accuracy of the Counter makes it an extremely valuable instrument to the scientist, engineer and experimenter. Its ruggedness and compactness make it practical for use by the hobbyist or in the field by service technicians. The low power consumption of less than 15 watts makes it practical to power the Counter by a DC-to-AC power Converter