Skip navigation
All Places > Keysight Blogs > Oscilloscopes Blog > Blog > Author: BoonCampbell
1 2 Previous Next

Oscilloscopes Blog

20 Posts authored by: BoonCampbell Employee

 Oscilloscopes are used to measure and evaluate a variety of signals and sources. They also play a major role in both design and manufacturing, providing a visual display of voltage over time. Many times oscilloscope users need more than a visual representation and want to validate the quality and stability of electronic components and systems. The Keysight Technologies mask test option, DSOX6MASK, for InfiniiVision Series oscilloscopes can save you time and provide pass/fail statistics in seconds. The mask test option offers a fast and easy way to test your signals to specified standards, as well as the ability to uncover unexpected signal anomalies and glitches. Mask testing on many industry oscilloscopes is based on software-intensive processing technology, which tends to be slow. Keysight’s mask test option is based on hardware-accelerated technology performing up to 270,000 real-time waveform pass/fail tests per second. This makes your testing throughout orders of magnitude faster. This Keysight InfiniiVision ground breaking Mask Technology is supported by their industry leading highest sampling rate at 1,000,000 waveforms per second allowing for improved display quality to catch subtle waveform details such as noise and jitter.

 

Mask Testing

Figure 1 shows a pulse-shaped mask using an input signal standard. You can easily specify horizontal and vertical tolerance bands in either divisions or absolute volts and seconds. You can set up the mask test to run continuously in order to accumulate valid pass/fail statistics. In this example, an infrequent glitch was quickly detected. In just six seconds, the mask test statistics show that the scope performed the pass/fail mask test on more than 1,000,000 waveforms and detected just two errors for a computed error rate of 0.0002%. In addition, you can see that this particular signal has a sigma quality relative to the mask tolerance of approximately 6.1 σ.

Mask Testing

Figure 1 - Mask testing uncovers an infrequent signal anomaly

 

Importing an industry-standard mask -

Figure 2 shows testing results of an industry standard imported eye-diagram mask. This particular polygon-shape mask is based on a published standard and was created using a simple text editor.

 Mask Testing

Figure 2 - Testing an eye-diagram with an imported industry standard mask.

 

You can also set up masks around areas of the signal that should be off-limits. That way you do not need a perfect signal; just an understanding of how a signal looks when it functions correctly. Mask testing provides more reliability than testing individual attributes because the entire signal can be evaluated against a correct signal to find glitches and/or errors. As a result, mask testing saves time and money in design and manufacturing and ensures customers receive higher-quality products sooner.

 

Keysight Mask Test Functions include -

  • Automatic mask creation using input standard
  • Easily download multi-region masks and setups based on industry standards
  • Detailed pass/fail statistics
  • Test to high-quality standards based on sigma
  • Multiple user-selectable test criteria

 

When setting up your specific mask test criteria, you can choose from multiple options including:

  • Run forever (with accumulated pass/fail statistics)
  • Run until a specified number of tests
  • Run until a specified time duration
  • Run until a maximum ideal sigma standard
  • Stop-on-failure
  • Save-on-failure
  • Print-on-failure
  • Trigger out-on-failure

 

Low Noise –

Lower inherent vertical noise is key when making quality and precise mask tests. Vertical noise can cause random increases (spikes) in all areas of the signal, including trigger timing issues. Figure 4 below shows test results in just 6 seconds, capturing 3 elusive failures out of one million waveforms sampled. A consistent trigger point is vital to prevent jitter and drift of the waveforms. If a signal drifts outside the mask limits due to noise and/or trigger jitter, mask testing will analyze the oscilloscope-induced error components as a failure, thereby corrupting the test results. A low noise floor is essential to attain the precision and reliability of Six Sigma testing.

 Mask Testing

Figure 3 - Mask testing to 6.2 sigma resolution takes only seconds using an InfiniiVision scope with a mask test rate of 270,000 waveforms/s

 

Conclusion –

Not only does Keysight provide the fastest, low-noise mask testing performance in the market, but the mask test application on InfiniiVision scopes also has a variety of features for customizing your measurements. The Keysight mask testing software allows you to setup the mask test to run for a specified time, until a certain sigma threshold is reached, continue running upon a failure, stop on a failure, or save the data on a failure. This allows you to run a complete six sigma test in around one second. If a failure occurs the waveform can be set to automatically stop or saved for viewing and later analysis work. The capability to assure the quality of products to Six Sigma in 1.1 seconds saves time and effort in R&D and manufacturing. In R&D environments, engineers can test signals they are developing through many waveform repetitions spending less time but still ensuring signal stability. The more waveforms they can test, the more confidence they have that the design functions correctly. In manufacturing environments engineers must test signals to ensure customers receive reliable products. Your company can save manpower hours and can invest the capital in profitable initiatives, such as new product design. At the same time, customer satisfaction will increase as you produce high-quality products and move them quickly into the market.

Lab benches are many times cluttered with multiple pieces of test equipment. Keysight’s InfiniiVision Oscilloscopes are equipped with a built in digital voltmeter, frequency counter and totalizer giving the oscilloscope user additional measurement options that can reduce the amount of test equipment needed. In addition, when you only measure the frequency of a signal, you rarely get the whole story. A repetitive signal can have spurs, intermittent spikes and noise that you need to see during design and or debug. The oscilloscope counter will show you all these attributes in addition to the frequency in one screen shot giving you the “big” picture. Keysight InfiniiVision oscilloscopes include both a 3 digit voltmeter (DVM) and a 5-10 digit Integrated Counter depending upon the oscilloscope model number (Figure 1 below).

 

 

Figure 1 – Functionality, options and specifications across the Keysight InfiniiVision family of Oscilloscopes

 

Digital Volt Meter

The DVM and Integrated Counter operate through the same probes as the oscilloscope channels. However, these measurements are decoupled from the oscilloscope triggering system measuring 100 points per second. This flexibility allows engineers to make DVM and triggered oscilloscope measurements with the same connection. DVM results are presented with an always-on seven-segment display keeping these quick characterization measurements at the engineers' fingertips. You get the added flexibility of measuring five types of DVM measurements depending upon your application: Peak-Peak, AC rms, DC, and DC rms. As a user you should also note that the oscilloscope DVM is designed for quick rough measurements as needed in design or debug and not meant to replace exact measurements you would get from a calibrated external DVM.

 

Standard 5 digit counter resolution

The traditional oscilloscope counter measurements offer only five or six digits of resolution, which may not be enough for the most critical frequency measurements are being made. With a 10 digit counter you can see your measurements with the precision you would normally expect only from a standalone counter. The Keysight integrated counter’s ability to measure frequencies up to a wide bandwidth of 3.2 GHz, allows it to be used in many high-frequency applications. This integrated hardware counter allows users to make much more accurate frequency measurements on signals. 4 digits is 1 part in ten thousand, or ~0.01% of the displayed number. In addition, relative to an industry standard oscilloscope frequency measurement, the Keysight counter measurement is designed to be very easy to use. It uses the trigger level of the oscilloscope as the trigger level for the counter independent of the cycles shown on the screen.

 

Up to 8 to 10 digit resolution with external time base

If an external 10MHz reference is used, the counter is as accurate as the externally fed 10MHz signal and the measurement resolution is increased. The 10MHz REF BNC connector on the rear panel is provided so you can supply a more accurate clock signal to the oscilloscope. To drive oscilloscope’s time base from external clock reference, connect a 10MHz square or sine wave reference signal to the 10MHz REF BNC input on the rear panel and go to the Utility -> Options ->Rear Panel menu and select Ref signal mode to 10 MHz input. The working 10MHz input voltage is 180mV to 1V in amplitude, with a 0V to 2V offset. To get the highest resolution, the time/div setting should be at 200mS/div or slower. With this setting, the resolution is increased up to 8 digits, which is what would be displayed if an external 10MHz reference is used. When the internal reference is used, the oscilloscope displays counter measurement in 5 digits. The counter measurements can measure frequencies up to the bandwidth of the oscilloscope.

 

Accuracy

Basically, the counter is as accurate as the time base reference that is used.  The oscilloscope’s time base uses a built-in 10MHz reference that has an accuracy of 1.6 ppm to 50 ppm depending upon the oscilloscope model number. This means that the number displayed is within 0.00016% to 0.0050% respectively of the actual signal measured. For example, if you are making a counter measurement of 32,768 Hz signal using a model 6000X with 1.6 ppm accuracy, you are measuring the signal at ~0.05 Hz accuracy (see calculation below).

32,768 Hz x 1.6 ppm (0.00016%) = ±0.0524288 Hz

 

Totalizer

The totalizer feature of the DSOXDVMCTR counter option adds another valuable capability to the oscilloscope. It can count the number of events (totalize), and it also can monitor the number of trigger-condition-qualified events. The trigger-qualified events totalizer does not require an actual trigger to occur. It only requires a trigger-satisfying event to take place. In other words, the totalizer can monitor events faster than the trigger rate of an oscilloscope, in some cases as fast as 25 million events per second. Keep in mind that the number of events is a function of the oscilloscope’s hold off time.

 

Summary

The voltmeter and counter functions discussed in this article are just two of the “6 instruments in one oscilloscope” of the Keysight InfiniiVision family. The six instruments are the oscilloscope, 16 digital channels (mixed signal), serial protocol analyzer, Dual channel 20 MHz function/arbitrary waveform generator, 3-digit voltmeter and 5 to 10-digit counter with totalizer.

 

The voltmeter operates through the same probes as the oscilloscope channels. However, the DVM measurements are made independently from the oscilloscope acquisition and triggering system so you can make both the DVM and triggered oscilloscope waveform captures with the same connection.

 

Traditional oscilloscope counter measurements offer only five or six digits of resolution. While this level of precision is fine for quick measurements, it falls short of expectations when critical frequency measurements are needed. With the integrated counter within the five Keysight oscilloscope families summarized in Figure 1, you can select between 5 and 10 digit counter options and see your measurements with the precision you would normally expect only from a standalone counter. Because the integrated counter measures frequencies up to a wide bandwidth of 3.2 GHz, you can use it for many high-frequency applications as well

Lab benches are many times cluttered with multiple pieces of test equipment. Keysight’s InfiniiVision Oscilloscopes are equipped with a built in digital voltmeter, frequency counter, and totalizer giving the oscilloscope user additional measurement options that can reduce the amount of test equipment needed. In addition, when you only measure the frequency of a signal, you rarely get the whole story. A repetitive signal can have spurs, intermittent spikes, and noise that you need to see during design and or debug. The oscilloscope counter will show you all of these attributes in addition to the frequency in one screen shot giving you the “big” picture. Keysight InfiniiVision oscilloscopes include both a 3-digit voltmeter (DVM) and a 5-10-digit integrated counter depending upon the oscilloscope model number (Figure 1 below).

 

 Digital Voltmeter

Figure 1 – Functionality, options and specifications across the Keysight InfiniiVision family of Oscilloscopes

 

Digital Volt Meter

The DVM and integrated counter operate through the same probes as the oscilloscope channels. However, these measurements are decoupled from the oscilloscope triggering system measuring 100 points per second. This flexibility allows engineers to make DVM and triggered oscilloscope measurements with the same connection. DVM results are presented with an always-on seven-segment display keeping these quick characterization measurements at the engineers' fingertips. You get the added flexibility of measuring four types of DVM measurements depending upon your application: Peak-Peak, AC rms, DC, and DC rms. As a user you should also note that the oscilloscope DVM is designed for quick rough measurements as needed in design or debug and not meant to replace exact measurements you would get from a calibrated external DVM.

 

Standard 5-digit counter resolution

The traditional oscilloscope counter measurements offer only five or six digits of resolution, which may not be enough for the most critical frequency measurements being made. With a 10-digit counter you can see your measurements with the precision you would normally expect, only from a standalone counter. The Keysight integrated counter’s ability to measure frequencies up to a wide bandwidth of 3.2 GHz allows it to be used in many high-frequency applications. This integrated hardware counter allows users to make much more accurate frequency measurements on signals. Four digits is one part in ten thousand, or ~0.01% of the displayed number. In addition, relative to an industry standard oscilloscope frequency measurement, the Keysight counter measurement is designed to be very easy to use. It uses the trigger level of the oscilloscope as the trigger level for the counter independent of the cycles shown on the screen.

 

Up to 8 to 10-digit resolution with external time base

If an external 10MHz reference is used, the counter is as accurate as the externally fed 10MHz signal, and the measurement resolution is increased. The 10MHz REF BNC connector on the rear panel is provided so you can supply a more accurate clock signal to the oscilloscope. To drive oscilloscope’s time base from external clock reference, connect a 10MHz square or sine wave reference signal to the 10MHz REF BNC input on the rear panel, and go to the Utility -> Options ->Rear Panel menu and select Ref signal mode to 10 MHz input. The working 10MHz input voltage is 180mV to 1V in amplitude, with a 0V to 2V offset. To get the highest resolution, the time/div setting should be at 200mS/div or slower. With this setting, the resolution is increased up to 8 digits, which is what would be displayed if an external 10MHz reference is used. When the internal reference is used, the oscilloscope displays counter measurement in 5 digits. The counter measurements can measure frequencies up to the bandwidth of the oscilloscope.

 

Accuracy

Basically, the counter is as accurate as the time base reference that is used.  The oscilloscope’s time base uses a built-in 10MHz reference that has an accuracy of 1.6 ppm to 50 ppm depending upon the oscilloscope model number. This means that the number displayed is within 0.00016% to 0.0050% respectively of the actual signal measured. For example, if you are making a counter measurement of 32,768 Hz signal using a model 6000X with 1.6 ppm accuracy, you are measuring the signal at ~0.05 Hz accuracy (see calculation below).

32,768 Hz x 1.6 ppm (0.00016%) = ±0.0524288 Hz

 

Totalizer

The totalizer feature of the DSOXDVMCTR counter option adds another valuable capability to the oscilloscope. It can count the number of events (totalize), and it also can monitor the number of trigger-condition-qualified events. The trigger-qualified events totalizer does not require an actual trigger to occur. It only requires a trigger-satisfying event to take place. In other words, the totalizer can monitor events faster than the trigger rate of an oscilloscope, in some cases as fast as 25 million events per second. Keep in mind that the number of events is a function of the oscilloscope’s hold off time.

 

Summary

The voltmeter and counter functions discussed in this article are just two of the “6 instruments in one oscilloscope” of the Keysight InfiniiVision family. The six instruments are the oscilloscope, 16 digital channels (mixed signal), serial protocol analyzer, Dual channel 20 MHz function/arbitrary waveform generator, 3-digit voltmeter, and 5 to 10-digit counter with totalizer.

 

The voltmeter operates through the same probes as the oscilloscope channels. However, the DVM measurements are made independently from the oscilloscope acquisition and triggering system, so you can make both the DVM and triggered oscilloscope waveform captures with the same connection.

 

Traditional oscilloscope counter measurements offer only five or six digits of resolution. While this level of precision is fine for quick measurements, it falls short of expectations when critical frequency measurements are needed. With the integrated counter within the five Keysight oscilloscope families summarized in Figure 1, you can select between 5 and 10 digit counter options and see your measurements with the precision you would normally expect only from a standalone counter. Because the integrated counter measures frequencies up to a wide bandwidth of 3.2 GHz, you can use it for many high-frequency applications as well.

What is the best oscilloscope for your application? The following areas will help you make an accurate and informed decision. Today’s complex electronics industries require a broad spectrum of test equipment, with oscilloscopes being one of the most fundamental tools used by engineers and technicians. Oscilloscopes provide design and manufacturing engineers with critical insights to signal properties suggesting additional design work needed, targeting manufacturing issues, or performing compliance and protocol testing per international standards.

Oscilloscopes fall into two groups, real-time oscilloscopes and sampling oscilloscopes (also called equivalent-time oscilloscopes) and it is important to understand the difference between the two types. Real-time oscilloscopes digitize a signal in real-time. Imagine a repetitive AC signal - the real-time oscilloscope acts like a camera, taking a series of frames of the signal during each cycle. The amount of frames the real-time oscilloscope captures depends upon the bandwidth, memory depth, and other attributes that we will soon discuss. A sampling oscilloscope, on the other hand, takes only one shot of the signal per cycle. By repeating this one shot, but at slightly different time frames, the sampling oscilloscope can reconstruct the signal with a high degree of accuracy.

The following topics can help you better evaluate which kind of oscilloscope will best suit your needs.

Trigger

Sampling oscilloscopes are designed to capture, display, and analyze repetitive signals. If your oscilloscope solution needs to capture a single random event within your waveform, a real-time oscilloscope should be selected. Whether you are looking at intermittent signals during product design or manufacturing, real-time oscilloscopes allow you to trigger on a specific event such as a rising voltage threshold, a set up and hold violation, or a pattern trigger. The real-time oscilloscope will capture and store continuous sample points around these triggers and update the display with the captured data.

 

Bandwidth

The frequency of your signal under test and the harmonics within it will determine the bandwidth of the oscilloscope that will fit your needs. Sampling and real-time oscilloscopes cover a wide bandwidth range and there is a lot of overlap. A sampling oscilloscope can acquire any signal up to the analog bandwidth of the oscilloscope regardless of the sample rate. But a real-time oscilloscope must gather a significant number of samples after the initial trigger to accurately display a waveform. A typical rule of thumb for a real-time oscilloscope bandwidth is 2.5 times your signal frequency to reproduce your signal with the best fidelity. So you can get by with an effectively lower bandwidth scope using a sampling scope as long as you have the trigger mentioned in the previous section.

 

Memory Depth

Oscilloscope memory depth is an important specification for only real-time oscilloscopes. A real-time oscilloscope captures an entire waveform on each trigger event. To do this the real-time oscilloscope captures a large number of data points in one continuous record. For a real-time oscilloscope, the memory is directly tied to the sample rate. The more memory you have, the more samples (sample rate) you can capture for each waveform.  The higher the sample rate, the higher the effective bandwidth of the oscilloscope.  There is a simple calculation to determine the sample rate given a specified time base setting and a specific amount of memory (assuming 10 divisions across screen): Memory depth / ((time per division setting) * 10 divisions) = sample rate (up to the max sample rate of the ADCs). This memory depth concept does not apply to sampling oscilloscopes because only one instantaneous measurement of waveform amplitude is taken at the sampling instant.

 

Analog to Digital Converter Bits

Sampling oscilloscopes can have as high as a 14-bit analog-to-digital converter (ADC). Consequently, they have a very large dynamic range, which enables viewing signals ranging from a few millivolts to a full volt without the need for attenuation. This allows sampling oscilloscopes to maintain very low noise levels at all volts per division settings. A real-time oscilloscope is limited in its dynamic range to 8 - 10 bits depending upon the model, but typically will show an effective bit number of around 6 – 8 bits respectively. Because of a real-time oscilloscope’s lower signal-to-noise ratio, it is designed with attenuators to correctly display signals at specific volts per division settings.

 

Frequency Response

Frequency response is another key consideration in your selection criteria. Sampling oscilloscopes do not use digital signal processing (DSP) correction, so the frequency response rolls off slowly and looks more Gaussian in shape. Real-time oscilloscopes can implement DSP to correct their frequency response. For instance, Keysight’s S-Series oscilloscope has a very flat frequency response across its bandwidth, which means its gain will not vary by more than 1 dB across the entire band.

 

Clock Recovery

The clock recovery component of an oscilloscope measurement is used for building real-time eyes, mask testing, and jitter separation. A recovered clock is a reference clock within the oscilloscope and used for measurement comparisons. Keysight’s Sampling oscilloscopes provide an accurate software-based clock recovery system. In many applications, real-time oscilloscopes have a software clock recovery and selectable hardware clock recovery frequencies. Please note that the advantage of a software clock recovery is that it is not prone to the hardware errors, and will land its edges where they need to be regardless of the data rate.

 

Applications

Sampling oscilloscopes, like real-time oscilloscopes, offer eye diagrams, histograms, and jitter measurements. With high bandwidths, modularity, and lower pricing, they typically fit manufacturing environments better than real-time oscilloscopes.

 

Many of Keysight’s sampling oscilloscopes have modular systems consisting of a mainframe and various electrical, optical and TDR modules. This allows the end user to customize measurement hardware to fit their solution. Sampling oscilloscope electrical and TDR channels can be integrated into a module to reduce cost or remote heads can be used to improve accuracy. Optical channels are always integrated creating a well-controlled 4th-order Bessel-Thomson frequency roll-off.

 

When making jitter measurements clock recovery systems play a large role. Understanding the clock recovery algorithm and the jitter transfer function used will help you determine your final oscilloscope selection. The sampling oscilloscope has a slightly lower jitter and a higher dynamic range making it ideal for characterization in a controlled environment assuming that your signal is repeatable. However, real-time oscilloscopes are great if you need to trigger on difficult to find events. Real-time oscilloscope users can choose from a long list of compliance, protocol triggering and decode, and analysis applications including jitter.

 

Form Factor

Your solution may require an oscilloscope solution with a specific size or configuration (form factor) to fit your needs. Keysight has both sampling and real-time scope solutions in a variety of form factors, from standard desk top and rack mountable frames to faceless (no screen) module solutions in a variety of AXI or PXI configurations. See the links below for sampling and real-time options.

 

http://fieldcom.cos.keysight.com/portal/Coll.php?cId=-32528

http://fieldcom.cos.keysight.com/portal/Coll.php?cId=-32546

 

Summary

On the surface there is a lot of overlap between sampling and real-time oscilloscopes but the differences in capabilities and performance that we have discussed can help you make an informed decision to tailor a selection to your specific application.

 

If you require measurements of a repetitive waveform with lower jitter and a higher dynamic range, a sampling oscilloscope is a good choice. In addition, sampling oscilloscopes have an advantage of a lower initial cost and modular upgrades, making them well suited for electrical and optical manufacturing test applications. Real-time oscilloscopes come in a variety of bandwidths, include the ability to capture single-shot events as well as repetitive signals.

 

Both Keysight sampling and real-time oscilloscopes are available in frequencies from 1 GHz to 50 GHz and beyond with a variety of modular and frame options to fit your specific requirements.

 keysight oscilloscopes samplingscope

Do you want to accurately measure signals with minimal effects from your oscilloscope probe? The oscilloscope probe is a critical link in the quest for accurate signal measurements. The probe is more than just a connection between the circuit under test and the oscilloscope. It can affect both your measurement results and the circuit under test. It’s important to select the right probe, understand its loading effects, and factor in signal fidelity impacts. This blog will cover probe selection, circuit loading, and factors effecting measurement accuracy.

 

Step 1: Selecting the Probe Type

Higher bandwidth is an obvious advantage of active probes over passive probes. Oscilloscope probe users often neglect the effect of the connection to the target. They tend to focus on the probe’s published specifications and do not factor in the effect of the probing accessories attached to the tip of the probe. The real-world, probe to system connection can drive the probe’s performance downward. Keeping this in mind, the system bandwidth of the probe and oscilloscope together should be three to five times greater than the frequency of the waveform being measured. This rule of thumb ensures adequate bandwidth for signals such as square waves with high-frequency content. In addition, when measuring rise times, you should ensure that the rise time of the oscilloscope and probe system should be three to five times faster than the pulse being measured. You may not always know the bandwidth of the target being measured but may know the rise time of the fastest signals to be measured. In these cases, there is a handy relationship for determining bandwidth: BANDWIDTH × RISE TIME = 0.35. Below is a chart to help with passive vs active probe selection.

 

Passive Probes

Active Probes

Features -

Features -

Higher Resistance

Low Loading

High Dynamic Range

High Bandwidth

Rugged

High Bandwidth

Low Cost

Least Intrusive

Tradeoffs -

Tradeoffs -

Bandwidth Limited to 500MHz

Higher Cost

Heavy Capacitive Loading

Limited Input Dynamic Range

Figure 1: Passive vs Active Probes

 

 

Step 2: Factoring Circuit Loading Effects –

All probes have a somewhat similar impact on the circuit under test. A probe draws a portion of the circuit energy and supplies this energy to the oscilloscope. All probes present a capacitive, resistive, and inductive loading element to the circuit under test. The challenge is to ensure that these effects do not impact the circuit and change the signal from its original state.

 

Capacitive - Capacitive loading is the main culprit of measurement errors. For general-purpose measurements less than 700 MHz, passive high-impedance resistor divider probes are good choices. These rugged and inexpensive tools offer wide dynamic range greater than 300 V and high input resistance to match an oscilloscope’s input impedance. These probes often come with the scope when you purchase it. But, they begin to impose heavier capacitive loading as the frequency of the signal being measured goes up. The input capacitance of the probe and oscilloscope combine to create an impedance between the signal being measured and ground. As the frequency of the signal goes up, the impedance created by the capacitance drops. If the impedance drops too low it can affect your signal being measured, this is known as capacitive loading. For example, a capacitance of 10 pF presents only 100 Ohms of impedance to a 150 MHz signal so it is important to know the input capacitance of the passive probe and the scope you are using. As a rule, high-impedance passive probes are a great choice for general purpose debugging and troubleshooting on most analog or digital circuits.

 

Resistive - Resistive loading is not as troublesome because it is the least likely to induce nonlinear behavior in your circuit. Most common is resistive loading consisting of the circuit’s output resistance and the probe’s own resistance forming a voltage divider circuit. This divider circuit distorts the signal being measured because the probe is seen as a load to the circuit under test. Even a probe with a small output capacitance of 1 pF can substantially affect the measured circuit.  A 1 pF probe looks like a 160-ohm load at 1 GHz, which is the highest frequency associated with a 0.5-ns rise or fall time.

 

Inductive - Many people think that probe input impedance is a constant number. Probes specifications may state that the probe has a kiloΩ, megaΩ or even a 10 megaΩ input impedance, but this is not constant over frequency because input impedance decreases over frequency. At DC and low frequency ranges, the probe’s input impedance starts out at the rated input resistance, say 10 MΩ for a 10:1 passive probe, but as the frequency goes up, the input capacitance of the probe starts to become a short, and the impedance of the probe starts to drop. Figure 2 below shows the limitations of the passive probe relative to an active probe. Note that the active probe in red tracks the non-loaded signal in blue very closely. But the passive probe in green has a slower slope or impulse response. This slower impulse response reflects the lower bandwidth and the ripple or lack of flatness because the displayed signal is missing the high frequency components needed for this measurement.

Figure 2: Passive vs Active Probe loading effects

 

Step 3: Understanding Signal Fidelity Impacts

Ground lead – Ground lead issues are a constant concern because of the difficulty in determining a true ground reference point. This difficulty arises from the fact that ground leads have inductance and become circuits of their own as signal frequency increases. In addition to being the source of ringing and other waveform aberrations, the ground lead can also act as an antenna for noise. Suspicion is the first defense against ground-lead problems. Always be suspicious of any noise or aberrations observed on the signal. The noise or aberrations may be part of the signal, or they may be the result of the measurement process.

 

Ground length – All probe ground leads have inductance, and the longer the ground lead the greater the inductance. Ground lead inductance combined with probe tip capacitance forms a resonant circuit that causes ringing at certain frequencies. In order to see this ringing or any other aberrations, the oscilloscope system bandwidth and the probe bandwidth must be high enough to handle the high-frequency content of the aberrations.

 

Accessory Impact – The convenience offered by some accessories can be attractive, but may come at a cost. Any accessory that is placed between the probe and the target system has the potential to adversely affect the target because of the extra capacitance and inductance of the accessory. Extra capacitance will load the target system and the larger or longer the accessory, the more impact it will have on the measurement being made. Therefore, it may be prudent to reserve the use of larger or longer accessories for slower signal measurements.

 

Passive probes usually specify an input capacitance of 10-13pF. The inductance will be mainly determined by the ground connection and as we have stated, the shorter the ground connection, the smaller the inductance. Reducing this inductance will increase the resonance frequency to a range outside the bandwidth of the oscilloscope. Figure 3 below shows the impact caused by increasing a ground accessory from a one half inch ground blade to a 9 inch ground. The best response is the blue line with a sharp rise and a small overshoot but flat response from there on. The longer ground lead, in red, causes over a 3 nS delay resulting in a lower bandwidth solution. A good rule of thumb when looking at data like this is that .35 divided by the total rise time equals the bandwidth of the probing solution. In this case the 2 nS difference reduces the calculated bandwidth by 350 MHz relative to the blue signal bandwidth. Also note the additional ripple in the red signal. This is due to the resonant circuit created by the added capacitance and inductance due to the longer ground lead. 

Figure 3: Loading Caused by a longer ground lead

 

Damping - As we have stated earlier, the performance of the probe and the resulting signal on the oscilloscope is highly affected by the probe connection. As signal speeds increase you may notice more overshoot, ringing, and other aberrations because probes and their accessories form a combined resonant circuit. If the resonance is within the bandwidth of the oscilloscope probe you are using, it will be difficult to determine whether the measured aberrations are due to your circuit or the probe resonating circuit. To reduce this effect, damping with the appropriate tip resistor can help. Figure 4 below shows the effects of damping on the measured signal reducing the aberrations on the output signal in blue. The resonance of the undamped circuit causes the oscillation seen in blue in the first screen shot using an active probe with a bandwidth greater than 1 GHz. However, this oscillation will not be seen using a 500 MHz passive probe because the frequency of the oscillation is above the bandwidth of the passive probe.

Figure 4: Undamped vs Damped waveforms

 

Summary:

The purpose of these 3 steps was to walk you through the path to accurate probing measurements. The resultant waveform on the oscilloscope will now most accurately represent the signal from your device under test. It is important to remember that there will always be a tradeoff between measurement flexibility, usability, and resulting bandwidth. As we discussed, added capacitance and inductance create RC circuits that resonate within the measured bandwidth of your signal. These circuits will reduce your bandwidth and the ripple will impact your amplitude readings as well as the loading effect. Remember, your probing solution should not load the signal source. It should pass on the signal accurately to the oscilloscope and be immune to noise. In addition, an oscilloscope probe is not a wire. Sure it’s an electrical connection between a node on a board and the oscilloscope. But its resistance, capacitance and inductance can have serious consequences as signal speeds increase.

 

Additional probing resources:

See all of Keysight’s oscilloscope probes

Learn more about Keysight probes with the Oscilloscope Probe Resource Center

See oscilloscope probing solutions in action on the Keysight YouTube channel