Brochure

Backgrounder
Thus the EQUAL TO pulse width trigger allows the oscilloscope do a logic analyzer’s job to
some extent.
BUILT-IN COUNTER TAKES FREQUENCY MEASUREMENTS TO THE NEXT LEVEL
Automated frequency measurements have been part of the DSO feature set almost from the
beginning. Normally these derive the period by examining the first full cycle of the captured
waveform. This is a powerful tool to measure the characteristics of one-time events, but lacks
the ability to extract a high accuracy average frequency value for a continuous waveform.
A different approach to frequency measurements is embodied in the common frequency
counter, generally available and often fairly inexpensive (though never free). This approach can
be implemented in a DSO by using the trigger signal as a source for average frequency readout,
a new feature now finding its way into some low-cost DSOs. Frequency counters use many
different means to make their measurements. The most common, simple solutions are the fixed
frequency counter, where the number of input cycles are counted over a fixed interval of time
(displaying the count) and the fixed period counter where the number of clock cycles are
counted during one period (displaying the reciprocal of the count). Both solutions offer excellent
accuracy when the number of accumulated counts is high, and poor accuracy when low. A
related solution divides the measurement interval in half. For the first half, it counts both time
base and stimulus counts. Once the half way point is reached, the next terminating transition
(same polarity as the starting transition) on either input completes the measurement. The result
is then calculated. This solution does not offer the maximum possible accuracy at the extreme
cases, but consistently provides approximately ½ the maximum possible accuracy, providing a
stable, at-a-glance indication (to six-digit precision) of the frequency of valid trigger events.
Since virtually any event (within reasonable amplitude limits) can be a trigger event, the
“readout” is in effect actually a useful general-purpose frequency counter.
In embedded system troubleshooting, it is often necessary to check the frequency of various
local clock signals, including the master crystal oscillator. Here, the oscilloscope’s trigger
counter offers a quick built-in solution for the problem, as in the TDS2000 Series. The
measurement is more accurate than that of the waveform-based automated frequency
measurement, and it eliminates the need to set up a separate instrument for frequency
counting.
The counter can also help in the search for crosstalk and noise sources. For example a noise
signal on a bus trace, identified by the counter as having frequency of 100 kHz, might point
toward crosstalk or grounding problems in the area of the switching power supply. Similarly, a
noise signal with a frequency ½ that of the master clock might indicate crosstalk from an
adjacent bus.
Because it is in fact derived from the trigger signal, the counter can detect the frequency of any
conditional trigger event, not just the once-per-cycle voltage triggers. Combined with the pulse
width trigger, for instance, the counter can determine the frequency with which a specific pulse
width occurred within a varying stream of pulses.
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