Time Interval and Frequency Counter

10 Front Panel Operationto about 200mV rms. Both positive and negativesettings of the knob have the same effect. Thesensitivity adjustment is useful because atmaximum sensitivity the prescalers will selfoscillatewith no signal input. With an input,however, this is not a problem, but by reducing thesensitivity slightly these oscillations will disappear.INPUT COUPLINGThe A and B inputs may be either AC or DCcoupled by pressing the AC/DC button. Thecoupling is independent of the input terminationimpedance. The EXT input is always DC coupled.REFERENCE OUTPUTThe front panel REF output puts out a 1kHz 50%duty cycle square wave synchronized to theSR620's internal 10MHz clock. This output mayprovide 4V into high impedance or 2V into 50Ohms, or ECL levels into 50 Ohms. An exampleof the use of this output is to set the mode tomeasure the time from REF to B. If a cable is thenconnected from the REF output to B the cabledelay may be directly measured.TIMEBASE INPUT AND OUTPUTA rear panel BNC outputs the SR620's 10MHzclock. This output supplies approximately 1V pkpkinto a 50 ohm load. Another BNC allows theinput of a 5 or 10 MHz external timebase. Thisinput presents a 1 kOhm load to the signal. TheSR620 can then phase-lock its internal timebaseto this external source. See theCONFIGURATION MENU chapter for detail onusing an external timebase.DVM INPUTSThe SR620 has two rear panel DVM inputs.These 1 MOhm differential inputs allow theSR620 to measure DC voltages on either a 2V or20V full scale range. The SR620 can eitherautorange the inputs (default) or they may be setto a fixed scale. See the CONFIGURATIONMENU chapter for detail on setting the DVMscales.SR620 Universal Time Interval Counter