SRS SR620 Time Interval/Frequency Counter

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The Stanford Research Systems SR620 Time Interval Counter performs practically all the time and frequency measurements required in the laboratory or ATE environment. The instrument's high single-shot timing resolution, low jitter, and outstanding flexibility make it the counter of choice for almost any application.

Time Measurements
You can measure time intervals with 25 ps rms resolution, making the SRS SR620 one of the highest resolution counters available. Time intervals up to ±1000 seconds can be measured with 50 ps relative accuracy, and 500 ps absolute accuracy.

Measurement Flexibility
The SR620 also measures frequency (up to 1.3 GHz), pulse-width, rise and fall time, period, and phase. Frequency can be measured with gates from 1 µs to 500 s, making the SR620 suitable for applications as varied as the measurement of short-term phase locked loop jitter, to the study of the long-term drift of atomic clocks. Up to 11 digits of frequency resolution for a 1 s measurement are provided. All measurement modes are supported by a wide variety of flexible arming and triggering options.

Complete Statistical Calculations
Statistics are automatically calculated on sample sizes from one to one million. The mean, standard deviation, Allan variance, minimum or maximum, can all be displayed on the front panel or quickly dumped to your computer via the standard GPIB and RS-232 interfaces.

Graphics and Hardcopy
Unlike conventional counters that only have numeric displays, the SR620 provides live, graphical displays of measurement results. Histograms and stripcharts can be displayed on any oscilloscope with an X-axis input, or output to HP-GL compatible plotters and dot-matrix printers. Convenient on-screen cursors and display annotation facilitate the analysis of measured data.

Measurement Modes
Time: In its most basic measurement mode, the SR620 measures the time interval between a start and a stop pulse. Either of the SR620's two inputs, or its REF output may be selected as the source of start and stop pulses. Internal and external gating signals can be used to holdoff the acceptance of either start or stop pulses. The SR620 can make both positive time measurements, (in which the stop pulse follows the start pulse), or negative time measurements (in which the stop pulse occurs before the start pulse).

Frequency: The SR620 measures frequency by the reciprocal frequency counting technique. In other words, the instrument measures the time interval for some integer number of input cycles, then computes frequency by dividing the number of cycles by the time interval. Since no fractional cycle measurements are involved (as would be the case if the instrument measured the number of cycles in a fixed time interval), extremely high frequency resolution can be achieved with up to 11 digits for a 1 second gate. The diagram below illustrates this method of computing frequency. Fixed and scanning internal gates are provided, or the gate can be triggered externally, or delayed by an adjustable amount from an external trigger.

Pulse Width: The width of pulses at either input can be measured. Separate start and stop voltages can be selected for pulse width measurements. Resolution and accuracy are the same as time measurement mode.

Transition Time: Rise and fall times of either input may be measured. Start and stop thresholds may be set between ±5 V with 10 mV resolution. The 350 MHz input bandwidth allows measurements of rise and fall times as small as 1 ns.

Period: The SR620 can also measure the period of waveforms. Period is measured similarly to frequency, but the reciprocal of frequency is computed and displayed.

Phase: The phase angle between signals on the A and B input can be measured with 0.001 degree resolution. You can measure the phase of signals (at the same frequency) from 0.001 Hz to 100 MHz in frequency. The counter actually makes two measurements: a frequency measurement of one channel, and a time measurement of the delay of the second channel with respect to the first. The phase is then computed as shown below.

Event Counting: The SR620 will also count transitions (events) at either of its inputs. As with all the other modes, event counting may be gated internally or externally, and both the voltage threshold and slope for a transition are adjustable. Event rates up to 300 MHz can be counted, with up to 12 digits of output. The unit also has a ratio mode which will compute the ratio of the number of events counted on the A and B inputs.

Reference Output
A precision, 1 kHz, 50% duty cycle square wave is available at the front panel REF output. The REF output can be used as a source of start or stop pulses for any of the SR620's measurement modes. For instance, the length of a cable connected between REF and the B input can be precisely determined by measuring the time delay between REF and B.

Statistical Calculations
The SR620 can make measurements on a single-shot basis, or calculate the statistics of a set of measurements. Sample sizes from 1 to one million can be selected. The SR620 will automatically calculate the mean, standard deviation or Allan variance, minimum, and maximum for each set of measurements.

Histograms and Strip Charts
The ability to generate live, graphical, output makes the SR620 stand out among time interval counters. Graphical output of measurement data is available in three formats: a histogram showing the distribution of values within a set of measurements, a stripchart of mean values from successive measurements, or a stripchart of jitter (standard deviation or Allan variance) values from successive measurements. A new histogram or stripchart point is generated after each set of measurements is completed. Up to 250 stripchart points, or histogram bins, can be displayed. Data for all three graphs is continuously saved, so you can view any of the graphs by cycling through selections on the front panel.

Both histograms and stripcharts can be displayed on any oscilloscope with an X-axis input (see the pictures below), or can be plotted on an HP-GL compatible plotter or dot-matrix printer. Convenient cursors allow you to read the value of any data point on the histogram or stripchart. Autoscale and zoom features make it simple to display all, or any portion, of the graphs.

Built-In Auto-Calibration
A sophisticated, built-in, auto-calibration routine nulls insertion delays between start and stop channels, and compensates for the differential nonlinearites inherent in analog time-measurement circuitry. The autocalibration routine takes about two minutes to perform and should be run every 1000 hours of operation.

Choice of Timebases
The choice of timebase affects both the resolution and accuracy of measurements made with the SR620. (See Application Note #2 for a detailed discussion of this issue.) The SR620 lets you choose between an economical standard timebase with an aging coefficient of 1x10-6 /year, or an optional ovenized-oscillator timebase with only 5x10-10/day aging, and about an order of magnitude better short-term stability than the standard timebase. A rear-panel input lets you connect any external 5 MHz or 10 MHz source as a timebase.

Configuration Menus and Scanning
Many measurement control parameters are typically set once and then rarely re-adjusted. These include the communications setup (GPIB and RS-232), the graphic output controls, the scan and D/A controls, and the self calibration menus. The SR620 lets you access these functions through convenient front-panel configuration menus. By scrolling through the different choices in each configuration menu, users can quickly and easily configure the instrument for their individual test conditions.

Computer Interfaces
Standard GPIB (IEEE-488) and RS-232 interfaces allow remote control of the instrument. All instrument functions and configuration menu settings are accessible via the interfaces. A fast binary dump mode outputs up to 1400 measurements per second to a computer. A parallel printer port allows direct printing from the instrument. Standard IEEE-488.2 communications are supported, and plotter outputs are provided in HP-GL format making interfacing simple and easy. For convenient debugging, the last 256 characters passed over the interfaces can be viewed on the front panel.

Rear Panel Features
Two rear-panel DVMs make measurements of DC voltages with 0.3% accuracy (up to 20 Volts). These values may be read via the interfaces or displayed directly on the front panel. Two rear panel outputs continuously provide voltages proportional to the mean and the jitter of the measurement sample. These 0 to 10V outputs can drive strip chart recorders, or can be set to provide fixed or scanned output voltages.