Computers and Technology

11 Frequently Asked Questions About Oscilloscope Probes

1. 100MHz “signal” using a 100MHz Oscilloscope probe

Oscilloscope probe bandwidth is specified in the same way as the oscilloscope bandwidth with which they are used, the -3dB point of the product’s response.

For example, if we measure 100 MHz 1Vpp sine wave with a probe with a 100 MHz bandwidth, the probe output will show an amplitude of 0.7 Vpp for the sine wave. Therefore, 100 MHz probes are not suitable for measuring 100 MHz signals. As a general rule of thumb, use the probe with 3x to 5x the clock frequency or the fastest trigger rate in a digital system to make measurements.

This provides the ability to capture the third or fifth harmonic of the fundamental frequency of a clock or digital signal, allowing the signal on the oscilloscope screen to more accurately represent the real signal with square edges. Another useful rule is BW*Tr=0.35 (for 10-90 Tr). Use this rule to determine the bandwidth required to measure a given rise time, and to determine the fastest edge a probe with a specific bandwidth can measure.

2. Oscilloscope probes that active one is only required for high bandwidth measurements

We often overlook the low loading of active probes, which is their advantage.

Whenever the probe makes contact with the target, the probe becomes part of the circuit it is measuring. Probe loading is the close contact effect between the probe and the circuit. The larger the load, the more probe interference it brings to the signal under test. Probe manufacturers specify input resistance and capacitance for probes.

A typical 500 MHz passive probe is 10 MΩ in parallel with 9.5 pf; a typical 1 GHz active probe is 1 MΩ in parallel with 1 pf. At DC, a passive probe will look like a 10 MΩ impedance to ground to the circuit under test, whereas an active probe will be 1 MΩ. Both are very large impedances, which means there is no noticeable effect on low frequency signals. At higher frequencies, probe capacitance will adversely affect the circuit under test.

For example, at 75 MHz, a passive probe capacitor will present 150 Ω impedance to ground, while an active probe capacitor will present 2.5 KΩ impedance to ground. The smaller capacitance of active probes will result in less loading of AC signal content above 10 kHz than passive probes.

3. Oscilloscope probes all have a 10:1 attenuation ratio

The probe attenuates the signal under test so that the signal presented to the oscilloscope does not exceed the oscilloscope’s input range.

Larger attenuations, such as 10:1, 50:1, 100:1, etc., are used to measure higher voltages, while small attenuations, such as 2:1 and 1:1, are suitable for lower voltages. The noise of the measurement system (oscilloscope noise plus probe noise) increases the probe attenuation ratio proportionally. This is an important consideration when selecting a probe. Both 10:1 passive probes and 1:1 passive probes can be used to measure typical signals of 1Vpp, but 1:1 passive probes result in a more favorable signal-to-noise ratio.

4. Just establish a stable connection to start measuring

When one sees the multitude of connection accessories included with oscilloscope probes, the misconception can arise that simply connecting them to the probe will accomplish the measurement. we design these accessories for the convenience of the user, allowing them to easily and quickly make qualitative measurements to check if the power supply is energized or if the clock is switching. Quantitative measurements include rise time, period, overshoot, etc. When performing quantitative measurements, it is best to remove accessories and use the shortest possible connections. Longer attachments add inductance to the probe’s signal path, greatly reducing its bandwidth and increasing the probe loading on the circuit under test.

5. Ground is ground

This statement is self-evident, but it may not be true for oscilloscope probes.

We ground the probe in the wrong way. The probe’s ground lead has inductive properties, and its impedance increases with frequency. The longer the probe ground lead, the greater its inductance and the lower the frequency, at which impedance problems can arise. Current returning down the probe’s shield encounters this impedance. This reduces the probe bandwidth, resulting in observable signal ringing. Also, the longer the ground lead, the larger the loop created by the lead, and it also becomes a larger antenna that picks up stray noise. It is best to always use the shortest possible ground connection.

6. Measure power using current and voltage probes

Power = Voltage * Current, so the above point seems reasonable. In fact, its mistake is that this statement is incomplete.

In order to accurately measure power with an oscilloscope, we need to offset correct voltage probes and current probes. Voltage probes and current probes usually have different electrical lengths. Cable length and equipment delay cause this, so that the signals from the two probes arrive at the oscilloscope at different times. As a result, for systems like switched mode power supplies, the voltage and current change dynamically, resulting in an incorrect voltage multiplied by current product. Biasing the probes removes the difference in signal transit time between the two probes and corrects errors.

Many oscilloscopes have built-in skew correction operations that automatically perform time alignment when we can detect a calibration signal.

7. Use DC Blocking/AC Coupling to Eliminate DC

Many times, AC signal is the useful signal waiting for analysis that sits on top of a relatively large DC signal.

A better approach is to utilize a probe with “probe offset” capability, such as the N7020A power supply probe. The probe offset is located where the oscilloscope and probe inject the zero voltage into the probe, preferably after the probe’s high value probe resistor. The advantage of using probe offset is that we can only eliminate. Using DC blocking can also filter out low frequency content. When measuring ripple and noise on a DC supply, DC blocking filters out low frequency supply drift and supply variations. Another advantage of probe offset is that the user adjusts the access offset and the oscilloscope knows DC blocking had removed how much DC and can display this information and use it in calculations or automated measurements.

8. Do not put Oscilloscope probes in the thermostat

Previously, this statement was not wrong. However, a variety of high-temperature options are now available to users.

For example, Utimemel Electronics now offers a range of voltage and current probes that can be used in thermostats with an operating temperature range of -50°C to +150°C. In addition to high-temperature capability, these probes also have longer cables so they can be connected from the inside of the chamber to the outside of the chamber where the test equipment is located.

9. Current probes don’t work when measuring “small” currents.

Many oscilloscope current probe users have had the unpleasant experience of trying to measure small currents (1-50mA) and have found that the current probe deviation from measurement to measurement is greater than the current they measured. This is due to a variety of factors, such as changes in the position of the leads passing through the probe, thermal drift of the probe, residual magnetization, or external signal coupling in the wire loop used to measure the current.

For the measurement of very small currents (uA and below), there is a new type of current probe that does not use the previous method of magnetic field sensing, but relies on Ohm’s law. This differential voltage probe measures voltage from a 1mΩ to 1MΩ sense resistor and displays the current measurement on an oscilloscope. This approach eliminates the aforementioned sources of error, allowing users to accurately measure very small currents with an oscilloscope.

10. You cannot use two oscilloscope probes while driving an oscilloscope

The probe manufacturer published a product presentation, people often overlook it because this one is about the probe holder and probe positioner. These handy accessories assist the user in operating the oscilloscope while probing multiple locations. They are of varying complexity, the probe attaches to some simple bipods to form a stable tripod, and the probe becomes the third “foot”; some are multi-axis, infinitely positionable mounts that are positioned to support horizontal targets and The orientation of the vertical target for detection.

 11. Detecting modern high-density targets is difficult

The detection of high-density targets is not as difficult as many users think. Probe manufacturers are always working on creating new lines of accessories or probes to make probing of high-density targets easier. They reduced the diameter of the new passive probe to make it easier to capture the target; in some cases, they added headlights to the active probe to illuminate the target. There’s even a new type of magnetic probe, the N2851A, where the user solders a small probe attachment point to the target, which connects to the probe and hold it in place by a small magnet inside the probe. In this way, we can easily move the probe from one position to another.

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