Full autoranging attenuators and timebase–a first for benchtop DSOs
Four digital storage scopes from Fluke Corp. (Everett, WA) become the first
benchtop DSOs to incorporate fully autoranging attenuators and time- base, a
feature that goes a long way toward allowing “hands-free” scope operation while
probing a circuit. These Fluke DSOs join the Tektronix TekMeter series of
handheld multimeter/DSO instruments as the only DSOs on the market offering
full autoranging capability.
The benchtop scopes are part of Fluke's series of CombiScopes (combination
DSO/analog oscilloscopes). Their autoranging capability works simultaneously on
more than one channel, making it particularly useful in measurement
applications where changes in input signals are continuously tracked.
Fully autoranging attenuators and timebase differ from “autoset,” a feature now
found on many scopes. Autoset operates only once when its corresponding
front-panel button is pressed. Autoranging, on the other hand, operates
continuously on both the attenuators and the timebase to maintain an optimal
signal display even when the scope's input signal changes.
Autoranging in the CombiScope is accomplished via a 40-MHz TM5320 digital
signal processor that analyzes the input signal's amplitude and the frequency
of the signal on the trigger channel. The resulting information is used to
update the scope's attenuators and timebase settings continuously.
The autoranging attenuator, operating independently of the autoranging time
base, keeps the input signal trace between 2 and 6 vertical divisions in the
traditional 1-2-5 steps. These settings are particularly well-suited for
probing unknown signals and for making adjustments to known signals.
The autoranging timebase operates in either of two modes. In the “steps” mode,
the scope autoranges in the traditional 1-2-5 timebase steps. As with
autoranging attenuators, this is very well-suited for making adjustments and
seeing their effect.
In the “variable” mode the timebase is used in variable time-per-division to
keep the same waveform display at all times. That is, the same number of signal
periods or part of a period on-screen, even when the signal frequency changes.
This can be very useful when testing at different frequencies, such as when
measuring frequency response.
A true variable timebase has been difficult to attain in DSOs because of the
“1-2-5 step” nature of traditional sample clock circuitry. In a DSO, the sample
rate and the timebase speed are interrelated. These benchtop CombiScopes have a
sampling system in which the clock frequency is programmable in very small
steps, making variable timebase operation similar to that of analog scopes
possible for the first time in a benchtop scope.
The CombiScope models that now incorporate full autoranging are two
four-channel benchtop models with 100 MHz and 200 MHz bandwidth (Models PM
3384A and PM 3394A, respectively) and two “2 + 2” channel benchtop models with
100- and 200-MHz bandwidth (Models PM 3382A and PM 3392A, respectively). Base
prices range from $5,175 for the 2 + 2 channel, 100-MHz PM 3382A to $6,675 for
the four-channel 200-MHz PM 3394A.
The TekMeter series of handheld, combination digital multimeter and DSO from
Tektronix, Inc. (Beaverton, OR) all incorporate full autoranging. The
TekMeter's autoranging 5-MHz DSO incorporates Tektronix' proprietary
signal-tracking technology that automatically finds, scales, and displays
signals continuously for hands-free operation. The TekMeter series DMM/DSOs,
which are focused heavily on the power and electrical industries, are quite
easy to use because of their autoranging capabilities, both in the DMM and
scope modes. Single- and dual-channel versions are available in the range from
about $800 to $1,200.
Will other scope manufacturers soon be introducing oscilloscope lines with
fully automatic ranging? While no others are talking about it yet, it seems to
be a good way to go in a test world where “easy to use” and “fast” are now
primary discripters of test equipment requirements.
For more information on the autoranging CombiScope, call Charles Holtom of
Fluke at 206-356-5802 or . For more on the TekMeter, call Tektronix at
800-426-2200, ext. 200 or .
–Warren Yates
CAPTION:
Full autoranging of attenuators and timebase make this combination
analog/digital storage oscilloscope from Fluke much easier to use.
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