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SR620 Time Interval & Frequency Counter SR620 Measurement Modes

Time Transition Time In its most basic measurement mode, the SR620 measures thetime interval between a start and a stop pulse. Either of the SR620's two inputs, or its REF output, may be selected as thesource of start and stop pulses. Internal and external gatingsignals 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) and negative time measurements (in which the stop pulse occurs before the start pulse). Frequency Rise and fall times of either input may be measured. Start andstop thresholds may be set between ±5 Vwith 10 mVresolution.The 300 MHz input bandwidth allows measurements of riseand fall times as small as 1 ns. The SR620 measures frequency by the reciprocal frequencycounting 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 (11 digits in 1 s). The diagram below illustrates this method of computing frequency. Both internal and external gates are supported.

RISE/FALL TIMES: +Time Arming

Pulse Width

STOPVSTARTVSTARTVSTOPV

The width of pulses at either input can be measured. Separatestart and stop voltages can be selected for pulse widthmeasurements. Resolution and accuracy are the same as timemeasurement mode.

SIGNAL ttrf

Period The SR620 can also measure the period of waveforms. Periodis measured similarly to frequency, but the reciprocal offrequency is computed and displayed. The phase angle between signals on the Aand B inputs can bemeasured with 0.001 degree resolution. You can measure the phase of signals (at the same frequency) from 0.001 Hz to 100 MHz. The counter actually makes two measurements: afrequency measurement of one channel, and a timemeasurement of the delay of the second channel with respectto the first. The phase is then computed as shown below.

FREQUENCY/PERIOD

Phase

SIGNALGATE
N cycles ∆
t 2nd pulse after falling edge2nd pulse after rising edge Freq = N/ ∆ t1
12 ∆ t/N 5ns
t 12

Event Counting

BGATEA ∆
tN cycles

The SR620 will also count transitions (events) at either of itsinputs. 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 resolution.The unit also has a ratio mode which will compute the ratio of the number of events counted on the Aand B inputs.

WIDTH: +Time Arming PHASE = 360 x ∆ t ∆ t /N
12 VV
STARTSTOP SIGNAL

T

W

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