Don't Attach Multiple Probes to the Same Place at the Same Time!

Figure 1. Response of two different probes to an
upper-side gate drive measurement.

Along with using single-ended passive probes for high-voltage measurements, another probing "no no" to avoid is attaching multiple probes to the same place at the same time.

You are probably aware that all measuring instruments, including oscilloscopes, are subject to conditions of observability.  As we discussed in a recent post on The Impact of the Interconnect, the very act of connecting the oscilloscope to the circuit with a particular probe affects the measurement in a particular way. Probes affect the circuit by applying additional resistance and capacitance loads in parallel with the circuit at the test point. Moreover, the probes themselves limit the fidelity of the measurement due to limits of bandwidth, slew rate and common mode response. So, it is always a good idea when making a measurement to compare how different probes/interconnects will affect the measurement…but don’t try to do it all at once.  

What happens if you connect two or more probes to the same test point at the same time? It should be no surprise that the probes affect each other’s response--they ‘talk’ to each other! 

Let’s consider how that might manifest by looking at how two different probes each affect an upper-side gate drive measurement in an LED driver.  One probe is a high-voltage fiber optically isolated probe (HVFO).  The other ‘probe’ is an old, differential amplifier (DA) that employs a matched pair of high-voltage probes to achieve a differential input.  

First, we’ll make the measurement with each probe separately, as seen in Figure 1 above. 

The measurement made with the DA has more anomalies, most of which can be attributed to poorer common mode rejection ratio (CMRR) compared with the HVFO probe. This is most apparent in the negative pulse just before the leading edge of the positive pulse, which occurs at the time the lower MOSFET changes state.  This is a high dV/dT event that creates significant common mode interference.  The HVFO probe with its higher CMRR shows a lesser effect: less than half the amplitude, compared to the DA. The trace tilt of the DA measurement, on both the baseline and top of the pulse, is likely caused by loading of its probe pair.

Figure 2. Two probes at the same test point load
each other, degrading the performance of both.

Now, let’s look at the result of making the measurement with the HVFO and the DA in circuit at the same time, shown in Figure 2.

When both probes are connected at the same time, they load each other, resulting in a change in their responses.  Note that the negative spikes, which were lower for the HVFO probe when the measurements were made independently, are now comparable in amplitude.  The additional loading of the DA has reduced the CMRR of the HVFO probe.  In addition, the loading of the circuit by the DA changes the waveshape at the test point, and the HVFO’s measurement echoes that change.  In essence, the loading of the DA has been coupled to the HVFO probe results.

The obvious conclusion is that you should not make a measurement with multiple probes connected to the same test point at the same time. If you wish to compare the results of different probes, the best way to proceed is to measure separately, storing acquisitions to an oscilloscope memory.  Then, you can recall the memory and compare it to a live acquisition (as we did in Figure 1) or to another memory, without the negative effects of the probes loading each other.

Learn more about the HVFO probes in the on-demand webinar, Probing in Power Electronics: What to Use and Why, Pt. 2.

See Also:

Don't Probe HV with Single-ended Passive Probes!

Situational Awareness: The Impact of the Interconnect

Probing Techniques and Tradeoffs, Part II

Don't Probe HV with Single-ended Passive Probes!

Figure 1. A simple 120 Vrms switch-mode power supply
has a +/- 170 V peak and a 340 V pk-pk, difficult for
most single-ended passive probes to ground safely.

The high-impedance passive probes distributed with oscilloscopes of every major brand are sturdy, reliable and accurate within their specification limits, but they’re not intended for all applications.  This is especially true when measuring switch-mode power devices or other (relatively) high-voltage systems. These applications require probes that are both rated for their high voltage levels and isolated from ground as a reference voltage.  

High-impedance passive probes generally have maximum voltage limits of about 500 V and are ground referenced—meaning, one side of the probe is connected physically to earth ground through the oscilloscope.  If you’re measuring a single-phase 120 V line input to a power supply or inverter, you should be careful when connecting the probe ground to power neutral, which may not always be at ground level.  Using a differential probe, which is not ground referenced, eliminates this concern.

If you want to measure three-phase line voltages, you have a similar problem.  Making line-to-neutral measurements must account for the neutral line possibly not being at ground in the presence of an unbalanced load, so measuring with a single-ended passive probe can be problematic.  

Line-to-line measurements are a definite “no-no” for the ground referenced single-ended probe. These measurements require a high-voltage differential probe to prevent shorting one phase to ground.  

Consider the simple 120 V input switch-mode power supply in Figure 1.  The supply rectifies the input voltage resulting from power rails that are at the peak voltages of the AC mains, namely +170 V and -170 V.  That’s a total of 340 V peak to peak. 

The voltage levels may be within the range of a single-ended passive probe, but the primary circuits are not referenced to ground.  Most of them use the negative rail (-170 V) as the reference.  Attaching a passive probe ground clip to that line would likely result in a pretty bad short circuit, one that could take out the input rectifiers and protection circuits on your oscilloscope.  

Some people try to get around this by “floating the scope”, meaning they cut the ground lead on the oscilloscope’s power cord.  Now, connecting the probe’s ground to the primary reference does not cause a short; but the oscilloscope case is tied to -170 V. Ouch! Try explaining that accident to your company’s Safety Officer. (Actually, please DON’T try this at home/work. We take no responsibility for the results of your science experiment.)

When measuring power systems, a far better solution is to leave the oscilloscope connected to ground (as intended) and use a high-voltage differential probe, such as one of the HVD3000 series. The HVD3000 probes cover the fullest range of applications, from 120/240 V switch-mode power supplies through 600 V class and 5 kV class electrical apparatus. Each model has the best available gain accuracy, widest differential and offset voltage range, and superior common mode rejection ratio (CMRR).

Another issue to consider with single-ended passive probes is the frequency derating of the maximum voltage these probes can handle.  Not all power devices operate at low frequencies. Some, like DC-DC converters, operate at switching frequencies up to 1 MHz or more.

Figure 2. Derating curve for a 500 Vrms single-ended passive probe.
Maximum input voltage is limited as a function of frequency.

Figure 2 shows the voltage vs. frequency derating curve for a 500 Vrms single-ended passive probe. Measuring signals switching at greater than 200 kHz with this probe requires derating the maximum voltage the probe can withstand. If the frequency being measured is 1 MHz, the maximum allowed input voltage is reduced to only 190 V. 

High-voltage differential probes like the HV3102A, HVD3106A and the new HVD3220 have similar derating characteristics, but they start out with a much higher maximum voltage limit of 1 kV or 2 kV, leaving quite a bit more headroom for reaching the 1 MHz derating breakpoint.

Learn more about the HVD3000 probes in the on-demand webinar, Probing in Power Electronics: What to Use and Why, Pt. 2.

See Also:

Probe Safety Demystified: Dynamic Range and Voltage Swing

Squeezing More Bandwidth from a 10x Passive Probe

Probing Techniques and Tradeoffs, Part II

Using Spectrograms to Visualize Spectral Changes

Figure 1. The spectrogram shows a history of
change in a spectrum and highlights variations
in frequency or amplitude.

In our last post, we discussed spectral analysis of RF in the lab as part of developing situational awareness. Another tool for spectral analysis is the spectrogram.

The spectrogram is a display composed of the most recently acquired 256 spectra all stacked in a persistence display. It is a feature of the SPECTRUM-1 and SPECTRUM-PRO-2R options that highlights variations in acquired spectra, making dynamic changes immediately visible. 

The spectrogram in Figure 1 shows the timing dynamics of a power rail load variation in a three-dimensional (3D) plot with color persistence. The same data could also be rendered in a flat, two-dimensional (2D) spectrogram display, or using monochrome instead of color persistence.  

Spectrograms are useful for any type of spectral analysis, although they shine wherever it is especially important to view dynamic changes, such as in switched-mode power and power rail analysis. For example, one of the common tests to perform on a power rail is load regulation. The test is quite simple: you monitor the rail voltage while changing the load current. Normally, the output voltage is measured as a function of the load current, and load regulation is computed. But the frequency spectrum of the signals emitted by the power supply change as a function of load, as well. 

Figure 2. Spectrums of a load regulation test
on a 5-volt USB power module with
load currents of 100 mA, 200 mA and 330 mA.

Figure 2 shows the spectral responses to load variation on a simple, five-volt USB power module at 100 mA, 200 mA and 330 mA load currents. The switching frequency of the power supply is shown as the spectral line at just above 20 kHz in the 100mA acquisition in the top grid in the figure. The spectral lines to the right of the switching frequency are harmonics at just above 40, 60, 80 and 100 kHz. As the load current increases, the power supply switching frequency increases and the conducted spectral amplitude increases.

From an electromagnetic compatibility standpoint, it is necessary to study these variations. However, the spectrums look…much the same.  Only by inspecting them carefully can we see the change between loads. Dynamic response to load changes can be difficult to read from traditional power or magnitude spectrum plots, because these plots represent a single acquisition—a single moment in time, not variation over time, which is the forte of the spectrogram.

The 3D spectrogram packs a good deal of the information in multiple spectrums into a compact display. The horizontal axis represents frequency, and the vertical axis represents the spectral amplitude—same as on any spectrum—but the third axis represents the timing of the acquired spectrums, with the newest acquisition in front and previous acquisitions behind it. The 2D spectrogram would eliminate the vertical deflection, displaying frequency and relative time.  

The encoding of the amplitude information depends on the selection of monochrome or color display. For the monochrome display, higher amplitudes are represented by more intense (brighter) shades of the same color.  The color display maps amplitudes as variations in color, with the higher amplitudes shown in the hotter colors (red the highest) and lower amplitudes shown in successively cooler hues (violet the lowest).

Figure 3. The colored ridges in the spectrogram
show the timing of frequency changes.

The spectrogram in Figure 3 shows the higher-level amplitudes from 10 Hz to about 10 kHz in red. As the spectrum amplitude fall offs, color varies from red through orange, yellow and green. The power supply switching frequency appears as the red ridge that varies from 20 to 30 kHz as the load is varied. The timing of the frequency changes is indicated by the position of the ridge in the spectrogram. The current spectrum in the front of the display shows a middle frequency value of about 26 kHz. Before that, the frequency was at its highest value about 30 kHz. Going further back in time, the frequency dropped to its lowest value of about 23 kHz. Harmonics also appear in each acquisition, their color varying between orange to yellow as they change over time. 

Because it shows variations in the spectrum over time, the spectrogram is a useful tool for visualizing spectral changes due to dynamic events, whatever the source of the acquisition.

Watch Dr. Eric Bogatin incorporate spectrograms into the Real-time Spectral Analysis of Power Rails in his on-demand webinar.

Situational Awareness: RF Noise in the Lab

Fig. 1. Time domain (top) and spectral (bottom) views of
signal shown in SPECTRUM-1 on a WaveSurfer 4000HD. 

Laboratories have multiple sources of RF that can affect measurements, like computers, cell phones, routers, local radio and TV stations, even a nearby airport. Knowing the RF background of your lab is another part of Situational Awareness. Only by knowing the background can you know what is actually due to the device under test.

One way to read RF is through spectral analysis of Fourier transforms (DFT and FFT). FFTs take a time domain view of a signal (e.g, amplitude versus time trace) and change it into a spectrum of amplitude plotted as a function of frequency. Frequency spectrums are great for observing signals than are asynchronous with the process being measured. They have a lower noise floor and offer better dynamic range than do time domain plots.  Consider the views of the same signal shown in the time domain and frequency domain in Fig. 1.

Situational Awareness: The Impact of the Interconnect

Fig 1. Coaxial cable has little effect on signal rise time,
but that's not true for every connection method.

How you connect a signal to your oscilloscope affects your measurements, and knowing the impact of different connection methods is an important part of your situational awareness.  

Using the same 40 ps fast edge signal we used for the risetime measurements in the last post, we’ll compare connections made using coaxial cable and a 10x passive probe, with and without different accessories.

Situational Awareness: Testing Oscilloscope Outer Limits

Fig 1. 40 ps signal measured full bandwidth on a
1 GHz oscilloscope shows visible over/undershoot.

Nothing is perfect. Every test instrument has its limits, and knowing the limits to your oscilloscope’s bandwidth in response to real-world signals helps to develop situational awareness when making measurements. This is especially true when testing signals that are at or very near the specified bandwidth limit of the instrument.

The measurements we’ll demonstrate were made on a WaveSurfer 4104HD, a 12-bit, 4-channel, 1 GHz bandwidth oscilloscope that samples at up to 5 GS/s.

Four Measurement Best Practices

To start the New Year right, we’re going to talk about four measurement "best practices", which will help you get the most out of any oscilloscope you have. These are important when doing any type of measurement—and you can get a good start on them simply by asking yourself the four questions in the sidebar.

1. Anticipate the results

Those who are familiar with Dr. Eric Bogatin’s Rule #9 will know this one. Before you do any measurement, anticipate what you expect the result to be, because that is the most important way of identifying if there is a potential problem.