The oscilloscope is a critical piece of the signal integrity measurement solution. The oscilloscope can provide detailed analysis of the digital signal to identify possible analog problems. For quality measurements, it’s important to look carefully at the performance of the oscilloscope and make sure it can meet the challenges of the signals being analyzed.
When choosing a scope, there are several key performance considerations that impact the quality of signal integrity measurements. These include bandwidth, rise time, sample rate, waveform capture rate, record length, and triggering flexibility.
The user must keep in mind that all digital oscilloscopes clarify the device bandwidth as the frequency at which a sine wave signal will be attenuated to 71% of its true amplitude (-3 Decibel point). In other words, the trace you see in the display will have a 29% error of the input. Before buying a digital oscilloscope you must refer the datasheet to note the BW defined for all voltage ranges. If your input waveform is not a pure sine wave, it is sure to include higher frequency harmonics. For example, a 500 MegaHertz pure sine wave, when seen on a 50 MegaHertz bandwidth oscilloscope will be shown as an attenuated and errored waveform. So theoretically, the user should go for an oscilloscope that has a BW five times higher than the userâ€™s input waveform.All Data communication buses in use on automotive applications are operating at not more than 100 Mbps and less in most cases so bandwidth at 20GHz is adequate for automotive systems.
In the digital world, rise time measurements are critical. Rise time may actually be a more appropriate performance consideration
than bandwidth when choosing an oscilloscope to measure digital signals like pulses and steps. Since semiconductor
device technology advances have brought faster edge performance to virtually every logic family, it’s important to
remember that many digital systems that are designed with slower clock rates may still have very fast edges The basis for the oscilloscope rise time selection is similar to that for bandwidth. In general, an oscilloscope with faster rise time will more accurately capture the critical details of fast
transitions. Just as with bandwidth, achieving this rule of thumb can be difficult when dealing with the extreme speeds of today’s high-speed serial buses. The rise time measured by the oscilloscope will depend on both the actual signal rise time and the oscilloscope rise time.
The faster the oscilloscope rise time, the more accurate the measured rise time will be.
A digital storage oscilloscope stores samples into the oscilloscope’s memory. The larger or deeper memory allows for more samples to be stored. The more samples stored, the higher the sample rate. In other words, deep memory allows users to maintain the DSO’s maximum sampling rate across a vast selection of time base settings. With this higher sustained sampling rate more reliable and more accurate measurements are obtained.
A typical challenge when using a digital storage oscilloscope is to capture adequate cycles of both fast and slow signals simultaneously while maintaining enough data points and resolution to zoom in and closely view signal details. Without sufficient resolution between data points, it is virtually impossible to determine what is actually going on with their project or design. Insufficient resolution means the user could be completely missing events like glitches and anomalies. Without an oscilloscope with fast real-time sampling and deep memory, serious issues like these can take hours or even days for an engineer or technician to eventually discover. An oscilloscope boasting fast real time sampling but with a shallow memory depth compromises real-world sampling performance and provides an incomplete picture of the digital and analog interactions in the subject design.
Sample rate – specified in samples per second (S/s) – refers to how frequently a digital oscilloscope takes a sample, or a visual snapshot, of the signal. A faster sample rate provides greater resolution and detail of the displayed waveform, making it less likely that critical information or
events will be lost. Waveform Capture Rate The waveform capture rate, expressed as wave forms per second (wfms/s), determines how frequently the oscilloscope captures a signal. While the sample rate indicates how frequently the oscilloscope samples the input signal within
one waveform, or cycle, the waveform capture rate refers to how quickly an oscilloscope acquires the whole waveform.
This amount of memory becomes important as you capture a longer period of time (more samples). The more memory available, the higher you can keep the sample rate of the scope. This means a higher “effective” bandwidth. As an example, we will look at memory depth as it applies to a typical DSO with a 1 GSa/sec sample rate at various time base settings.
Sample rate = memory depth/(time per division*10)
So using an example of a sweep rate of 100 us/div with a 1 Mpts memory depth we are at the maximum sample rate possible of 1 GSa/sec. But if we slow the sweep rate down to 1 ms/div we now get an equivalent sample rate of 100 Msa/sec. Modern DSO's will automatically adjust the Memory Depth to maximize the sample rate. Note that scope with a 2 Mpts memory depth would double the equivalent sample rate at this slower time base setting. Memory depth can be an important parameter when viewing low frequency of slowly changing signals but may not be as critical at faster time base settings.Oscilloscopes with high waveform capture rates provide significantly more visual insight into signal behavior. They can dramatically increase the probability that the oscilloscope will quickly capture transient anomalies like jitter, runt pulses, glitches, and transition errors. Record length is the number of samples the oscilloscope can digitize and store in a single acquisition. Since an oscilloscope can store only a limited number of samples, the waveform duration – or length of “time” captured – will be inversely proportional to the oscilloscope’s sample rate. Today’s oscilloscopes allow the user to select the record length for an acquisition to optimize the level of detail needed for the application. If a very stable sinusoidal signal is being analyzed, a 500-point record length may be sufficient. However, if a complex digital data stream is being analyzed for the causes of timing anomalies, a record length of over a million points may be required. A longer record length enables a longer time window to be captured with high
resolution (high sample rate).
The triggering functions in an oscilloscope are just as critical as those in a logic analyzer. Like a logic analyzer, the
oscilloscope’s trigger is proof that a specified type of event occurred. Modern oscilloscopes offer triggers for a host of analog events:
Edge levels and slew rate conditions
Pulse characteristics, including glitches,
low-amplitude events and even width conditions
Setup and hold time violations
Serial digital patterns
All of these trigger types can assist in detecting and isolating signal integrity problems. There are also
various combinations of voltage, timing, and logic triggers, as well as specialty triggers, for applications such as serial
data compliance testing.