I own two analog
scopes: a B+K Precision 100 MHz dual trace portable that I bought back in the
1980s and a Tektronix analog storage tube mainframe scope with high-performance
plug-ins acquired at an electronics flea market in the early 1990s for pennies
on the dollar. The Tektronix is a very heavy beast.
The other day,
I got an email from a test equipment dealer about a line of Chinese-designed
test equipment under the trade name Rigol. This company offers test equipment
at drop-dead prices compared to the big players in the U.S. and Europe. Its
product line runs from entry-level to mid-level equipment including DSOs
(digital storage oscilloscopes).
DSOs not much larger than multi-meters were introduced, they became an instant
must-have toy for some field service engineers. I was never impressed with
them. Most had hard-to-see monochrome LCD displays, as well as poor resolution
and frequency response due to insufficient memory and processing speed. Even
the early bench DSOs suffered from dead-band issues and other digitization
problems, making them less suitable than traditional analog scopes with CRTs
for many measurement needs.
matured, and it’s rumored that Rigol is the OEM manufacturer for Tektronix’s
entry-level DSOs among other test equipment products. A 100 MHz dual channel
DSO with a 1 Gigahertz sample rate, serious memory depth and lovely color LCD
display selling for just short of $400 would be amazing if it really worked.
DIGITAL VS. ANALOG
Rigol DS1102E Digital Storage Oscilloscope
automatic waveform acquisition
on screen measurements negate the need for a multi-meter
and setup storage to internal or external memory
accurate color LCD display
inexpensive; price/performance ratio is amazing
channels share one set of controls
display can be a bit choppy due to segmentation
is a little noisy
storage pouch for probes or other accessories
Contact: Rigol USA at (440) 232-4488 or (877) 4-RIGOL-1
or visit www.rigolna.com
When it arrived,
I put it through some paces that evening without even cracking the manual. It
is outstanding, The on-screen measurement information, the automatic scaling
for whatever signal you throw at it, the intuitive control layout and menus
make it a winner. It effortlessly displays 2–3 cycles of a 100 MHz RF carrier
while my so-called 100 MHz analog scopes struggle to paint viewable images.
have a fairly complex set of controls to set triggers, signal gain, position on
the CRT screen and relative calibration to a scale bezel. Try something a bit
more involved such as a measurement requiring dual time-base with delay so you
can simultaneously view a large and a small portion of a wave form, and you
have lots of controlsto adjust to
get it right: sweep start offset, sync level, trace delay, trace position,
intensity, etc. Even the simple act of displaying two waveforms at once
requires operator decisions on the analog scope. For low sweep rates, you need
to choose chop, rather than alternate to share the one CRT with two or more
input channels. For high sweep rates, you need to choose alternate else the
waveforms will be chopped into broken dashes. And if you need to store a
transient signal, your choices can be limited. With an analog storage scope,
the image would blur and fade as the seconds went by unless you had a screen
camera and film.
Or you might have had an analog scope
with an expensive but shallow digital memory and limited speed digitizer. The
top-of-the-line Tektronix analog scopes had measurement cursers and on-screen
display of channel gain and sweep delay if you equipped them with the deluxe
versions of their plug-ins.
everything from the get-go. The signals are digitized and stored. Changing
sweep rate is a memory record and readout issue. As a result, DSOs show lots of
on-screen numerical measurements in the voltage and time domains, and they can
do advanced math routines such as peak-to-average ratios, sums and differences
between signals, multiplication of signals and fast Fourier transforms
converting time domain to frequency domain displays. Press a button and in
seconds a DSO will acquire a signal and adjust all of its parameters for best
results. You can even view unrelated signals on both inputs by selecting
alternate in the trigger source menu. Then each waveform will have its own
sweep rate suited to its frequency.
If you are an
old-school engineer who learned how to explore electrical signals with a
conventional oscilloscope, a well-designed DSO will knock your socks off, but
only after you forget what you know about analog scopes. After a half-hour of
experimenting, I was able to put my new DSO through its paces without ever
cracking the full manual, which is not delivered in print form. Controls and
menus are quite intuitive.
MEASUREMENT SHOOTOUT PART I
with setting up a simple dual time-base mode on my analog scopes. I’ll admit to
being a bit rusty. It took me 10 minutes to get a stable pair of displays. The
same signals fed into the DSO took about five seconds to acquire including the
time to press the correct button in the right menu screen. No contest!
The front panel
of the Rigol DS1102E looks like a normal analog scope. It even has two groups
of identical looking controls that would normally be two signal input channels
of an analog instrument. One group controls horizontal activity, sweep rate or
display magnification in the time domain while the other controls amplitude of
the displayed signals. Waveform channel 1 or 2 amplitudes and positions are
controlled one at a time by selecting the CH1 or CH2 button. Note other models
give you two sets of vertical amplitude/scale knobs rather than sharing one,
but at the cost of other controls or increased front panel size.
horizontal scale knob and the display instantly changes to dual sweep mode,
where you can home in on a small portion of the waveform and magnify it and
display it below the main trace. No need to reset vertical position as it
happens automatically. There is hardly any need to fool with the trigger sync
levels, unless you want to intentionally offset the trigger point. It locks to
the waveform as stored in its buffer and the display parameters are calculated
mathematically on the fly.
Rigol looks like my B+K scope but with 3/4ths of its rear body sawn off. See
Fig. 1. The front panel has a similar size screen. See Figs. 2 and 3.
Fig. 1: Side View of Rigol DSO vs. Analog Scope
Fig. 2: Rigol Scope With Dual Waveform Dual Sweep
Fig. 3: B+K Scope With Two Waveforms
So, how does
its signal capture compare to an analog scope? My 100 MHz test signal is full
of AC hum due to some tired electrolytics in the generator’s power supply. The
B+K analog scope shows it as a fuzzy 100 MHz sine wave whose thickness varies
as you change the lock-in position of the trigger circuit. The Rigol DSO shows
it as a sharp 100 MHz sine wave that is drifting up and down at a leisurely
rate as though it were beating slowly with a 60 Hz component. Trigger point does
not change this display. Force the sweep down to a low frequency and switch the
sync to line frequency and voilà, you see a stable 60 Hz envelope on the RF. When
you make an adjustment there might be a very slight screen lag; after all, the
signal is being digitized and processed before it is converted into an image.
Most of the time, there is no noticeable lag. The only glaring exception is
when you remove a signal. The last sweep will remain on the screen for a couple
of seconds as the scope goes into hold mode while it waits for the signal to
come back. I suppose you could press the stop button and then save it before it
MSO PART II
There is one
drawback with DSOs, at least the low-cost instruments. The LCD screen
resolution cannot compete with an analog CRT. As a result, unless you display
as large a waveform sample as possible with a single cycle showing, you will
see stair steps in the trace, which could mask tiny glitches or make a clean
smooth signal look a bit dirty. Should you continue to keep an analog scope
around? Maybe. But the benefits of a DSO are so overwhelming it’s worth making
Fig. 4 shows a
bitmap capture from the DSO. Notice the raster image jaggies compared to the
continuous vector beam drawings of the analog instrument in Fig. 5. Notice the
DSO’s improved screen draw when selecting a faster time-base, which results in
a larger waveform and reduced screen aliasing, Fig. 6.
Fig. 4: Rigol DSO exhibiting ‘steps’ in waveform.
Fig. 5: Even ancient analog scope shows smooth
waveform trace — advantage analog.
Let’s try a more
complex setup, two unrelated waveforms being measured and viewed at the same
time on our dual trace scopes. We’d like to use different sweep time-bases so
we can acquire sufficient wave-shape detail for both images even though one is
a much higher frequency than the other. I chose a 1 kHz square wave from a
scope calibrator and a 9.6 kHz sine wave from an audio generator.
Here’s how the
scopes fared. The DSO images (Figs. 2 and 7) made the process look easy.
Setting the trigger menu to alternate and dialing in the desired time-base rate
for the second channel is all that it took. Notice all the detailed measurement
data on the display, instrument is in stop mode to hold the image for storage,
both waveform triggers are set for zero crossing 0.00 uV. Peak-to-peak voltages
of both waveforms are shown along with each channels amplitude value per grid
line and sweep times per grid line.
Fig. 6: Expanding the waveform trace to show fewer
cycles improves the image on the DSO.
Fig. 7: Effortless display of dual timebase traces
referenced accurately to 0 volts — advantage digital.
Fig. 8: Same signals on the analog storage scope — not
easy to read this.
analog mainframe was able to do much the same, but only by showing duplicates
of both channels, first at one time-base and then at the second time-base.
Careful reduction of waveform height and screen position was required to be
able to capture a similar but much more cluttered display. See Fig. 8. Note the
on-screen readouts of both time-bases and the vertical scale of the first
the B+K analog scope failed to duplicate the screen on my first attempt as the
dual time-bases are linked such that the B time-base cannot be slower than the
A. See Fig. 9. As you can see, Time-base B would be too fast for the square
wave on channel 2 if we sped up time-base A to permit only a few cycles of the
sine wave on channel 1 to display. You cannot randomly select any time-base
rate. You have to assign the lower frequency waveform to the first input
channel. Thus, when I reversed the scope probes, we get the results depicted in
Fig. 10, which more closely matches the DSOs display.
the B+K scope when alternating sweep time-bases does so at less than the lower
sweep rate. This is why a camera shot of the screen shows the faster sine wave
as being much dimmer as it is blanked much longer than its writing speed and it
repeats less often. So it appears darker to the photographic camera, which is
not integrating the image over as long a time as the human eye. To the naked
eye, both waveforms are alternately flashing on and off.
Fig. 9: Upper trace unable to show higher
resolution due to time base limitations.
Fig. 10: The display now shows what the DSO does but the
high frequency sine waves are dimmed — advantage digital.
LEARN ANALOG, BUY DIGITAL
So which scope
would you prefer most of the time? Analog or DSO?
The digital age
tends to remove us from the touch and feel of the real world that we learned to
understand with our own senses and analog tool extensions. For that reason, I
firmly believe analog scopes should continue to be used in education to
acquaint students with the concepts they are measuring and the functions used
to acquire those waveform measurements. After students get a physical
understanding of the process, they can graduate to DSO tools with all of their
hidden processes and measurement advantages.
Like all modern
digital devices, the Rigol can output measurement files and screen shots to a
host computer or even directly to a “Pict-Bridge” capable color printer. It can
also be used on an assembly line where it’ll provide a go/no go contact closure
to automatic test equipment. Physically the unit has little depth. No need for
it as there is no long-necked CRT in the cabinet. The case is made of sturdy
plastic, but the innards are housed within a shielded metal chassis. It has a
good heft to it for something as small as
a table radio. Even so, it weighs a
tiny fraction of a traditional oscilloscope.
It comes with
two switchable 1X/10X probes, amazing when you consider that a pair of name
brand probes would set you back almost half the price of the DSO itself.
Ira Wilner is director of engineering at
Monadnock Radio Group, Saga Communications in Keene, N.H.