The Basic Functions of Counting Systems

The Basic Functions of Counting Systems

There are five general classes of counting systems: counters, timers, ratemeters, multichannel scalers and time digitizers, and digital signal averagers. Counters simply count the number of input pulses received during the counting period. ORTEC offers four of the five. Counters simply count the number of input pulses received during the counting period. Timers count pulses generated by an internal clock and are used to measure elapsed time or to establish the length of the counting period. Ratemeters provide a meter readout of the pulse count rate and convert this frequency to a DC voltage or current proportional to the average count rate per unit of time. This is normally expressed in counts per second (counts/s).

Two types of ratemeters are available from ORTEC: linear scale (Model 661), or combined logarithmic and linear scales (Models 449 and 9349). The Model 449 has positive and negative inputs with an optional audible alarm. The Model 9349 has a rear-panel, fast, negative input.

All ORTEC NIM timers contain preset controls to establish the duration of the counting period. When counting is initiated, the internal clock pulses are counted until the preset condition is reached; at that time, counting is stopped in all counters connected to the common gate line of the master timer. If the external input is used, the preset control will apply to counting of the pulses at the external input and will result in preset count operation.

Multichannel Scalers counts the number of events that occur during the time interval t to t + Δt as a function of time. The interval Δt is called the dwell time. The time t is quantized into channels or bins by the relation t = i Δt, where i is the bin number (an integer). Dwell times can be selected from fractions of a nanosecond to hours, and the total number of bins ranges from 4 to 67 million. When the scan is started, the MCS begins counting input events in the first channel of its digital memory. At the end of the selected dwell time the MCS advances to the next channel of memory to count the events. This dwell and advance process is repeated until the MCS has scanned through the preset number of memory channels. The display shows a graph of counts versus dwell channels from zero time on the left to end of the scan on the right.

Time digitizers measure the arrival time of individual events with a quantization precision, Δt, selectable from picoseconds to nanoseconds. The time spectrum is represented by t = i Δt, where i is the bin number (an integer). The number of bins is typically on the order of 65,000 to 67 million. Time digitizers can measure the time spectrum with a quantization precision that is at least three orders of magnitude smaller than is offered by multichannel scalers. This exceptional advantage in time resolution is sometimes gained at the expense of pulse-pair resolution. For example, the MCS-pci delivers a 6.7-ns pulse-pair resolving time, whereas the Model 9308-pci time digitizer is limited by interpolator dead time to 50 ns.

Both multichannel scalers and time digitizers are triggered by a start pulse that defines zero time. Typically, the start pulse corresponds to the stimulation of a process. The “products” of the process are counted as a function of time by the MCS, or their emission times are measured by the time digitizer. In a single pass through the selected time span, both instruments can record multiple events. If the process is repeatable, the data from multiple passes through the time span can be summed to improve the statistical precision. The final result is a histogram showing the probability of observing the product events as a function of time.

Multichannel scalers often have the advantage over time digitizers when dwell times much larger than a few nanoseconds are required, particularly in applications that demand pulse-pair resolving times much better than 50 ns. A time digitizer is the preferred solution when the measurement demands sub-nanosecond time quantization.

The Model 9353 straddles the time digitizer and MCS functions. It behaves like a time digitizer with 1-ns pulse-pair resolving time for 100-ps digital resolution. For digital resolution >1 ns it acts like an MCS, counting multiple stop pulses in a bin on each scan.