Measurement Computing WBK17 - Manual (Page 14)

WBK17, pg. 14 987996 WBK17, Counter/Encoder Module
Period Mode
This mode allows for period measurement of the channel input. The measurement period is the time from
edge-to-edge, either both rising or both falling. Period data is latched as it becomes available and the data
is sent to the WaveBook at the scan rate. Therefore, if the scan period is much faster than the input
waveform, there will be a great deal of repetition in the period values. This repetition is due to the fact that
updates take place only when another full period becomes available.
Period Mode
An example: One channel’s acquired data might be: 0,0,0,0,80,80,80,80,79,79,79,79,81,81,81,81,…..
This data represents the number of ticksize intervals counted during the period measurement. The first
value(s) returned will be zero since the counters are cleared at the beginning of the acquisition. The data
comes in sets of four since the scan period is about one-fourth as long as the input channel’s period. Every
time the period measurement is latched from the counter, the counter is immediately cleared and begins to
count the time for the subsequent period.
If the scan period is a lot slower than the input period, the acquired data will be missing some periods.
To obtain greater resolution, you can increase the scan period, or use an averaging option (see OPT[1:0]).
The data returned is interpreted as time measured in ticks. There are four timebase settings: 20 ns, 200 ns,
2 µs, and 20 µs. These are often referred to as tick-sizes. The WBK17 uses a 50 MHz, 10 ppm oscillator
as a timing source.
PERIOD: OPT[1:0]: Determines the number of periods to time, per measurement. This makes it
possible to average out jitter in the input waveform, sampling error, noise, etc. There are four options:
(1) The channel’s measurement is latched every time one complete period has been observed.
(2) The channel’s measurement is latched every time that 10 complete periods have been observed,
the value that gets returned is equal to 10 consecutive periods of the input channel.
(3) The number returned is 100 consecutive periods.
(4) The number returned is 1000 consecutive periods.
PERIOD: OPT2: Determines whether the period is to be measured with a 16-bit (Counter Low), or
32-bit (Counter High) counter. Since period measurements always have the “stop at the top” option
enabled, this option dictates whether the measurement has a range of 0 to 65535 ticks or 0 to 4,294,967,295
ticks.
PERIOD: OPT4: Allows the mapped channel to gate the counter if desired. When the mapped
channel is high, the counter is enabled. When the mapped channel is low, the counter is disabled, but still
holds the count value. The mapped channel can be any other input channel or a detection signal.