Overlooked Probe Usage Mistakes

Misconception1. 100 MHz of“signal”, using 100 MHz oscilloscope probe

Oscilloscope probe bandwidth is specified in the same way as the bandwidth of the oscilloscope with which they are used, namely, the -3dB point. For example, if using a 100 MHz bandwidth probe to measure 100 MHz 1 Vpp sine wave, then the probe output will display a sine wave 0.7 Vpp amplitude. Therefore,100 MHz probe is not suitable for measuring 100 MHz signal. A common rule of thumb is to use a 3 times to 5 times the clock frequency or the probe with the fastest trigger rate in the digital system for measurement. This provides the ability to capture the third or fifth harmonic of the fundamental frequency of the clock or digital signal, so that the signal on the oscilloscope screen can more accurately represent the real signal with square edges. Another useful rule is BW*Tr=0.35(for 10-90 Tr). This rule can be used to determine the bandwidth required to measure a given rise time, and can also be used to determine the fastest edge that a probe with a specified bandwidth can measure.

Misconception2.Only high-bandwidth measurements require active probes

The low loading of active probes is their most frequently overlooked advantage. Whenever the probe contacts the target, the probe becomes part of the circuit it measures. This close contact effect between the probe and the circuit is called probe loading. The greater the loading, the more probe disturbance is introduced into the signal under test. Probe manufacturers specify the input resistance and capacitance of probes. Typical 500 MHz passive probe has a parallel resistance of 10 MΩ, capacitance 9.5 pF; whereas a typical 1 GHz active probe has a parallel resistance of 1 MΩ, capacitance 1 pF. At DC, to the circuit under test, a passive probe appears as a 10 MΩ impedance to ground, whereas an active probe will be 1 MΩ. Both are very large impedances, which means there is no significant effect on low-frequency signals. At higher frequencies, probe capacitance will adversely affect the circuit under test. For example, at 75 MHz frequency, the passive probe capacitance will present 150 Ω impedance to ground, while the active probe capacitance will present2.5 KΩ impedance to ground. The smaller capacitance of the active probe will result in less loading of AC signal content at 10 kHz and above than a passive probe.

Misconception3.All oscilloscope probe attenuation ratios are 10:1

Probes attenuate the signal under test so that the signal presented to the oscilloscope does not exceed the oscilloscope input range. Larger oscilloscope probe attenuation ratios, such as 10:1、50:1、100:1 etc., are used to measure higher voltages, while small attenuation ratios such as 2:1 and 1:1are suitable for lower voltages. The measurement system noise (oscilloscope noise plus probe noise) will increase in direct proportion to the oscilloscope probe attenuation ratio. This is an important consideration when selecting a probe.10:1 passive probe and 1:1 Passive probes can all be used to measure 1Vpp typical signal, but 1:1 passive probes will provide a more favorable signal-to-noise ratio.

Misconception4.Only a stable connection is needed to begin measurement

When people see the many connection accessories included with an oscilloscope probe, they may develop this misconception that simply attaching them to the probe is sufficient to achieve the measurement objective. These accessories are intended to provide convenience for users, enabling simple and fast qualitative measurements, such as checking whether power is present or a clock is toggling. Quantitative measurements include rise time, period, overshoot, and so on; when making quantitative measurements, it is best to remove the accessories and use the shortest possible connection. Longer accessories add inductance to the probe's signal path, greatly reducing its bandwidth and increasing the probe loading on the circuit under test.

Misconception5.Ground is ground

This statement is self-evident, but for oscilloscope probes it may not be accurate. Probe grounding can be done incorrectly. 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 its impedance becomes problematic. The return current flowing down along the probe shield encounters this impedance. This reduces probe bandwidth and causes observable signal ringing. In addition, the longer the ground lead, the larger the loop formed by the lead, and it also becomes a larger antenna for picking up stray noise. It is best to always use the shortest possible ground connection.

Misconception6.Measuring power using a current probe and a voltage probe

Power= Voltage* Current, so the above view seems reasonable. In fact, it is wrong because this statement is incomplete. To accurately measure power with an oscilloscope, the voltage probe and current probe need to be deskewed. The electrical lengths of voltage probes and current probes are usually different. This is caused by cable length and device delay, so that the signals from the two probes arrive at the oscilloscope at different times. As a result, for systems such as switch-mode power supplies, the voltage and current change dynamically, resulting in an incorrect product of voltage multiplied by current. Deskewing the probes removes the signal propagation time difference between the two probes and corrects the error. Literature on oscilloscope probe use will contain detailed information on this process; it usually requires probing a known signal, such as the deskew fixture supplied by the manufacturer with the probe, and aligning it in time by adjusting the channel delay on the oscilloscope. Many oscilloscopes have a built-in deskew operation function that can automatically perform time alignment when a calibration signal is detected.

Misconception7.Using DC blocking / AC coupling to eliminate DC

In many cases, the useful signal to be analyzed is an AC signal riding on top of a relatively large DC signal. Measuring the ripple and noise of a DC power supply is a common example.“The old-school”method is to connect a large capacitor in series with the probe to remove the DC component, allow the signal to be centered on the screen, and amplify it for analysis. Another better method is to use a probe with“probe offset”capability, such as N7020A power-rail probe. Probe offset is located where the oscilloscope and probe inject a zeroing voltage into the probe, preferably after the probe's high-value tip resistor. The advantage of using probe offset is that only DC is eliminated. When using DC blocking, low-frequency content is also filtered out. When measuring ripple and noise on a DC power supply, DC blocking can filter out low-frequency power-supply drift and supply variations. Another advantage of probe offset is that, as the user adjusts the applied offset, the oscilloscope knows how much DC has been eliminated and can display this information and use it in calculations or automatic measurements.

Myth8.Do not place oscilloscope probes inside a temperature chamber

At one time, this statement was not wrong. However, multiple high-temperature options are now available to users. For example, Keysight Technologies offers a range of voltage and current probes that can be used inside a temperature chamber, 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 connect from inside the temperature chamber to the outside of the temperature chamber where the test equipment is located.

Myth9.Current probes fail when measuring“small”currents

Many oscilloscope current probe users, when attempting to measure small currents (1-50mA) have had unpleasant experiences, finding that the current-probe offset in different measurements is larger than the current under test. This is caused by multiple factors, such as variation in the position of the lead passing through the probe, thermal drift of the probe, residual magnetization, or external signal coupling in the wire loop used to measure current. For very small currents (uA and below) measurements, there is a new type of current probe, such as N2820A high-sensitivity current probe. This probe does not use the previous magnetic-field sensing method but relies on Ohm's law. This differential voltage probe can measure the voltage across a sense resistor from 1mΩ to 1MΩ and display the current measurement result on an oscilloscope. This method eliminates the error sources mentioned above and enables users to accurately measure very small currents with an oscilloscope.

Misconception10.You cannot use two probes while driving an oscilloscope

There is a product brochure from a probe manufacturer that has not yet been made public; this brochure concerns probe holders and probe positioners and is therefore often overlooked. These convenient accessories can assist users, enabling them to operate the oscilloscope while probing multiple locations. They vary in complexity; some are simple two-legged stands that connect to the probe to form a stable tripod, with the probe becoming the third“leg”; others are multi-axis, infinitely positionable holders positioned to probe horizontal and vertical targets.

Misconception11.Probing modern high-density targets is difficult

Probing high-density targets is not as difficult as many users think. Probe manufacturers are continuously working to create new accessories or probe product lines to make probing high-density targets easier. They have reduced the diameter of new passive probes, making it easier to capture targets; in some cases, they add a headlight to active probes to illuminate the target. A new type of magnetic probe has even emerged N2851A, with which users can solder a small probe connection point to the target; the probe connects to the probing site and is held in place by a small magnet inside the probe. In this way, by moving the magnet, the probe can be easily moved from one position to another.

Reprinted from Keysight Technologies