Page 349 - The Mechatronics Handbook

FIGURE 17.1 An oscillating sine wave.

FIGURE 17.2 Measurement using a time interval counter.

The test signal for time measurements is usually a pulse that occurs once per second (1 pps). The
pulse width and polarity varies from device to device, but TTL levels are commonly used. The test signal
for frequency measurements is usually at a frequency of 1 MHz or higher, with 5 or 10 MHz being
common. Frequency signals are usually sine waves, but can also be pulses or square waves. If the frequency
signal is an oscillating sine wave, it might look like the one shown in Fig. 17.1. This signal produces one
cycle (360∞ or 2π radians of phase) in one period. The signal amplitude is expressed in volts, and must
be compatible with the measuring instrument. If the amplitude is too small, it might not be able to drive
the measuring instrument. If the amplitude is too large, the signal must be attenuated to prevent
overdriving the measuring instrument.
This section examines the two main specifications of time and frequency measurements—accuracy
and stability. It also discusses some instruments used to measure time and frequency.

Accuracy
Accuracy is the degree of conformity of a measured or calculated value to its definition. Accuracy is
related to the offset from an ideal value. For example, time offset is the difference between a measured
on-time pulse and an ideal on-time pulse that coincides exactly with UTC. Frequency offset is the difference
between a measured frequency and an ideal frequency with zero uncertainty. This ideal frequency is
called the nominal frequency.
Time offset is usually measured with a time interval counter (TIC), as shown in Fig. 17.2. A TIC has
inputs for two signals. One signal starts the counter and the other signal stops it. The time interval
between the start and stop signals is measured by counting cycles from the time base oscillator. The
resolution of a low cost TIC is limited to the period of its time base. For example, a TIC with a 10-MHz
time base oscillator would have a resolution of 100 ns. More elaborate TICs use interpolation schemes
to detect parts of a time base cycle and have much higher resolution—1 ns resolution is commonplace,
and 20 ps resolution is available.

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