In order to accurately measure the frequency response and fast rising edge, the oscilloscope and probe must have sufficient bandwidth.A good rule of thumb is that the bandwidth of the oscilloscope and probe (which also has a bandwidth limit) should be three to five times the highest frequency of the signal being measured. The bandwidth attenuation of -3db will introduce a measurement error of 30%, so the wider the bandwidth of oscilloscope and probe, the better.
There is a very common situation, people use X company's oscilloscope but Y company's probe for measurement.In fact, oscilloscopes and probes are not always interchangeable or compatible.It is best to use oscilloscopes and probes from the same company to rule out any potential conflicts.
One of the most overlooked steps in measuring with an oscilloscope is calibration.Calibration is an easy way to ensure that every measurement you make is from scratch, unaffected by the last measurement.Manual calibration should be performed before starting the measurement. If the oscilloscope has a self-calibration function, you should run this function before measuring.The uncompensated or overcompensated probe will lead to serious errors in amplitude, rise time and distortion measurement of measured signal waveform.
The best results are always obtained by selecting the right probe for the specific measurement task.Usually, for general measurements, 10:1 probe is sufficient;For low amplitude signal measurements, however, you may want to consider a 1:1 probe. The probe capacitance should be taken into account when making high - speed measurements.Probes with large capacitance values can slow down the rise and fall edges, and even cause oscillations in some devices (such as high-speed operational amplifiers) when examining the input or output ends. Therefore, another consideration for measuring high-speed circuits is the use of active FET probes.Active probes usually have a low contact capacitance (usually several pF's) and a very high impedance, so the active probes have a very small load on any node under test.
The probe ground
The biggest mistake that can be made in high speed measurement is using probe earthing clamp, which causes the probe problem of oscilloscope.Using an earthing clamp is equivalent to adding a series inductor to the ground path.This series inductor works with the probe capacitance to introduce oscillations and overshoot. The best way to connect the probe to earth is to use the earthing shielding network inside the probe, but you have to disassemble the probe.But it's not hard to do.First, loosen the plastic skin from the probe probe and remove it from the probe.The probe's metal earthing screen is exposed.Then remove the grounding clip from the probe and you are done. It is now possible to measure a node simply by connecting the metal earthing shield of the probe to the nearest ground point of the circuit under test.This method eliminates any series inductance, eliminating almost all oscillations and overshocks. If you cannot find the nearest ground, place a piece of bus on the metal ground shield of the oscilloscope probe several times, and then ground it again.You can use almost any small metal object for a ground connection. I've used screwdrivers, paper clips, and tweezers.My favorite tweezers are tweezers, because the tip of the tweezers can be poked into the ground plane or plugged into the area where the PCB board is packed with components.
When measuring time difference or transmission delay, be sure to use two probes of the same length. The transmission delay of the cable is about 1.5 ns/ft. Different cable lengths can get you in trouble. For example, using a 3-foot and 6-foot cable oscilloscope probe to measure transmission delay, the cable length difference results in an error of about 4.5 nanoseconds (ns), which is a considerable error when it comes to resolving measurements in units of 1 ns.
While these tips and tricks may not seem noteworthy by themselves, taken together they can significantly improve the accuracy of your measurements.Even if you use only a few of these methods in your measurements, they still ensure that you get fast and reliable measurements every time you enter the lab.