Tips to improve oscilloscope measurement accuracy

The oscilloscope is a commonly used electronic measuring instrument, which is widely used in many industries. When we use the oscilloscope, its measurement accuracy is very important for users. In fact, when using the oscilloscope, using some simple methods can greatly improve the measurement accuracy of the oscilloscope.

bandwidth

To accurately measure frequency response and fast rising edges, the oscilloscope and probe must have sufficient bandwidth. A good rule of thumb is that the bandwidth of the oscilloscope and probe (probes also have bandwidth limitations) should be 3 to 5 times the highest frequency of the measured signal. -3dB bandwidth attenuation will introduce a 30% amplitude measurement error, so the wider the bandwidth of the oscilloscope and probe, the better.

compatibility

It is very common to see people using an oscilloscope from Company X with a probe from Company Y. The reality is that oscilloscopes and probes are not always interchangeable or compatible. It is best practice to use oscilloscopes and probes from the same company to eliminate any potential conflict issues.

calibration

One of the most overlooked steps when taking measurements with an oscilloscope is calibration. Calibration is a simple and easy way to ensure that each measurement you make is from scratch and not affected by the last measurement. Manual calibration should be performed before starting a measurement, and if the oscilloscope has a self-calibration function, you should run this function before taking a measurement. Under-compensated or over-compensated probes can introduce significant errors in the measurement of amplitude, rise time, and waveform distortion of the measured signal.

Probe

The best measurement results are always obtained by choosing the right probe for the specific measurement task. Usually, for general-purpose measurements, a 10:1 probe is sufficient; but for low-amplitude signal measurements, you may want to consider using a 1:1 probe. When making high-speed measurements, probe capacitance should be considered. Probes with large capacitance values ​​will slow down the rising and falling edges, and even cause some devices (such as high-speed op amps) to oscillate when probing the input or output of these devices. Therefore, another consideration for measuring high-speed circuits is to use active FET probes. Active probes usually have low contact capacitance (usually a few pF) and very high impedance, so active probes present very little loading to any node being measured.

Probe grounding

The biggest mistake you can make when making high-speed measurements is to use the probe ground clip, which can cause problems with your oscilloscope probe. Using the ground clip is like adding a series inductor to the ground path. This series inductance, combined with the probe capacitance, can introduce ringing and overshoot. The best way to make a ground connection is to use the ground shield inside the probe, but you have to disassemble the probe. However, this is not difficult to do. First, loosen the plastic cover from the probe tip and peel it off the probe. The metal ground shield of the probe is now exposed. Then, remove the ground clip from the probe and you are done. Now that you can make a measurement, simply measure a node and connect the metal ground shield of the probe to the nearest ground point of the circuit being measured. This method eliminates any series inductance and almost eliminates all ringing and overshoot. If you can't find the closest ground point, wrap a section of bus wire around the metal ground shield of the oscilloscope probe a few times and then connect it to ground. You can use almost any small metal object to make a ground connection. I have used screwdrivers, paper clips, and tweezers. My favorite tool is tweezers because the tips of the tweezers can poke into ground planes and reach into densely populated areas of the PCB.

Differential Measurement

When making a time difference or propagation delay measurement, make sure you are using two probes of the same length. The propagation delay of a cable is about 1.5ns/ft. Cable length differences can get you in trouble. For example, using a 3-foot and a 6-foot cable oscilloscope probe to measure propagation delay, the cable length difference can cause an error of about 4.5 nanoseconds (ns), which is a considerable error when trying to resolve measurements in 1ns units.

Although each of these tips and tricks may seem insignificant individually, together they can significantly improve the accuracy of your measurements. Even if you only use a few of these techniques in your measurements, they will ensure that you get fast and reliable results every time you enter the lab.