Measurement Computing DBK Part 2 - Manual (Page 115)

The figure shows the effect of 10 ms debouncing on a noisy signal. To be counted, a rising edge must be
preceded by a low sustained for at least 10 ms without any other edges. Rising edges a and f are counted
because they are preceded by low signal levels sustained for at least 10 ms (the debounce time). All other
rising edges (b, c, d, and e) are ignored. Any falling edge makes (or keeps) the debounced output low,
regardless of preceding edges. Thus, the DBK55 can detect short pulses even with debouncing.
Frequency Measurement
After debouncing, the signal’s frequency is ready to be measured. Frequencies are measured to 12-bit
accuracy between a minimum frequency (F
min
) and maximum frequency (F
max
). This frequency range can
be programmed individually for each channel. The limitations on F
min
and F
max
are:
• The frequency range must be within 0 to 1 MHz.
• F
max
- F
min
must be at least 1 Hz.
• F
max
/ F
min
must be at least 100/99 (1.010101).
Based on F
min
and F
max
, the DBK55 measures the frequency by counting input cycles during a variable
time interval. The length of the interval depends on the difference between F
min
and F
max
.
• For a wide range (when F
min
and F
max
are far apart), each bit of the 12-bit result represents a
large frequency change and can be measured quickly.
• For a narrow range (when F
min
and F
max
are close together), each bit of the 12-bit result
represents a small frequency change and takes longer to measure.
The following equation determines the time interval needed to measure a frequency:
Minimum Measurement Period (sec) = (4096 x 0.5
µs) [F
max
/(F
max
- F
min
)]
In this equation: 4096 derives from 12-bit precision; 0.5 µs is the resolution of the DBK55’s timing
circuits; and F
max
/ (F
max
- F
min
) is the ratio the measurement time must be increased to achieve 12-bit
accuracy over the selected range.
To see how the measurement period varies, consider two examples:
• To measure frequencies from 59 to 61 Hz, the measurement period is at least
4096 x 0.5 µs x 61/2 = 62.5 ms, or about 16 measurements per second.
• To measure frequencies from 1 to 61 Hz, the measurement period is at least
4096 x 0.5 µs x 61/60 = 2.1 ms. Note that as the DBK55 only measures frequency once per
cycle, it would take from 1 to 61 measurements per second.
Thus, measuring frequencies over a narrow range takes longer than over a wide range as the ratio of
F
max
/(F
max
- F
min
). The actual measurement time is the sum of several items: the minimum measurement
period (from the equation above), the actual input period, and a variable processing time of 0 to 4 ms.
Note: If the Sequence Rep Rate is set faster than the measurement rate, multiple readings of the
same measurement will occur.
After the frequency (F) is measured to the required accuracy, it is scaled to a 12-bit number (D) for use by
the Digital to Analog Converter (DAC). This 12-bit number is determined by the formula:
D = 4096 [(F - F
min
) / (F
max
- F
min
)]; where: 0 < DAC < 4096
DBK Option Cards and Modules 988793 DBK55, pg. 5