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www.thinkSRS.com Measurements with the SR620 Application Note #2 In order to realize the full potential of the SR620 Universal Pulse Width Measurement Time Interval and Frequency Counter, a brief look at its several modes of operation and some of their applications is Magnetic and optical memory disk data is stored using appropriate. It is also important for a user to understand the different modulation schemes to minimize disk real-estate and performance specifications of the SR620, in order to draw maximize signal-to-noise ratio. For example, compact disk reasonable conclusions from experimental data. How accurate players use 3-11 modulation to obtain very high disk density. is a particular measurement? What errors can be expected? This scheme produces data patterns with nine different pulse This application note defines, in detail, the performance widths (corresponding to 3, 4, 5, ...11 consecutive 0's). The specifications and terminology for each mode of operation. SR620 can be used to measure these pulse widths and their This information will enable the user to fully understand the variations, and display them graphically in histogram form on capabilities and limitations of the instrument. an X-Y oscilloscope. The first section of this application note describes the type of Rise and Fall Time Measurement measurements that the SR620 can perform and gives some application examples. It will give the reader an appreciation When analyzing the transition time required for a for the wide range of applications in which the SR620 least-significant-bit change in a DAC (digital to analog Universal Time Interval and Frequency Counter can be used. converter), the 10 % to 90 % rise time of that transition is of importance. Once the rise time has been established, the The second section (SR620 Specification Guide) explores the small-signal frequency response of the DAC can be calculated specifications of the SR620 in detail. A precise definition of (bandwidth=0.35/rise time). The SR620 allows the user to set measurement accuracy (resolution and error), along with the start and stop voltage thresholds, in rise and fall time explanations of technical terminology, is given for each mode measurements, so that any part of a transition may be of operation. analyzed. Applications of the SR620 Frequency and Period Measurement The SR620 Universal Time Interval and Frequency Counter When measuring the quality of a reference frequency source, has been used to measure everything from the propagation the jitter (standard deviation or Allan variance) is often of delays of integrated circuits, to the distance to the moon. Its