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Detailed introduction
The positive input accepts and counts signals in the +150 mV to +10 V amplitude range. These signals can be positive unipolar pulses or bipolar pulses. For bipolar pulses, only the positive lobe portion is counted. The positive input includes a discriminator with an adjustable threshold from 150 mV to 10 V. In many cases it can be used as a precision discriminator. Only those pulses whose amplitude exceeds a positive threshold are counted.
A negative input is provided to count NIM standard fast negative logic pulses in the -600 to -1800 mV amplitude range. The negative input threshold is fixed at -250 mV. Pulses as narrow as 4 ns can be counted via this input.
A front panel switch selects the rate meter's response time, which determines the random error in the measurement. There are three response times: fast, medium, and slow. When measuring the steady-state count rate of randomly arriving pulses, the standard deviation of the instantaneous meter reading is <1% at the "Slow" response setting, <3% at the "Medium" response setting, and <10% at the "Fast" response setting.
At low count rates, the settling time for 1% accuracy with slow response times can be very long. To overcome this limitation, the Model 661 Count Rate Meter includes a special fast response circuit. Using this feature, measurements can be started when the response switch is in the "fast" position. After the meter has stabilized, move the response switch to the "Medium" position and then to the "Slow" position. Because fast, medium and slow response times are maintained at a ratio of 1:9:100, this technology significantly reduces the time required to stabilize to 1% accuracy.
The rear panel analog output is used to connect a strip chart recorder. Full-scale output is selectable from 100 mV, 1 V, or 10 V. A ±10% trimpot is provided for calibration of this output.
