The digital storage oscilloscope: a brief introduction
Oscilloscopes are the basic tool for anyone designing, manufacturing or repairing electronic equipment. A digital storage oscilloscope (DSO, which this guide concentrates on) acquires and stores waveforms. It can show high-speed repetitive and single-shot signals across multiple channels to capture elusive glitches and transient events.
A scope shows the signal’s frequency, whether a malfunctioning component is distorting the signal, how much of the signal is noise, whether the noise changes with time, and much, much more.
In short, whatever scope you choose it must not only match how and where you work but also:
- Accurately capture your signals.
- Have features that expand your capabilities and save you time.
- Offer guaranteed not just typical specifications.
Accuracy. you will need a pretty good idea of what signals you’re going to need to look at: whether (analog) audio and transducer signals or (digital) pulses and steps. If you’re looking at digital signals, will you be measuring rise times, or just looking at approximate timing relationships? Will you use the scope to qualify elements of your design, or mostly for debugging? Either way, accurate signal capture at the outset is more important than any later signal processing – your decisions rely on accurate information, and you can always process the information on a computer.
Capability. you need to consider not just your present generation of designs, but future generations too. A high-quality scope will give you many years’ reliable service.
Guaranteed specs. Ensure that all the parameters you need to measure are detailed as “guaranteed specifications” in the oscilloscope datasheet. Parameters listed as “Typical” are simply an indication of oscilloscope performance, and cannot be used to make meaningful measurements, that comply with recognized quality standards.
#1 BANDWIDTH
System bandwidth determines an oscilloscope’s fundamental ability to measure an analog signal - the maximum frequency range that it can accurately measure.
What you need
- Entry level scopes will often have a maximum bandwidth of 100 MHz. They can accurately (within 2%) show the amplitudes of sine-wave signals up to 20 MHz.
- For digital signals, oscilloscopes must capture the fundamental, third and fifth harmonics or the display will lose key features. So, the bandwidth of the scope together with the probe should similarly be at least 5x the maximum signal bandwidth for better than ±2% measurement error – the ‘five times rule’. This is also needed for accurate amplitude measurements.
- High-speed digital, serial communications, video and other complex signals can therefore require scope bandwidths of 500 MHz or more.
#2 RISE TIME
While analog engineers look at bandwidth, digital engineers are more interested in the rise time of signals like pulses and steps.
What you need
- The faster the rise time, the more accurate are the critical details of fast transitions. Fast rise time is also needed for accurate time measurements.
- Rise time is defined as k/bandwidth, where k is between 0.35 (typically for scopes with bandwidth <1 GHz) and 0.40 to 0.45 (>1 GHz).
- Similar to bandwidth, an oscilloscope’s rise time should be < 1/5 x fastest rise time of signal. E.g. a 4-ns rise time needs a scope with faster than 800 ps rise time. Note: As with bandwidth, achieving this rule of thumb may not always be possible.
- TTL and CMOS may need 400 to 300 ps rise times.
#3 MATCHING PROBES
Precision measurements start at the probe tip. The probe’s bandwidth must match that of the oscilloscope (the ‘five times rule’ again), and must not overload the Device Under Test (DUT).
What you need
- Probes actually become a critical part of the circuit, introducing resistive, capacitive and inductive loading that alters the measurement. To minimize the effect it’s best to use probes from the same manufacturer as the scope, forming an integrated solution.
- Loading is critical. Resistive loading of standard passive probes is usually an acceptable 10 MΩ or better. Capacitive loading of 10, 12 or even 15 picoFarads (pF) at high frequencies is a real problem though.
- When selecting a mid-range scope choose probes with capacitive loadings of < 10 pF. The best passive probes offer 1GHz bandwidth with a capacitive load <4 pF.
#4 ACCURATE INPUT CHANNELS... AND ENOUGH OF THEM
Digital scopes sample analog channels to store and display them. In general, the more channels the better, although adding channels adds to the price.
What you need
- Whether to select 2, 4, 8 or 16 channels depends on your application. Two or four analog channels will allow you to view and compare signal timings of your waveforms, while debugging a digital system with parallel data needs an additional 8 or 16 digital channels or more.
- A Mixed Signal Oscilloscope adds digital timing channels, which indicate high or low states and can be displayed together as a bus waveform. The latest Mixed Domain Oscilloscopes add a dedicated RF input for making high frequency measurements in the frequency domain.
- Whatever you choose, all channels should have good range, linearity, gain accuracy, flatness and resistance to static discharge.
- Some instruments share the sampling system between channels to save money. But beware: the number of channels you turn on can reduce the sample rate.
- Isolated channels simplify floating measurements. Unlike ground-referenced oscilloscopes, the input connector shells can be isolated from each other and from earth ground.
#5 FAST SAMPLE RATE
The sample rate of an oscilloscope is similar to the frame rate of a movie camera. It determines how much waveform detail the scope can capture.
What you need
- Sample rate (samples per second, S/s) is how often an oscilloscope samples the signal. Again, we recommend a ‘five times rule’: use a sample rate of at least 5x your circuit’s highest frequency component.
- The minimum sample rate may also be important if you need to look at slowly changing signals over longer periods of time.
- Most entry-level scopes have a (maximum) sample rate of 1 to 2 GS/s, while mid-range ones can have 5 to 10 GS/s.
- The faster you sample, the less information you’ll lose and the better the scope will represent the signal under test. But the faster you will ill up your memory, too, which limits the time you can capture.
#6 VERSATILE TRIGGERING
Triggering gives a stable display and lets you zero in on specific parts of complex waveforms.
What you need
- All oscilloscopes provide edge triggering, and most offer pulse width triggering.
- To acquire anomalies and make best use of the scope’s record length, look for a scope that offers advanced triggering on more challenging signals.
- The wider the range of trigger options available the more versatile the scope (and the faster you get to the root cause of a problem!):
- A & B sequence triggering; delay by time or delay by events
- video triggering on line/frame/HD signals, etc.
- Logic triggering: slew rate, glitch, pulse width, time-out, runt, setup-and-hold
- Communications triggers: embedded system designs use both serial (I2C, SPI, CAN/LIN, USB…) and parallel buses.
#7 LONG RECORD LENGTHS
Record length is the number of points in a complete waveform record. A scope can store only a limited number of samples so, in general, the greater the record length the better.
What you need
- Time captured = record length/sample rate. So, with a record length of 1 Mpoints and a sample rate of 250 MS/sec, the oscilloscope will capture a signal 4 ms in length.
- Today’s scopes allow you to select the record length to optimize the level of detail needed for your application.
- A good basic scope will store over 2,000 points, which is more than enough for a stable sine-wave signal (needing perhaps 500 points). But to ind the causes of timing anomalies in a complex digital data stream you should consider, for example, a DPO (Digital Phosphor Oscilloscope) with a record length of 1 Mpoints or more.
- To search for infrequent transients such as jitter, runt pulses and glitches, select at least a mid-end scope that combines long record length with a high waveform capture rate.
#8 POWERFUL WAVEFORM NAVIGATION AND ANALYSIS
Searching for specific waveform errors can be like searching for a needle in a haystack. you need tools that automate the process and accelerate the “time to answer”.
What you need
- Zoom & Pan allows you to zoom in on an event of interest, and pan the area backwards and forwards in time.
- Play & Pause automatically pans the zoom window across the waveform. That allows hands-free playback so you can concentrate on what’s important – the waveform itself.
- Marks lets you mark events of interest while you’re looking for a problem. you can use front-panel controls to rapidly jump between each mark for quick and easy timing measurements (see panel).
- Search & Mark lets you search through the entire acquisition and automatically mark every occurrence of a user-specified event.
- Advanced search lets you define various different criteria, similar to trigger conditions, which will be automatically detected and marked in the captured waveform.
#9 AUTOMATED WAVEFORM MEASUREMENTS
Automated waveform measurements make it easier to obtain accurate numerical readings.
What you need
- Most scopes offer front-panel buttons and/or screen-based menus to take accurate automated measurements.
- Basic choices on most scopes include amplitude, period and rise/fall time.
- Many digital scopes also provide mean and RMS calculations, duty cycle, and other maths operations.
- Advanced mathematics functions are found on some scopes, improving the ‘time to answer’ even further. Some examples:
- FFT, Integrate, Differentiate, Logarithm, Exponent, Square root, Absolute
- Sine, Cosine, Tangent, Radians, Degrees
- Scalars, with user-adjustable variables and results of parametric measurements.
#10 ADVANCED APPLICATION SUPPORT
Advanced scopes have application software for optical and electrical design troubleshooting and standards compliance.
What you need
- Signal integrity and jitter measurement packages: provide insight into signal integrity-related problems in digital systems, their causes, characteristics and effects.
- RF applications: view signals in the frequency domain and analyze using spectrograms, amplitude, frequency and phase versus time traces.
- Support for debug of embedded systems with mixed analog & digital, parallel & serial technologies such as CAN/LIN, I2C, SPI, FlexRay, MOST and others.
- Education: electrical engineering students need to understand complex circuits and electronic designs to develop next generation technologies.
- Power measurement (SMPS, for example): automated measurements for power quality, switching loss, harmonics, safe operating area, modulation, ripple, slew rate and more.
Others include optical communications, memory system verification, communications standards testing, disk drive measurements, video measurements, and more.
#11 EASY, RESPONSIVE OPERATION
Oscilloscopes should be easy to operate, even for occasional users. The user interface is a large part of the ‘time to answer’ calculations.
What you need
- Frequently used adjustments should have dedicated knobs.
- AUTOSET and/or DEFAULT buttons will make for instant setup.
- The scope should be responsive, reacting quickly to changing events.
- There should be support for your own language, with templates for the dials.
#12 CONNECTIVITY AND EXPANSION
Connecting a scope to a computer directly or transferring data via portable media allows advanced analysis, and simplifies documenting and sharing results.
What you need
- Consider a scope that allows you to access a Windows desktop and provide network printing and file sharing resources.
- Check if it can run third-party analysis, documentation and productivity software.
- Is it helpful to provide internet access, and share measurements with colleagues real-time?
- Can it meet your needs as they change? For example, can you add:
- Memory to channels to analyze longer record lengths
- Application-specific measurements and application modules
- A full range of probes and modules
- Accessories like battery packs and rack mounts
- Software to control the scope from your PC, take automated measurements, waveform data logging and export waveforms live.
Ask about interfaces
LAN, Display, and Printer interfaces enable you to integrate your oscilloscope with the rest of your working environment:
- Ethernet port for network connectivity, plus compatible software to capture screen-shots, waveform data and measurement results
- USB Host port: quick & easy data storage, printing, and connecting a USB keyboard
- USB device port for easy connection to a PC or direct printing to a printer
- Video port to export the oscilloscope display to a monitor or projector
... AND FINALLY, CONSIDER LOW COST OF OWNERSHIP AND PEACE OF MIND!
