2 THINGS TO CONSIDER WHEN CHOOSING AN OSCILLOSCOPE
12 THINGS TO CONSIDER WHEN CHOOSING AN OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 12 CONTACT 2 12 things to consider when choosing an oscilloscope This is a quick guide to the most important criteria for choosing your next scope. For a scope with a bandwidth above 1 GHz, or if you need one for special-purpose testing, you should probably talk to an applications engineer to help you make the right choice.
12 THINGS TO CONSIDER WHEN CHOOSING AN OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 12 CONTACT 3 CONTENTS PA G E PA G E The digital storage oscilloscope: a brief introduction 4 Bandwidth 5 Long record lengths 17 Rise time 7 Powerful waveform navigation and analysis 19 Matching probes 9 Automated Waveform Measurements 21 Accurate input channels ...
12 THINGS TO CONSIDER WHEN CHOOSING AN OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 12 CONTACT 4 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.
12 THINGS TO CONSIDER WHEN CHOOSING AN OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 12 CONTACT 5 Bandwidth System bandwidth determines an oscilloscope’s fundamental ability to measure an analog signal - the maximum frequency range that it can Amplitude error (%) ß 100 85 accurately measure. What you need ~2% Amplitude degration 30% Amplitude degration! 70.7 0.1 0.5 -3 dB 1.0 Frequency (GHz) E ntry level scopes will often have a maximum bandwidth of 100 MHz.
12 THINGS TO CONSIDER WHEN CHOOSING AN OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 12 CONTACT 6 ß Remember the ‘five times rule’ When selecting bandwidth, use the ‘five times rule’. If bandwidth is too low, your oscilloscope will not resolve high-frequency changes. Amplitude will be distorted. Edges will vanish. Details will be lost. Signals captured at 250 MHz, 1 GHz and 4 GHz bandwidth. www.tek.co.
12 THINGS TO CONSIDER WHEN CHOOSING AN OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 12 CONTACT 7 Rise time ß While analog engineers look at bandwidth, digital engineers are more interested in the rise time of signals like pulses and steps. What you need T he faster the rise time, the more accurate are the critical details of fast transitions. Fast rise time is also needed for accurate time measurements. R ise time is defined as, where k is between 0.
12 THINGS TO CONSIDER WHEN CHOOSING AN OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 12 CONTACT 8 ß Accurate rise time measurements are key Many logic families have faster rise times (edge speeds) than their clock rates suggest. A processor with a 20 MHz clock may well have signals with rise times similar to those of an 800 MHz processor. Rise times are critical for studying square waves and pulses.
12 THINGS TO CONSIDER WHEN CHOOSING AN OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 12 CONTACT 9 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 P robes actually become a critical part of the circuit, introducing resistive, capacitive and inductive loading that alters the measurement.
12 THINGS TO CONSIDER WHEN CHOOSING AN OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 12 CONTACT 10 ß Use a range of probes To start with, select passive probes that have high bandwidth and low loading. Active ground-referenced probes offer 1 to 4 GHz bandwidth, and differential active probes 20 GHz or more. Adding a current Probe enables the scope to calculate and display instantaneous power, true power, apparent power, and phase. High voltage probes measure to 40 kV peak.
12 THINGS TO CONSIDER WHEN CHOOSING AN OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 12 CONTACT 11 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 W hether to select 2, 4, 8 or 16 channels depends on your application.
THINGS TO CONSIDER WHEN CHOOSING AN OSCILLOSCOPE CONTENTS Choose enough channels INTRO 1 2 3 4 5 6 7 8 9 10 11 12 CONTACT 12 ß Analog The more time-correlated analog and digital channels your scope has, the more points in a circuit you can measure at the same time and the easier it is to decode a wide parallel bus, for instance. The example shows 2 analog, 8 digital and 1 decoded bus waveforms. Digital Bus www.tek.co.
12 THINGS TO CONSIDER WHEN CHOOSING AN OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 12 CONTACT 13 Fast sample rate ß Sine Wave Reproduced 100 using Sine x/x Interpolation 90 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. Sine Wave Reproduced using Linear Interpolation What you need S ample rate (samples per second, S/s) is how often an oscilloscope samples the signal.
12 THINGS TO CONSIDER WHEN CHOOSING AN OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 12 CONTACT 14 ß To capture glitches you need speed! Nyquist said that a signal must be sampled at least twice as fast as its highest frequency component to accurately reconstruct it and avoid aliasing (showing artefacts that are not actually there). Nyquist however is an absolute minimum – it applies only to sine waves, and assumes a continuous signal.
12 THINGS TO CONSIDER WHEN CHOOSING AN OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 12 CONTACT 15 Versatile triggering Triggering gives a stable display and lets you zero in on specific parts of complex waveforms. Triggered display What you need Untriggered display See how it works All oscilloscopes provide edge triggering, and most offer pulse width triggering.
12 THINGS TO CONSIDER WHEN CHOOSING AN OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 12 CONTACT 16 ß Advanced triggers find the right information Triggering lets you isolate a group of waveforms to see what is going wrong. Specialized triggers can respond to specific conditions in the incoming signal – making it easy to detect, for example, a pulse that is narrower than it should be. www.tek.co.
12 THINGS TO CONSIDER WHEN CHOOSING AN OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 12 CONTACT 17 Long record lengths ß 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 T ime 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.
12 THINGS TO CONSIDER WHEN CHOOSING AN OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 12 CONTACT 18 ß See the bigger picture Capturing enough detail to decode this USB serial data stream requires high resolution sampling (200ps). Capturing multiple packet contents needs a long time (200µs). An oscilloscope with long record length (1 Mpoints) is needed to display both. www.tek.co.
12 THINGS TO CONSIDER WHEN CHOOSING AN OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 12 CONTACT 19 Powerful waveform navigation and analysis ß 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.
12 THINGS TO CONSIDER WHEN CHOOSING AN OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 12 CONTACT 20 ß Consider advanced search tools The industry’s fastest tool for automated navigation, search and analysis is Wave Inspector®, a proprietary technology. It allows you to specify search criteria to automatically find every occurrence in an acquisition that violates some specified criteria such as setup and hold time. Wave Inspector® marking every pulse that crosses 300mV in a long acquisition. www.
12 THINGS TO CONSIDER WHEN CHOOSING AN OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 12 CONTACT 21 Automated Waveform Measurements Examples of fully automated waveform measurements: Period Duty Cycle + High Frequency Duty Cycle - Low Width + Delay Minimum Width - Phase Maximum Automated waveform measurements make it easier to obtain accurate Rise time Burst width Overshoot + numerical readings.
12 THINGS TO CONSIDER WHEN CHOOSING AN OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 12 CONTACT 22 ß Look for fast answers Once again, extra functions shorten the time to answer. Digital Signal Processing techniques can automate measurements – making them faster, more accurate and more repeatable than is possible with cursors. You can even write your own formulae for specific maths functions. www.tek.co.
12 THINGS TO CONSIDER WHEN CHOOSING AN OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 12 CONTACT 23 Advanced application support ß 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.
12 THINGS TO CONSIDER WHEN CHOOSING AN OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 12 CONTACT 24 Think about your future needs Complex electronic designs are driving innovation across many industries today. Your scope should have all the features your application needs – now and in the future. www.tek.co.
12 THINGS TO CONSIDER WHEN CHOOSING AN OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 12 CONTACT 25 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.
12 THINGS TO CONSIDER WHEN CHOOSING AN OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 12 CONTACT 26 ß Controls that match your way of working Oscilloscopes should give you different ways to operate the instrument. Built-in help can provide a convenient, built-in reference manual, while smart menus give easy access to multifunction, context-sensitive commands. An icon-rich graphical user interface helps you understand and intuitively use advanced capabilities. www.tek.co.
12 THINGS TO CONSIDER WHEN CHOOSING AN OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 12 CONTACT 27 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 C onsider a scope that allows you to access a Windows desktop and provide network printing and file sharing resources.
12 THINGS TO CONSIDER WHEN CHOOSING AN OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 12 CONTACT 28 Ask about interfaces LAN, Display, and Printer interfaces enable you to integrate your oscilloscope with the rest of your working environment: E thernet 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
12 THINGS TO CONSIDER WHEN CHOOSING AN OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 12 CONTACT 29 ... and finally, consider low cost of ownership and peace of mind! In the end, the scope you choose will have a price tag – but what is the real cost of ownership? Check out the manufacturer’s support options to see how far they add value to your purchase and contribute to extending your scope’s useful life.
12 THINGS TO CONSIDER WHEN CHOOSING AN OSCILLOSCOPE CONTENTS For Further Information To access product information and related literature please visit www.tek.co.uk Freephone Number: 00800 2255 4835* (available in Austria, Belgium, France, Germany, Ireland, Italy, Netherlands, Spain, Sweden, Switzerland, UK) * If Freephone numbers are not accessible from your phone, or for any other countries, dial: +41 52 675 3777 Literature reference number: “12 Things to Consider When Choosing an Oscilloscope”.
