What’s All This About DSOs (Digital Storage Oscilloscopes)?
     

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).

When handheld 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.

DSOs have 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.

It does.

DIGITAL VS. ANALOG

Product Capsule

Rigol DS1102E Digital Storage Oscilloscope

PROS
Quick automatic waveform acquisition
Multiple on screen measurements negate the need for a multi-meter
Measurement and setup storage to internal or external memory
Easy-to-read accurate color LCD display
Compact and lightweight
Very inexpensive; price/performance ratio is amazing

CONS
Input channels share one set of controls
Waveform display can be a bit choppy due to segmentation
Fan is a little noisy
No storage pouch for probes or other accessories
No hard-copy manual

Price: $399

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.

Analog scopes 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.

DSOs do 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

I experimented 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.

Press the 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.

Physically the 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 With CRT
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 vanishes forever.

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 the jump.

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.

The Tektronix 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 channel.

In comparison, 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.

Unfortunately 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.


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