0 FACTORS IN CHOOSING A BASIC OSCILLOSCOPE
10 FACTORS IN CHOOSING A BASIC OSCILLOSCOPE CONTENTS INTRO 10 Factors in Choosing a Basic Oscilloscope Basic oscilloscopes are used as windows into signals for troubleshooting 1 2 3 4 5 6 7 8 9 10 11 CONTACT There are several ways to navigate this interactive PDF document: Click on the table of contents (page 3) circuits or checking signal quality.
10 FACTORS IN CHOOSING A BASIC OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 CONTACT 3 CONTENTS PA G E PA G E THE DIGITAL STORAGE OSCILLOSCOPE: A BRIEF INTRODUCTION 2 BANDWIDTH 4 AUTOMATED MEASUREMENTS AND ANALYSIS 16 SAMPLE RATE 6 EASY OPERATION 18 ENOUGH INPUT CHANNELS – AND THE RIGHT ONES 8 CONNECTIVITY 20 SERIAL BUS DECODING 22 TRIGGERING 12 SUPPORT: THE 11TH FACTOR 24 RECORD LENGTH CONTACT INFORMATION 25 COMPATIBLE PROBES 10 14 www.tektronix.
10 FACTORS IN CHOOSING A BASIC OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 CONTACT 4 BANDWIDTH System bandwidth determines an oscilloscope’s ability to measure an analog signal. Specifically it determines the maximum frequency that the instrument can Amplitude error (%) ß 100 85 accurately measure. Bandwidth is also a key determining factor in price. Determine what you need For example, a 100 MHz oscilloscope is usually guaranteed to have less than 30% attenuation at 100 MHz.
10 FACTORS IN CHOOSING A BASIC OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 CONTACT 5 ß When selecting bandwidth, use the ‘five times rule’. Oscilloscope Bandwidth ≥ 5 x Maximum frequency of interest If bandwidth is too low, your oscilloscope will not resolve high-frequency changes. Amplitude will be distorted. Edges will slow down. Details will be lost.
10 FACTORS IN CHOOSING A BASIC OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 CONTACT 6 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. Determine 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.
10 FACTORS IN CHOOSING A BASIC OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 CONTACT 7 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 (undersampling). Nyquist however is an absolute minimum – it applies only to sine waves, and assumes a continuous signal.
10 FACTORS IN CHOOSING A BASIC OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 CONTACT 8 ENOUGH INPUT CHANNELS – AND THE RIGHT ONES ß Digital oscilloscopes sample analog channels to store and display them. In general, the more channels the better, although adding channels adds to the price. Determine what you need Whether to select 2 or 4 analog channels depends on your application. Two channels let you compare a component’s input to its output, for example.
10 FACTORS IN CHOOSING A BASIC OSCILLOSCOPE Choose enough channels CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 CONTACT 9 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, 2 digital and 1 decoded bus waveforms.
FACTORS IN CHOOSING A BASIC OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 CONTACT 10 COMPATIBLE PROBES Good measurements begin at the probe tip. The scope and probe work together as a system, so be sure to consider probes when selecting an oscilloscope. During measurements probes actually become a part of the circuit, introducing resistive, capacitive, and inductive loading that alters the measurement.
10 FACTORS IN CHOOSING A BASIC OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 CONTACT 11 ß Use the right probe for the job n Passive probes Probes with 10X attenuation present a controlled impedance and capacitance to your circuit, and are suitable for most ground-referenced measurements. They are included with most oscilloscopes – you’ll need one for each input channel.
10 FACTORS IN CHOOSING A BASIC OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 CONTACT 12 TRIGGERING Triggering gives a stable display and lets you zero in on specific parts of complex waveforms. Determine 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.
10 FACTORS IN CHOOSING A BASIC OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 CONTACT 13 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. Consider a higher-performance oscilloscope Sequence triggering can help capture more elusive signal events.
10 FACTORS IN CHOOSING A BASIC OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 CONTACT 14 RECORD LENGTH 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. Determine 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 4 ms.
10 FACTORS IN CHOOSING A BASIC OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 CONTACT 15 ß See the bigger picture Long record length lets you see a long time window, and lets you zoom in to see signal detail with good resolution. Consider a higher-performance oscilloscope Oscilloscopes are available with millions of points of data and manually searching for events can become impractical. Some scopes include search functions that let you search long records for specific events. www.tektronix.
10 FACTORS IN CHOOSING A BASIC OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 CONTACT 16 AUTOMATED MEASUREMENTS AND ANALYSIS Automated waveform measurements make it easier to obtain accurate numerical readings. Examples of fully automated waveform measurements: Time Determine 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.
10 FACTORS IN CHOOSING A BASIC OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 CONTACT 17 ß A Window into the Frequency Domain Basic oscilloscopes often include an FFT function that lets you view the spectrum of the acquired waveform. This is useful for trying to determine the source of noise, for example. Consider a higher-performance oscilloscope A basic oscilloscope does not have enough dynamic range to look at small RF signals like a spectrum analyzer can.
10 FACTORS IN CHOOSING A BASIC OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 CONTACT 18 EASY OPERATION Oscilloscopes should be easy to operate, even for occasional users. The user interface is a large part of the ‘time to answer’ calculation. Determine 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.
10 FACTORS IN CHOOSING A BASIC OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 CONTACT 19 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.tektronix.
10 FACTORS IN CHOOSING A BASIC OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 CONTACT 20 CONNECTIVITY Connecting a scope to a computer directly or transferring data via portable media allows advanced analysis, and simplifies documenting and sharing results. Determine what you need W ill you need to produce reports? Many oscilloscopes can produce .JPG, .BMP, or, PNG files that can easily be incorporated. Check for compatibility with third-party analysis, documentation software.
10 FACTORS IN CHOOSING A BASIC OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 CONTACT 21 Ask about interfaces Interfaces enable you to integrate your oscilloscope with the rest of your working environment: 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 Ethernet port for network connectivity, plus compatible software to capture screen-shots, waveform data and measurement results
10 FACTORS IN CHOOSING A BASIC OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 CONTACT 22 SERIAL BUS DECODING ß Most system-level (computer to computer) communication is transferred over serial data links. Even on today’s circuit boards, much of the chip-to-chip data is transferred over serial buses. Determine what you need Some scopes are able to decode serial buses and display data time-correlated with other waveforms.
10 FACTORS IN CHOOSING A BASIC OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 CONTACT 23 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.
10 FACTORS IN CHOOSING A BASIC OSCILLOSCOPE CONTENTS INTRO 1 2 3 4 5 6 7 8 9 10 11 CONTACT 24 SUPPORT: THE 11TH FACTOR After you’ve studied all the specs and features, take some time to look into the after-sale support you can expect.
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