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The Stanford Research SR625 Frequency Counter is a NIST traceable frequency counting standard for calibrating base stations, transmitters and many other types of communication systems. It combines the high resolution and wide variety of features found in the SR620 counter with the atomic accuracy of a rubidium timebase. The SR625 Frequency Counter consists of a frequency counter (SR620), a high-accuracy rubidium timebase (PRS10), and a 2 GHz input prescaler. The combination of the SR620 and the prescaler allows direct frequency measurements up to 2 GHz with twelve digits of resolution in a 100 s measurement. The rubidium timebase ensures excellent short-term stability (< 2 × 10^-11 Allan variance (1 s)) and long-term drift (< 5 × 10^-11/month). The SR625's warmup time is less than ten minutes making it ideal for field applications. An additional back-panel output provides a rubidium stabilized 10 MHz signal which can be used to drive other test equipment (e.g., synthesizers or spectrum analyzers). The standard GPIB and RS-232 interfaces allow for complete control and data acquisition from any laboratory computer. The SR625's performance makes it the standard for remote applications or laboratory calibration. Specifications. Rubidium Timebase; Frequency: 10.000 MHz. One day stability 4 × 10^–11/day. Prescaler; Frequency ratio: 10:1. Input impedance: 50 ohm. Max. input level: +23 dBm. Input freq. range: 50 MHz to 2.2 GHz. Time Interval, Width, Rise and Fall Times; Range: -1000 to +1000 s in +/-TIME mode; -1 ns to +1000 s in all other modes. Trigger rate: 0 to 100 MHz. Display: LSD 4 ps single sample, 1 ps with averaging. Frequency; Range: 0.001 Hz to 300 MHz via comparator inputs. 40 MHz to 1.3 GHz via internal UHF prescalers. RATIO A/B range: 10^-9 to 10^3 . Period; Range: 0 to 1000 s, RATIO A/B range: 10^-9 to 10^3. Phase: Definition: Phase = 360 × (Tb - Ta) / Period A. Range: -180 to +180 degrees, 0 to 100 MHz. Counts; Range: 10^12, RATIO A/B range: 10^-9 to 10^3. Count: Rate 0 to 300 MHz.
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