9 Factors to Consider When Choosing an Oscilloscope

SDS Front

If you’re involved in electronics, you’ll probably have an oscilloscope on your bench. As electronics become more complex almost daily, sooner or later a new oscilloscope will be in order. How to choose the right one for your applications?

Factors to consider:

Remember that the bandwidth specification of an oscilloscope is the frequency of the “-3 dB point” of a sine-wave signal of a particular amplitude, e.g. 1 Vpp. As the frequency of your sinewave goes up (while keeping the amplitude constant), the measured amplitude goes down. The frequency at which this amplitude is -3 dB lower, is the instrument’s bandwidth. This means that an oscilloscope of 100MHz would measure a 1Vpp sinewave of 100MHz at only (approx.) 0.7Vpp. That is an error of about 30%! In order to measure more correctly, use this rule of thumb: BW/3 equals about 5% error; BW/5 equals about 3% error. In other words: if the highest frequency you want to measure is 100 MHz, choose an oscilloscope of at least 300MHz, a better bet would be 500MHz. Unfortunately, this has the most influence on the price…

Understand that today’s signals are no longer pure sine waves, but most of the time square waves. These are built by “adding” the odd harmonics of the fundamental sine wave together. So a 10 MHz square wave is “built” by adding a 10MHz sine wave + a 30MHz sine wave + a 50MHz sine wave and so on. Rule of thumb: get a scope that has a bandwidth of at least the 9th harmonic. So if you’re going for square waves, it’s better to get a scope with a bandwidth of at least 10x the frequency of your square wave. For 100MHz square waves, get a 1GHz scope… and a bigger budget…

Consider rise (fall) time. Square waves have steep rise and fall times. There’s an easy rule of thumb to get to know what bandwidth your scope needs to be if these times are important to you. For oscilloscopes with bandwidths below 2.5GHz, calculate the steepest rise (fall) time it can measure as 0.35/BW. So an oscilloscope of 100MHz can measure rise times up to 3.5ns. For oscilloscopes above 2.5GHz up to about 8GHz, use 0.40/BW, and for scopes above 8GHz use 0.42/BW. Is your risetime the starting point? Use the inverse: if you need to measure rise times of 100ps, you’ll need a scope of at least 0.4/100ps = 4 GHz.

Choose your sample speed. Today’s oscilloscopes are almost all digital. The above steps involved the analog part of the instrument, before it gets to the A/D converters to get “digitized”. Here the bandwidth-to-rise time calculation can help you out: an oscilloscope of 500MHz has a calculated rise time of 700ps. To reconstruct this, you need at least 2 sample points on this edge, so at least a sample each 350ps, or 2.8Gsa/s (gigasamples per second). Scopes don’t come in this flavor, so choose a model with a faster sampling speed, e.g. 5Gsa/s (resulting in 200ps “time resolution”).

Decide on the number of channels. This is easy: most scopes come with 2ch or 4ch configurations, so you can choose what you need. Fortunately prices don’t double from 2ch to 4ch, but it does have a big impact on the price of the instrument. High-end scopes (>=1GHz) have always 4ch.

Calculate how much memory you’ll need. Depending on how much of your signal you want to see in a “single shot acquisition”, get your math right: at 5Gsa/s, you have a sample each 200ps. A scope with a memory of 10.000 sample points, can store 2µs of your signal. A scope with 100M samples (they do exist!) can store 20 seconds! Looking at repetitive signals or “eye-diagrams”, memory is less important.

Think about repetition rate. A digital oscilloscope uses a lot of time calculating. Between the moment of triggering (see next step), having the captured signal on the display, and capturing the next triggered event, most digital scopes “consume” several milliseconds. This results in only a few “photos” of your signal each second (waveforms per second), typically about 100-500. One vendor solved this problem with so called “Digital Phosphor” (from about 4.000 wfms/s to >400.000 wfms/s for the top models), others followed with similar-like technologies (but not always sustained/continuous, rather in bursts). This repetition rate is important because those rare errors and faults in your signal might occur just then when the scope is not acquiring, but busy calculating the last taken acquisition. The higher the repetition rate (wfms/s rate), the higher your chances are of capturing that rare event.

Check what kind of errors you expect to be looking for. All digital scopes have some sort of intelligent triggers on board, meaning you can trigger on more than just the rising or falling edge of your signal. If your repetition rate is high enough, you’ve probably seen that rare glitch every other second. Then it’s nice to have a Glitch trigger.

Think about resolution of LCD display. Small screens with poor resolution can make your life miserable if you cannot see results easily. Buy the largest screen with the best definition your budget will allow.

Some Final Tips

  • Triggering, repetition rate and memory: once you found the rare event with a high wfms/s rate, having the right trigger available is more important than repetition rate, as your scope will trigger only on the (rare) event, which occurs… right: rarely. So you don’t need high rep-rate anymore. Memory can become more important, as to be able to analyze what happened before or after the event.
  • Remember: garbage in is garbage out, so get the bandwidth and rise time issue sorted out first!

Established in 2004, NorthTree Associates (Cologne, MN) is a North American distributor that specializes in providing unique Electronic Test & Measurement tools for design engineers, test engineers and production engineers. You can visit our website at http://www.northtreeassociates.com

Advertisements

Choosing the Right Diagnostic Automotive Oscilloscope for Your Auto Repair Shop

vds3104 right

Searching for Answers: Choosing Your Oscilloscope

Every shop will have different needs and uses for an oscilloscope, so it’s important to identify your facility’s specific needs from the equipment. Here are six steps to cover so your choice is the most correct one.

Step 1: What kind of vehicles are worked on? Take a look at the shop’s mix. What kind and type of vehicles are repaired the most? What makes, models and years are seen most often? “The more specific and ‘specialized’ the work that can be provided, the better off the shop will be. Choose the 10 most worked-on vehicles, and figure out the needs for those.

Step 2: Consider what isn’t worked on. Obviously, an oscilloscope should be able to help in bringing in additional business, jobs, revenue and, as is the overall goal – profitability. Will it be a tool that can help in bringing in vehicles in the area that the shop is missing out on? Are there any other shops working on a specific vehicles? If not, can the right oscilloscope help the shop take advantage of this opportunity?

Step 3: Research oscilloscopes. Here’s where shops often get off track or go the easy route. But if the proper approach is used and the correct observance of the shop’s work mix (Steps 1 and 2), then it narrows it down quite a bit. Here are six things to consider:

Coverage. What software does the tool come with? What updates? What vehicles does the software cover? Makes and model years? Because of changes in vehicle design and capabilities, how often is the software updated? It needs to be understood what each software package is capable of diagnosing.

Training/Ease of Use. Most oscilloscopes are “plug & play” aftermarket tools. Higher-end oscilloscopes often come with a steeper learning curve for first-time users. Try to get a feel for how long it will take shop technicians to master the equipment, and what training or support is offered.

Compatibility. Some oscilloscopes are Windows-based PC or laptop-based, and that often means one oscilloscope with powerful software can provide a wide range of coverage.

Technical Support. Got hotline? Some oscilloscope manufacturers provide hotlines of sorts to call for additional information or for support for difficult diagnoses. Understand how each oscilloscope is supported.

Upgrades/Updates. Oscilloscopes are constantly being upgraded and updated. Research the companies you’re considering and see what they offer in terms of upgrades. Not only for the purpose of the software but also for the oscilloscope.

Cost. An obvious point. Do you want a high-end do it all oscilloscope, then get ready to pay significantly. There’s going to be a large discrepancy in price between oscilloscope makers. This is why understanding the work mix of the shop is important to grasp the value of the tool.

Step 4: Analyze the return. There are a lot of ways to try to analyze how valuable a diagnostic oscilloscope is for a shop. One way to analyze the return is to low-ball the return and only compare the cost of the tool (including subscriptions and upgrades) to the amount of profit made on diagnostic charges. This will gave an absolute minimum that can serve to directly pay off the tool.

Step 5: Demo the tools. Be wary of any company that isn’t confident enough in its oscilloscope to let you have it for trial period. What is their return policy? If the oscilloscope doesn’t fit your needs, can you return it NQA? Using the oscilloscope on your own is important in making the right decision.

Step 6: Implement the tools. Although this step must come after you selected and purchased a tool, it will also help to confirm your decision. Don’t just simply buy diagnostic equipment and hand it off to the technician. Create processes and systems for your shop to use it correctly, Seyfer says, and make sure to market your capabilities.

Keep It Simple

Choosing an oscilloscope for the shop can be a difficult task. The most important thing to remember, is to find the best fit for your shop. Get as much information as possible. Speak with other shops, talk with vendors, ask about it in association gatherings, and on message boards—anywhere you can. There’s plenty of information out there about each tool.

In the end, try to make the process as simple as you can.

Should you have any questions, you can contact us directly by filling out the form below.

5 Features To Consider When Choosing A Digital Oscilloscope

vds3104 right

For anyone designing, manufacturing, or repairing electronic equipment, a digital storage oscilloscope is a must-have tool. It lets you see high-speed repetitive or single-shot signals across multiple channels to capture elusive glitches or transient events. An oscilloscope is equally as useful a tool for qualifying elements of a new design as it is for isolating problem components in an existing system under repair.
When it comes to evaluating oscilloscopes, many engineers focus on one specification: bandwidth. The assumption is generally that the faster oscilloscope is the better oscilloscope. And while bandwidth is an important thing to consider, it falls well short of telling the whole story or in ensuring that the oscilloscope you’re considering will truly meet your needs. With that in mind, here are five other things you’ll want to consider when choosing your next oscilloscope.

1. Rise time — Accurate rise-time measurements are key to making accurate measurements in the time domain. 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 important for studying square waves and pulses. Square waves are standard for testing amplifier distortion and timing signals for TVs and computers. Pulses may represent glitches or information bits — too slow a rise time for the circuit being tested could shift the pulse in time and give a wrong value.

2. 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. To capture glitches you need speed. A signal must be sampled at least twice as fast as its highest frequency component to accurately reconstruct it and avoid aliasing (showing artifacts that are not actually there). This is however an absolute minimum. What’s more, it applies only to sine waves and assumes a continuous signal. Glitches are by definition not continuous, and sampling at only twice the rate of the highest frequency component is usually not enough. A high sample rate increases resolution, ensuring that you’ll see intermittent events. As a rule of thumb, look for a sample rate of at least 5x your circuit’s highest frequency component.

3. Versatile triggering — All oscilloscopes provide edge triggering, and most offer pulse width triggering. But more advanced triggering capabilities can save you time and shorten the time to answer when working with more challenging signals. The wider the range of trigger options available, the more versatile the scope. Some of the triggers available include A & B sequence triggering; video triggering on line/frame/HD signals, etc.; logic triggers such as slew rate, glitch, pulse width, time-out, runt, setup-and-hold; and communications triggers for serial and parallel buses.

4. Powerful waveform navigation and analysis — Searching for specific waveform errors can be like searching for a needle in a haystack. Tools that automate the process can be a big time saver. For instance, oscilloscopes with record lengths in the millions of points can show thousands of screens worth of signal activity, essential for examining complex waveforms. Capabilities such as search and mark speed up the process by letting you search through the entire acquisition and automatically mark every occurrence of an event you specified. Other capabilities include zoom and pan, play and pause, and advanced search.

5. Matching probes — Precision measurements start at the probe tip. The probe’s bandwidth must match that of the oscilloscope, and must not overload the Device Under Test (DUT). Probes actually become a part of the circuit, introducing resistive, capacitive and inductive loading that alters the measurement. It’s important to have a range of probes available. To start with, select passive probes that have high bandwidth and low loading. Active ground-referenced probes offer one to four GHz bandwidth while active differential probes support 20 GHz or more. Adding a current probe enables the scope to calculate instantaneous power, true power, apparent power and phase. High voltage probes measure to 40kV peak. Specialty probes include logic, optical and environmental types.

Cost of ownership
Any scope you choose will need to fit within the constraints of a capital acquisition budget. While cost of ownership isn’t a feature per-se, it’s an important consideration.  This means you should compare support options to see to whether they add value to your purchase or can help extend the scope’s useful life. On-site education and training, as well as design, system integration, project management, and other professional services can help maximize productivity and ensure reliable measurements. Support packages such as these, along with options like extended warranty can save money in the long term.

Contact us with your questions or if you would like to visit our online store to shop for digital oscilloscopes – visit http://www.northtreeassociates.com / sales@northtreeassociates.com

The Basics Of Oscilloscopes – What Is An Oscilloscope – Part 2

Instek GDS-2000

What other factors should be considered when purchasing a digital storage oscilloscope?

Beyond the basic four specifications, it is common to consider:

  • Number of channels (typically two or four). If you need to record multiple high-speed signals beyond four, you might want to look at a dedicated recorder.
  • Size of the display is often a consideration. Larger, clearer screens make it easier to see multiple signals at once. Luckily today’s digital storage oscilloscope also has different color lines for each signal.
  • How you capture a signal is also important. This is where triggers come into play. It is often important to see only signals with specific characteristics among the many captured. With most digital storage oscilloscopes, a variety of different trigger types are available to find particular events that happen during signal analysis.
  • If you are looking at packets of serial data, you may also find it useful to decode the signal to make sure that the correct instructions are being sent. Protocols such as I2C, SPI, CAN, LIN, RS232 are commonly used to communicate between devices. It is important to make sure that the right commands are communicated when a specific event happens.

When graphing a signal, what do you want to find out?

  • The time and voltage value of a signal
  • The frequency of an oscillating signal
  • How much of a signal is direct current (DC) or alternating current (AC)
  • How much of the signal is noise and if the noise is changing over time
  • To see the “moving parts” of a circuit represented by the signal
  • To tell if a malfunctioning component is distracting the signal

Oscilloscopes come in many different versions

  • Digital
  • Analog
  • Mixed signal
  • Portable
  • PC based versions

If the recording of a waveform is required, a digital scope will be applicable. If you need to see the waveform in real time, or to see the original intensity an analog scope would better suit that requirement.  The higher the input signal frequency is, the higher the bandwidth that will be required. If you do not have the appropriate amount of bandwidth, you risk the possibility of not getting accurate results.

If there is doubt about the amount of bandwidth that is required, then you should go the next step up. The bandwidth can usually be calculated by this formula: BANDWIDTH = (0.35 / rise time of the signal)

The higher the sampling rate, the more accurate and precise the captured waveform is. As the sampling rate increases, it allows for more samples a captured waveform has, for any given period of time.

In almost every electric application, including lab use, research and development, and product development there is a need for an oscilloscope to provide waveform analysis.

Mixed Signal Oscilloscopes (MSO)

Mixed Signal Oscilloscopes (MSO) can capture both analog and digital signals at once. A mixed signal scope usually combines 2 or 4 analog channels with either 8 or 16 digital ones. This is useful when looking at logic signals after a specific input has occurred when developing a system that combines physical input and computer controls.

There are both digital and analog channels that provide the ability to accurately time-correlate both signals. The measurements are compiled by using a single time base on a single display. Any combination of these measurements can be used to trigger the scope.

The key advantage of the MSO is that only one unit is required for conducting tests that you would normally need two units for.

An oscilloscope is a test and measurement instrument used primarily to measure voltage over time. The input signal is converted from an analog wave to a series of digital signals. Once it is digitized, oscilloscope can then store the information in memory and display it on the screen. The faster the signal is processed, the better the display will be.

Refer to the above general and digital oscilloscopes discussions that cover the common features of an MSO with a DSO.

Applications for Mixed Signal Oscilloscopes

  • Aerospace
  • Defense
  • Industrial Electronics
  • Communications
  • Research and Development

The MSO treats the oscilloscope and logic channels in different ways

  • Logic Channels: These channels are converted to digital format, where no analog information is shown
  • Oscilloscope Channels: These channels use an analog to digital converter to allow the analog input to show in digital format

MSO scope vs. Logic Analyzer factors to consider

  • State Analysis

o             MSO- Yes. Separate channels for clocks

o             Logic Analyzer- No. No provision for clock input

  • Triggering

o             MSO- Single events on both the analog and digital channels

o             Logic Analyzer- Advanced sequential capabilities

  • Channel Count

o             MSO- 16 / 32

o             Logic Analyzer- 64 – 204 +

  • Timing Analysis

o             MSO- Yes

o             Logic Analyzer- Yes

What other factors should be considered when purchasing a mixed signal oscilloscope?

Beyond the basic four specifications, it is common to consider:

  • Number of analog channels (typically two or four). If you need to record multiple high-speed signals beyond four, you might want to look at a dedicated recorder.
  • Number of digital channels, usually 8 or 16
  • Size of the display is often a consideration. Larger, clearer screens make it easier to see multiple signals at once. Luckily today’s digital storage oscilloscope also has different color lines for each signal.
  • How you capture a signal is also important. This is where triggers come into play. It is often important to see only signals with specific characteristics among the many captured. With most digital storage oscilloscopes, a variety of different trigger types are available to find particular events that happen during signal analysis.
  • If you are looking at packets of serial data, you may also find it useful to decode the signal to make sure that the correct instructions are being sent. Protocols such as I2C, SPI, CAN, LIN, and RS232 are commonly used to communicate between devices. It is important to make sure that the right commands are communicated when a specific event happens.

PC Based Oscilloscopes

PC based oscilloscopes are the modern alternative to the traditional bench top oscilloscope. All data and configurations measured on these oscilloscopes can be saved into a PC for further data analysis.

One of the key factors involved is the USB connection. The USB (universal serial bus) is intended for communications between interfaces, such as the oscilloscope and the PC in this instance.

PC based oscilloscopes come in either internal or external versions.

The external version(s) is a small unit that connects to a PC, usually by a USB. They can be used by a laptop or a desktop computer.

The internal version(s) usually come with a plug in card that is PCI format. This does not allow for portability and being that the card is placed in the actual PC, there is a lot of noise which could interfere with the results that are being measured.

Advantages of PC Based Oscilloscopes:

  • Easy to Use
  • Portable
  • Cost Effective
  • Large Display
  • Uses already “off-the-shelf” equipment- USB and PC

Portable Oscilloscopes

Portable Oscilloscopes are otherwise known as handheld oscilloscopes. They are typically used for on- site contractor maintenance and either in the industrial or electronic field.

If you need to move your oscilloscope around to many locations or from bench to bench in your lab, then the portable oscilloscope would be ideal for you.

Advantages of a Portable Oscilloscope

  • Lightweight
  • Easy to Use
  • Turn On and Off Quickly

To read Part 1 – click here:  http://wp.me/p42MUZ-6Z

Use the contact form below to contact NorthTree Associates with any questions you may have on this article or about oscilloscopes in general.

The Basics of Oscilloscopes – What Is An Oscilloscope – Part 1

GDS-3000 Series Instek Oscilloscope

An Oscilloscope is an instrument that is used as a graph displaying device of an electrical signal. The graph will show how signals change over time. The vertical (Y) axis represents voltage and the horizontal (X) axis represents time. The horizontal sweeps at a constant rate. The (Z) axis, although not that common, can display brightness or intensity of the display. With a proper transducer, an oscilloscope can measure just about anything. A transducer is a device that creates an electrical signal in response to physical stimuli such as sound, pressure, light, heat, etc.

When graphing a signal, what do you want to find out?

  • The time and voltage value of a signal
  • The frequency of an oscillating signal
  • How much of a signal is direct current (DC) or alternating current (AC)
  • How much of the signal is noise and if the noise is changing over time
  • To see the “moving parts” of a circuit represented by the signal
  • To tell if a malfunctioning component is distracting the signal

Oscilloscopes come in many different versions

  • Analog
  • Digital
  • Mixed signal
  • Portable
  • PC based versions

If the recording of a waveform is required, a digital scope will be applicable. If you need to see the waveform in real time, or to see the original intensity an analog scope would better suit that requirement.  The higher the input signal frequency is, the higher the bandwidth that will be required. If you do not have the appropriate amount of bandwidth, you risk the possibility of not getting accurate results.

If there is doubt about the amount of bandwidth that is required, then you should go the next step up. The bandwidth can usually be calculated by this formula: BANDWIDTH = (0.35 / rise time of the signal)

The higher the sampling rate, the more accurate and precise the captured waveform is. As the sampling rate increases, it allows for more samples a captured waveform has, for any given period of time.

In almost every electric application, including lab use, research and development, and product development there is a need for an oscilloscope to provide waveform analysis.

Analog Oscilloscopes

An Analog Oscilloscope will draw waveforms on its display by deflecting an electron beam that sweeps across its screen horizontally. The beam is vertically deflected in proportion to the applied voltage, which allows the shape to reproduce the shape of the target trace. Analog equipment works with continuous variable voltages. Analog oscilloscopes can display signals as they happen.

The immediate representation of a waveform on the CRT (panel) display results in a faster update rate. The CRT’s have a higher resolution and faster waveform update rate. The analog oscilloscope displays the signal in real-time with less risk of modifying any part of the original signal. The original intensity of the waveform can be observed with the CRT. Depending upon application, the intensity of the waveform can be critical.

Analog oscilloscopes are sometimes preferred by some users. They can both interpolate and aliasing the points between the waveforms. Although the analog oscilloscopes do not allow for digital storage and analysis, pre or post triggering information, and high frequency limitations, they can still be used.

The basic difference between the digital and analog oscilloscopes is the display signals. The digital scope can display signals that may happen only once and analog scopes can display signals as they happen, or in real-time.

To use an analog oscilloscope, there are three basic settings to adjust an incoming signal

  • Time base: Set the amount of time per division represented on the screen.
  • Triggering: Use a trigger level to stabilize a repeating signal, or trigger one event.
  • Attenuation: Adjust the amplitude of the signal before it is applied.

What is trace storage?

Trace storage allows for direct-view storage CRT’s. It will display a trace pattern that would normally disappear in seconds to stay on the screen for several minutes.

Digital Oscilloscopes

An oscilloscope is a test and measurement instrument used primarily to measure voltage over time. A Digital Storage Oscilloscope (also known as a DSO) takes the input signal and converts it from an analog wave to a series of digital signals. Once it is digitized, the digital storage oscilloscope can then store the information in memory and display it on the screen. The faster the signal is processed, the better the display will be.

The digital oscilloscope uses graphical grid called a graticule to display a signal. The Y-axis (vertical) is usually the voltage (though it could also be current, pressure, or another type of signal that is then converted into voltage). The X-axis (horizontal) is usually time (though it could also be frequency). Some digital storage oscilloscopes also use signal brightness as their Z-axis.

To make it easier to read the graticule, it is typically broken into 8 squares (or divisions) going vertically and 10 squares (or divisions) going horizontally. This can change from manufacturer to manufacturer, but it is fairly standard. The reason why it is done this way is that as long as you know what each division is, it makes it easier to read the values on screen. Luckily, digital storage oscilloscopes can also display the exact value at a given point on the screen. You have to move your marker, or cursor, to the spot in question and you should be able to see the specific value. A scope with multiple cursors will allow you to measure the difference between two spots, which can be very handy.

Some common measurements digital storage oscilloscopes are used for include

  • Looking at the shape of a signal (also known as a waveform)
  • Checking the amplitude (strength) of a signal
  • Checking the frequency (timing) of a signal
  • Checking the amount of time between events
  • Looking for problems (noise) with a signal

What are the key specifications in selecting a digital storage oscilloscope?

There are typically four parameters that should be considered when choosing your instrument:

  • Bandwidth
  • Sample Rate
  • Rise Time
  • Recording Length

How much scope bandwidth do I need?

The amount of times a signal repeats itself in one second (Hertz or Hz) is its frequency. Oscilloscopes can view signals occurring anywhere from 1Hz (or less) up to 1GHz (1,000,000,000 (that’s one billion) times per second) or more. Select an digital storage oscilloscope that can see more than the fastest signal you want to measure. In theory, you want your signal to be no faster than 71% of your maximum. The rule of thumb is that the bandwidth be five times (5x) greater than the maximum signal. So, if your signals to be observed are a maximum of 100MHz (100,000,000 times per second) then choose a model with a 500 MHz bandwidth.

How much sampling do I need?

As mentioned, a digital storage oscilloscope converts an analog signal into a digital one. This is done through a process known as sampling. The faster the sample rate, the more information about the original signal is captured and converted. This a common specification you see on data sheets for these types of scopes. It is measured in Samples per Second (S/s). For high-speed samples, you will often see it measured in MS/s (Mega Samples per second) or GS/s (Giga Samples per second).

There is something called Nyquist’s Theorem, which states that in order to properly slice up an analog signal (so you have enough information to recreate it back again) you need to have a sample rate at least twice the fastest signal you are looking at. That is of course, a minimum amount. In practice, most scopes are built to sample at least 5 times the highest speed it can capture. So, for example, a 200MHz signal would be best sampled at a rate of at least 1GS/s.

How fast does a scope need to be? 

The speed at which a signal goes from 10% of its level (in amplitude) to 90% of its top value is called the rise time. In order to see the maximum amount of each signal edge (vertical) both the scope and the probe must have a fast enough rise time. This is especially true for when the signal changes. Once again, the practical scenario calls for the rise time on the instrument to be five times faster than the signal. If your fastest signal has a rise time of 5 usec (micro second), then you want a scope/probe combo to have a rise time of 1 usec.

How deep should the memory be?

The last major specification to be considered is memory depth or record length. The major benefit of the digital storage oscilloscope is the storage part. This gives you the ability to recall, compare, and perform math functions on a captured signal. The record length is measured in samples or points. The total amount of time you can record for is determined by the number of points available and the sample rate (each sample being a point). You would simply divide the number points by the sample rate to get your acquisition time. If you have a total memory depth of 1 Mpoints and a sample rate of 250 MS/second, then you can record a signal that is 4 msec (millisecond) long.

(To be continued in Part 2…)

Use the contact form below to contact NorthTree Associates with any questions you may have on this article or about oscilloscopes in general.