FIGURE 1. An oscilloscope’s
display reveals signal
details of a largely analog
nature, such as rise and
fall times, amplitude, and
other subtle characteristics.
capture many data points
over a short span. This
allows measurements of
signal transitions (edges),
transient events, and small
time increments. While
DSOs can view the same
digital signals as a logic
analyzer, most DSO users
concentrate on analog
measurements such as rise- and fall-times, peak amplitudes and the elapsed
time between edges. Figure 1 illustrates
the oscilloscope’s strengths. The waveform (though taken from a digital circuit)
reveals the analog characteristics of the
signal, all of which can have an effect on
the signal’s ability to perform its
function. Ringing, overshoot, roll off in
the rising edge, and other aberrations
appearing periodically exist here.
With a modem oscilloscope’s
built-in tools such as cursors and
automated measurements, it’s easy to
verify signal integrity problems impacting your design. Purely analog signals —
such as the output of a microphone or
digital-to-analog converter — can only
be monitored with an instrument that
records analog details (see Figure 2).
Logic Analyzer Details
More on the Digital
The DSO is for general-purpose
signal viewing. Its sample rate (up to 20
Gs/sec) and bandwidth enable it to
FIGURE 2. A family of modern high
performance Tektronix DSOs.
The most obvious difference
between these two instruments is the
number of channels (inputs). Typical digital oscilloscopes have up to four signal
inputs. Logic analyzers have between 34
and 136 channels. Each accepts one
digital signal. Some complex system
designs require thousands of input channels. Appropriately scaled logic analyzers are available for those tasks, as well.
A logic analyzer detects logic
threshold levels (see Figure 3). When
the input is above the threshold voltage,
the level is said to be “high” or “1;” conversely, the level below the threshold
voltage is a “low” or “0.” When a logic
analyzer samples an input, it stores a 1
or a 0, depending on the level of the
signal relative to the voltage threshold.
A logic analyzer’s waveform timing
display is similar to that of a timing diagram found in a data sheet or produced
by a simulator. All of the signals are time-correlated and effectively show progressive system “snapshots” through time.
A logic analyzer’s digital design
verification and debugging features —
such as sophisticated triggering —
allow you to specify the conditions
under which the logic analyzer
acquires data. High-density probes
and adapters simplify connecting to
the SUT. Analysis capabilities translate
captured data into processor instructions and correlate it to source code.
FIGURE 3. The logic analyzer
reveals a more limited picture
of a signal’s characteristics —
just logic levels relative to a
threshold voltage level.
Logic Analyzer Architecture
There are four steps to using a