Alternative oscilloscope probes can be easily selected with the right approach

While engineers know that oscilloscope probes need to be used with care because not all damage is immediately visible, sometimes probes can fail even with careful use. Unfortunately, this usually occurs in the most important tests.

Author: Art Pini

Earlier in my career, I managed a test department where we had 22 engineers and technicians, and about 35 oscilloscopes. The most common failure in an oscilloscope is a probe defect. Broken probe tips, broken connectors, broken cables—oscilloscope probes can fail endlessly. Therefore, we probably replace more than 30 probes a year.

When looking for a replacement probe, there is no secret to knowing how the probe works and its specifications. Almost all oscilloscopes have a passive probe per channel, so probes are a very common test accessory. Although passive probes have undergone eight decades of technological development, the basic concepts remain simple. As shown in Figure 1, a 10:1 passive probe is basically a compensation attenuator that connects to the oscilloscope’s 1 MΩ input termination. An oscilloscope’s 1 MΩ input can be modeled as a 1 MΩ resistor with a small capacitor in parallel.

Alternative oscilloscope probes can be easily selected with the right approach
Figure 1: A circuit model of a 10:1 high impedance passive probe that can be connected to the oscilloscope’s 1 MΩ input. (Image credit: Art Pini)

10:1 attenuation is achieved by placing a 9 MΩ resistor in series with the oscilloscope input (RIN in Figure 1). This combination will attenuate low frequency signals at the probe input by a factor of 10 at the oscilloscope input. For higher frequency signals, the shunt capacitance (Cin) of the oscilloscope input acts as a low-pass filter for the input signal along with the capacitance of the coaxial cable.

Equalization is required to achieve a flat frequency response. Adding a series high-pass filter with the same corner frequency as the low-pass filter can flatten the frequency response of the probe-oscilloscope combination. This is achieved by making the time constant RIN x CIN equal to the time constant of Ro x (CSCOPE in+CCABLE+C COMP). Since the input capacitance of the oscilloscope changes very little, a variable capacitor CCOMP is added to adjust the time constant of the low-pass components. This capacitor can be adjusted to set the low frequency compensation according to the instructions in each oscilloscope manual.

Let’s look at an example of finding a replacement probe. Let’s say we’re looking for a replacement probe for our Teledyne LeCroy HDO4104A 1 GHz four-channel oscilloscope. This oscilloscope usually ships with four PP018 500 MHz probes. One piece of information we need about our oscilloscope is the input capacitance, CSCOPE in, which can be found in the datasheet. The input capacitance is listed as 16 pF in the datasheet; if not explicitly listed, a tolerance of ±20% can be assumed.

Start by searching for oscilloscope probes on the Digi-Key website. One of the search results, shown in Figure 2, is the Test Leads – Oscilloscope Probes page.

Alternative oscilloscope probes can be easily selected with the right approach
Figure 2: Search criteria highlighted via the Test Leads – Oscilloscope Probes page of the Digi-Key website: passive probe, 10:1 attenuation, 500 MHz bandwidth, and 10 MΩ probe input resistance. (Image credit: Art Pini)

The search results in 16 products, as shown in Figure 3.

Alternative oscilloscope probes can be easily selected with the right approach
Figure 3: The search results list 16 probes, the first 8 are in stock. One of the options is the exact same replacement PP018, but there are also suitable probes from other manufacturers. (Image credit: Art Pini)

Of the first 8 search results, 6 were Teledyne LeCroy probes, including the PP018 probe provided with the oscilloscope. In addition, there are probes from two other manufacturers: the CT4203 from Cal Test Electronics and the P500-010 from Carlisle Interconnect Technologies. In general, it is best to evaluate other options.

By consulting each probe’s data sheet, we can compare the key specifications shown in Table 1.

Alternative oscilloscope probes can be easily selected with the right approach
Table 1: Comparison of probe products from three different manufacturers using eight key probe specifications. (Table source: Art Pini)

All three probes are highly matched. A key spec that wasn’t mentioned before is the compensation range. This is the range of oscilloscope input capacitance that the probe can match. Since the oscilloscope’s input capacitance is 16 pF ±20%, all of these probes are matched.

The second issue is the diameter of the probe tip. The smaller 2.5 mm diameter allows for tighter probe spacing without physical interference. The larger 5 mm probe tip is stronger and less prone to breakage. This engineering decision can only be made by the user.

Summarize

While engineers know that oscilloscope probes need to be used with care because not all damage is immediately visible, sometimes probes can fail even with careful use. Unfortunately, this usually occurs in the most important tests.

Using Digi-Key’s product search engine and basic selection guide provided, as shown above, makes it easier to obtain alternatives. Aside from the probe tip diameter, the main difference between the three probes selected here is cost – a final engineering decision.

Alternatively, you can “borrow” from a neighboring table.

The Links:   2DI200MC-050 GT30F123