Oscilloscopes were once a rarity among hobbyists, and those that were available were mostly outdated military or industrial surplus. For around the price of those secondhand scopes, which were once the envy of every major research facility, you can now get a nice scope.
Although they resemble an old analog scope, this new breed of the instrument is primarily digital, and the way they operate can result in some strange gotchas, which [Arthur Pini] describes in a recent post.
Some of his advice is obvious, yet it’s simple to overlook. For instance, it makes sense if you arrange your four input channels so that each takes up a fourth of the screen. However, [Arthur] notes that by losing two bits of dynamic range, you can significantly skew a sensitive test.
Additionally, he offers advice on how to reduce measurement noise and maximize cursor data. Again, if you stop to consider how everything functions, it becomes very clear: displaying millions of samples on a screen with perhaps 2,000 pixels means that each pixel contains a significant amount of data.
It is an interesting illustration of how what your eyes tell you isn’t always the reality, even though some of the elements he cites may not be in your scope. The article serves as a wonderful reminder to consider the scope’s operation when taking measurements.
1 – Passive or Active Probe?
Passive High-Impedance Resistor Divider Probes Are Suitable for General-Purpose Mid-To-Low-Frequency (less than 600 M Hz) Measurements. These Tough and Affordable Instruments Have a High Input Resistance to Match the Input Impedance of A Scope, a Broad Dynamic Range (more than 300 V), and Low Cost.
They Have Lower Bandwidths and A Larger Capacitive Loading than Low-Impedance (z0) Passive Probes or Active Probes, Though.
Overall, High-Impedance Passive Probes Are a Fantastic Option for General Debugging and Troubleshooting on The Majority of Analogue or Digital Circuits.
- For High-Frequency Applications that Demand Precision (more than 600 M Hz),
- Active Probes Are the Preferred Method Over a Wide Frequency Range. They Are Dearer.
- Compared to Passive Probe and Have a Lower Input Voltage, but They Provide More Precise Information on Rapid Signals Due to Their Significantly Reduced Capacitive Loading.
2 — Probe Loading Check with Two Probes
Connect Your First Probe Tip to A Spot on The Circuit Before Probing It, and Then Attach Your Second Probe to That Same Location. Your Signal Should Ideally Remain Unchanged. if You See a Change, the Probe Loading Is to Blame.
In a Perfect World, a Scope Probe Would Be a Non-Intrusive Wire Attached to The Circuit of Interest that Has Infinite Input Resistance, Zero Capacitance, and Zero Inductance, and It Would Deliver an Identical Replica of The Signal Being Monitored.
the Probe, However, Becomes a Component of The Measurement in The Actual World and Adds Stress to The Circuit.
3 — Compensate Probe Before Use
The Majority of Probes Are Made to Fit the Inputs of Particular Oscilloscope Types. There Are, However, Small Differences Between Scopes and Even Between Various Input Channels Within the Same Scope.
when Connecting a Probe to An Oscilloscope Input for The First Time, Make Sure to Check the Probe Compensation Because It Might Have Already Been Set to Correspond with Another Input. the Majority of Passive Probes Feature Built-In Compensatory Rc Divider Networks to Address This.
in order for The Probe to Maintain Its Attenuation Ratio Over the Probe’s Rated Bandwidth, the Rc Divider Must Be Adjusted for Probe Compensation.
4 — Low Current Measurement Tips
Engineers Developing Cell Phones and Other Battery-Powered Devices Have Recently
To Help Them Make Sure Their Current Usage Is Within Reasonable Bounds, Devices Have Sought Higher-Sensitivity Current Metering. an Easy Technique to Measure Current without Having to Break the Circuit Is to Use a Clamp-On Current Probe with An Oscilloscope.
However, as The Current Levels Drop Into the Low Milliampere Range or Below, the Procedure Becomes Challenging.
5 — Make Safe Floating Measurements with A Differential Probe
Users of Scopes Frequently Need to Do Floating Measurements in Which neither Measurement Point Is at Earth Ground Potential. Assume, for instance, that You Observe a Voltage Drop Between the Input and Output of The Series Regulator U1 of A Linear Power Supply. Either the Voltage in Pin or The Voltage out Pin on The Regulator Is Not Grounded.
the Signal Difference Between the Test Point and Earth Ground Is Really Measured by A Normal Oscilloscope Measurement in Which the Probe Is Attached To A Signal Point and The Probe Tip Ground Lead Is Coupled to Circuit Ground. the Signal Ground Terminals (or BNC Interface’s Outer Shells) of The Majority of Scopes Are Connected to The Protected Earth Ground System.
This Is Done to Provide a Single Connecting Point for All Signals Applied to The Scope. the Majority of Scope Measurements Are Made in Relation to “earth” Ground. the Probed Point Is Basically Pulled Down to The Earth Ground when The Ground Connector Is Connected to Any of The Floating Points, Which Frequently Results in Spikes or Circuit Faults. What Is the Solution to The Floating Measurement Issue?
6 — Check the Common Mode Rejection
Common Mode Rejection’s Potential to Lower Measurement Quality Is One of The Most Poorly Understood Problems with Probing. It Is Always Worthwhile to Connect Both Probe Tips to The Dut’s Ground when Using a Single-Ended or Differential Probe to Check for Signals.
7 — Check the Probe Coupling
Move the Probe Cable and Grab It with Your Hands when Your Probe Is Attached To A Signal. if There Are Noticeable Variations in The Waveform Displayed on The Screen, Energy Is Being Coupled onto The Probe Shield and Producing This Variation. by Lowering the Common Mode Noise Currents on The Cable Shield, a Ferrite Core on The Probe Cable May Enhance Probing Accuracy.
a Parallel Series Impedance Is Produced by A Resistor in The Conductor and A Ferrite Core on The Probe Wire. Because the Signal Travels Through the Core on The Centre Conductor and Returns Through the Core on The Shield, There Is No Net Signal Current Flowing Through the Core, the Addition of The Ferrite Core to The Probe Cable Hardly Ever Influences the Signal.
8 — Damp the Resonance
The Probe Connection Has a Significant Impact on A Probe’s Performance. when Connecting an Oscilloscope Probe, You Can See Increased Overshoot, Ringing, and Other Alterations as The Speeds in Your Design Grow.
Where They Are Connected to The Device, Probes Create a resonant circuit. It will be challenging to tell if the detected disturbances are caused by your circuit or the oscilloscope probe if this resonance falls within the bandwidth of the oscilloscope probe you are using.