How to perform jitter testing and analysis using a real-time oscilloscope

　　Current digital oscilloscopes fall into two categories: waveform viewing instruments and waveform analyzers. Oscilloscopes designed for viewing waveforms are typically used in testing and troubleshooting applications where the waveform image provides all the information the user needs.

　　In waveform analysis applications, features such as the Microsoft Windows operating system and advanced analysis capabilities can apply an additional level of abstraction to determine the performance of the system under test. In this regard, it is also difficult to determine whether an oscilloscope can meet user needs based solely on product data sheets. Real-time demonstrations in the lab are required to determine whether the oscilloscope under investigation can display what the user needs to see.

6. What trigger functions do you need?
　　Many general-purpose oscilloscopes use edge triggering. However, in some applications, you may need to use other trigger functions. Advanced triggering functions enable you to isolate the events you want to view. For example, in digital applications, triggering on a certain pattern in the channel will be very helpful. As mentioned earlier, mixed signal oscilloscopes can trigger on logic channels and oscilloscope channel patterns, while in oscilloscope/logic analyzer combination solutions, users can only cross-trigger the two instruments by connecting their respective input/output trigger signal cables together.

　　For serial designers, some oscilloscopes even come with serial trigger protocols for standards like SPI, CAN, USB, I2C, and LIN. This is where advanced triggering options can save a ton of time in everyday debugging tasks. What if you need to capture a rare event? Glitch triggering allows you to trigger on positive-going or negative-going glitches. Or trigger on pulses that are greater or less than a specified width. These features are especially useful when diagnosing a problem. You can trigger on the problem, look back in time (using the Delay or Horizontal Position knobs), look back in time (using the Delay or Horizontal Position knobs), and see what caused the problem.

　　Many oscilloscopes on the market today also offer triggering capabilities for TV and video applications. By using the TV triggering feature of an oscilloscope, you can trigger the system on the specific line where you need to view it.

7. What is the best way to probe a signal?
　　Signals start changing at rates above 1 GHz. Since passive probes are generally limited to 600 MHz, getting the full bandwidth of the oscilloscope can be a problem. The system bandwidth (and therefore the combined oscilloscope/probe bandwidth) is the lower of these two bandwidths. For example, consider a 1 GHz oscilloscope with a 500 MHz passive probe. The combined system bandwidth is 500 MHz. It is not worth buying a 1 GHz oscilloscope if you get 500 MHz bandwidth because of the probe!

　　Additionally, every time you connect a probe to a circuit, the probe becomes part of the circuit being measured. The probe tip is essentially a short transmission line. The transmission line is an LC resonant circuit, and at a frequency that is 1/4 the frequency of the transmission line, the impedance of the LC resonant circuit will become low, approaching zero, and will load the device under test. The loading of the LC resonant circuit can be easily seen in the slow rise time and ringing of the signal.

　　Active probes not only provide greater bandwidth than passive probes, but they also eliminate some of the transmission line effects when connecting the probe to the device under test (DUT). By using resistive "attenuated" probe tips and accessories in active probes, Agilent Technologies minimizes signal loading and resulting signal distortion. These attenuated accessories prevent the impedance of the LC resonant circuit from becoming too low, thereby preventing ringing and signal distortion caused by the loaded signal.

　　In addition, the attenuation accessories enable the probe's frequency response to remain flat across the entire probe bandwidth. With a flat frequency response, signal distortion is prevented across the probe's entire bandwidth.

　　Now that signal distortion has been addressed, if you are probing high-speed signals, the next step is to ensure that full bandwidth is achieved even when using probe accessories. Agilent InfiniiMax probes optimize probe bandwidth by using controlled transmission lines between the probe amplifier and the probe tip. By using one amplifier, you can connect a variety of differential or single-ended probes, including browsing probes, probes with sockets, solder probes, and SMA probes, and get full system bandwidth. In addition, because the probe amplifier is actually separated from the probe tip by a controlled transmission line, it is easy to access tight probe spaces.

　　The key here is to understand the bandwidth rating of the probe when using various probes and accessories. Accessories can degrade the performance of the probe, and you certainly don't want to unnecessarily spend thousands of dollars on a high-bandwidth active probe that will seriously degrade system performance when used in your preferred probing configuration.

8. What archiving and connectivity features do you need?
Many digital oscilloscopes now come with the same interfaces as personal computers, including GPIB,
RS-232, LAN, and USB. It is much easier to send images to a printer or transfer data to a PC or server than it was in the past. Do you often transfer oscilloscope data to a PC? It is very important that the oscilloscope has at least one of the interface options listed above. A built-in floppy or optical drive can also help you transfer data, but using a floppy or optical drive usually requires more work than sending files from the oscilloscope via a USB or LAN connection. For economical oscilloscopes that do not have more advanced interface options such as LAN and USB, oscilloscope manufacturers usually provide software that allows waveform images and data to be easily transferred to a PC via GPIB or RS-232. If the PC does not have a GPIB card installed, or the user wants a simpler way to transfer waveforms to a laptop, you may want to consider a GPIB to USB converter. Many oscilloscopes also come with a hard drive of several GB, which the user can also use to store data. You should determine in advance what level of connectivity and archiving capabilities you need from the oscilloscope. If you need to connect the oscilloscope as part of an automated test system, be sure to ensure that the oscilloscope is equipped with sufficient software and driver level to adapt to your programming environment.

9. How do you analyze waveforms?
Automatic measurements and built-in analysis functions can save users time and make work easier. Digital oscilloscopes usually come with a range of measurement functions and analysis options that are not available on analog oscilloscopes.

Math functions include addition, subtraction, multiplication, division, integration, and differentiation. Measurement statistics (minimum, maximum, and average) can characterize measurement uncertainty, which is an important resource when characterizing noise and timing margins. Many digital oscilloscopes also provide FFT capabilities.

For the "high-demand user" who is focused on waveform analysis, oscilloscope manufacturers are providing greater flexibility in mid-range and high-end oscilloscopes. Some manufacturers offer software that allows for the customization of complex measurements, math functions, and post-processing directly from the oscilloscope user interface. For example, a measurement program can be written in C++ or Visual Basic and then executed from the oscilloscope graphical user interface (GUI). This feature eliminates the need to transfer data to an external PC, which can save a lot of time for users who are focused on waveform analysis.

10. Last but not least: demos, demos, and more demos!
　　If you have considered the previous nine factors, you may have narrowed down the field to a small number of oscilloscopes that meet your criteria. Now is the time to try out these oscilloscopes and compare them side by side. Borrowing an oscilloscope for a few days will give you time to fully evaluate these oscilloscopes. Some of the factors to consider when using each oscilloscope include:

Ease of Use: During your experiments, evaluate each scope for ease of use. Does the scope have easy-to-use, dedicated knobs for commonly used adjustments such as vertical sensitivity, timebase speed, trace position, and trigger level? How many buttons do you have to press to get from one operation to another? Can you run the scope intuitively while keeping your focus on the circuit under test?
Display Response Speed: When evaluating oscilloscopes, pay attention to the scope's responsiveness, which is a critical factor whether you are using the scope to diagnose a problem or collect large amounts of data. Does the scope respond quickly when you change V/div, time/div, memory depth, and position settings? Take another look at the scope's responsiveness when you turn on measurement functions. Is there a noticeable drop in response speed?

ConclusionAfter
thoroughly examining these questions and evaluating oscilloscopes, you should have a good idea of ​​which model will truly meet your needs. If you are not sure at this point, you may want to discuss product selection with other oscilloscope users or call the manufacturer's technical support.