Secret world of oscilloscope probes

Secret world of oscilloscope probles article written by Doug Ford and published by Silicon Chip magazine, describes how high frequency oscilloscope probes really work. Most textbooks treat scope probes as a combination of a resistive divider in combination with capacitors to provide an extended frequency response. But as will be revealed, the reality is that they are much more complex in principle. The hidden secret to designing a good 10x oscilloscope probe seems to be to use lossy transmission line cable! Usually the coax cable on probles has been made deliberately lossy, to reduce the effects of end-to-end transmission-line reflections!

scopeprobes

9 Comments

  1. Tomi Engdahl says:

    Scope bandwidth, rise time, and the “rule of five”
    http://www.edn.com/electronics-blogs/other/4391632/Scope-bandwidth–rise-time–and-the–rule-of-five-?cid=EDNToday

    You might easily think that a 500-MHz oscilloscope would be good for measuring sine waves up to 500 MHz. As I noted in an earlier post, however, this isn’t the case. There are at least two approaches you can take to make sure get the right scope bandwidth for your application.

    First, if you have a maximum sinusoidal frequency in mind, you can work with the bandwidth specification of the scope. However, you’ll need to account for the roll-off of the scope.

    Second, if you’re looking at non-sinusoidal signals, such as digital pulses, the highest frequency in your signals will be a multiple of the highest repetition rate. You’ll need to use a scope bandwidth much more than five times the repetition rate to account for harmonics. If you’re working on digital systems, it’s often easier to consider the rise-time specifications for scopes.

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  2. Tomi Engdahl says:

    Make Your Own Probes
    http://www.scopejunction.com/author.asp?section_id=1835&doc_id=253846&cid=reg-email-12-18

    You may never have thought to make your own scope probes, but sometimes, they’re just what the doctor ordered.

    The first inkling I might want to make my own probes came when I was pondering how to use the 50Ω inputs on some of my scope modules for general probing, as opposed to a direct connection to a 50Ω source. OK, so, maybe I haven’t built mine yet, but let’s consider the options when it comes to rolling your own.

    Glen created 500Ω 10:1 probes by soldering 50Ω coax to a 450Ω (453Ω is a standard value) resistor, which is soldered to the circuit under test. The scope is set to 50Ω input mode, of course.

    SJ user clockman (Hugh Houtman) has also mentioned some homemade probes, which I’ve further discussed with him. Hugh’s goal was to achieve fast risetime along with extremely low capacitive loading, so his 100:1 probe consists of 100MΩ of series tip resistance paralleled with 1pF, driving his scope’s 1MΩ input.

    Because of the impedance mismatch between the coax cable and the scope’s input, there are some reflections,

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  3. Tomi Engdahl says:

    Oscilloscope probes: Understand and optimize
    http://www.edn.com/design/test-and-measurement/4420010/Oscilloscope-probes–Understand-and-optimize

    Editor’s Note: This article is a guide to predicting the performance of your probe for your specific test setup, including tips on getting the best performance out of your probe.

    Summary
    Passive oscilloscope probes are very sensitive to ground lead inductance and source impedance. It is critical to take these factors into account when determining the connection scheme for probing your signal. By estimating values for ground lead inductance and source impedance you can easily plot the transfer function to get a good idea of what performance you can expect from your probe. Pay special attention to devices under test with very high source impedance, this will essentially turn you’re your probe into an RC filter. For a source impedance of 10k Ohms the bandwidth will be reduced to 1.67 MHz. By understanding the effects of these different variables you can use your passive probe with confidence in a wide variety of environments.

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  4. Tomi Engdahl says:

    Oscilloscope Mistakes, Part 2
    http://www.eetimes.com/author.asp?section_id=36&doc_id=1319495&

    The engineering community uses oscilloscopes more than any other piece of equipment, yet many of the published results are questionable at best. Some errors are very common, so we can eliminate a great deal of bad data by considering a few simple but key points. This second part of this four-part series covers probe issues: choosing an incorrect probe or using it improperly.

    The three most common probe issues are not calibrating the probe (either for capacitance or for time skew), ringing due to the ground wire, and using 50-Ohm coaxial cable connected to a high-impedance input.

    The scope probe is represented as a high impedance in parallel with a high Q capacitance. Any series inductance (either connected to the tip or the ground) will result in a high Q tank circuit and ringing. The ringing frequency will generally be lower than the stated bandwidth of the probe.

    An unterminated coaxial cable offers shielding, but it will still resonate with the scope input capacitance and with high Q.

    Recommendations
    For all high-fidelity measurements up to 100 MHz, remove the ground clip from the measurements. Above 100 MHz, consider an active probe.

    A typical oscilloscope probe presents 10-15 pF of capacitance. Many circuits cannot tolerate this, and, at the least, this capacitance can cause significant ringing. In most cases, an active probe is required.

    Sometimes the best probe is no probe at all, and a coaxial cable is perfect. This solution also maximizes the sensitivity of the measurement. However, remember that a 50-Ohm coaxial cable must be terminated into 50 Ohms.

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  5. Tomi Engdahl says:

    Comment from http://www.eetimes.com/author.asp?section_id=36&doc_id=1319495&

    Source impedance often dictates what type of probe you can and should use. I often find myself creating my own resistor divider probes with simple coax and a couple SMT resistors. Very low capacitance and minimal loading on high impedance outputs. Active probes are great but sometimes you don’t need them or want them. Been fooled lots of times relying on an active probe. Particularly nasty if you think you can use them for power supply ripple and droop analysis.

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  6. Tomi Engdahl says:

    How An Oscilloscope Probe Works, And Other Stories
    http://hackaday.com/2017/03/15/how-an-oscilloscope-probe-works-and-other-stories/

    The oscilloscope is probably the most versatile piece of test equipment you can have on your electronics bench, offering a multitude of possibilities for measuring timing, frequency and voltage as well as subtleties in your circuits revealed by the shape of the waveforms they produce.

    On the front of a modern ‘scope is a BNC socket, into which you can feed your signal to be investigated. If however you simply hook up a co-axial BNC lead between source and ‘scope, you’ll immediately notice some problems. Your waveforms will be distorted. In the simplest terms your square waves will no longer be square.

    Crucial to the operation of an oscilloscope is a very high input impedance, to minimise current draw on the circuit it is investigating.

    This high resistance does its job of presenting a high impedance to the outside world, but comes with a penalty. Because of its high value, the effects of even a small external capacitance can be enough to create a surprisingly effective low or high pass filter, which in turn can distort the waveform you expect on the screen.

    The majority of passive oscilloscope probes contain an attenuator to both isolate the circuit under test from the capacitance of the cable, and compensation capacitors in parallel with each of the resistances to cancel out the effect of the capacitance of the cable. The attenuator is usually chosen to divide the input voltage by ten, hence you will see “10x” probes.

    One of the compensation capacitors will be adjustable, to fine-tune the response.

    However there are times when it is necessary to measure an output that expects a low impedance, such as a 50 ohm source.

    Some ‘scopes have a 50 ohm input mode

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  7. Tomi Engdahl says:

    Home> Test-and-measurement Design Center > How To Article
    Build your own oscilloscope probes for power measurements (part 1)
    http://www.edn.com/design/test-and-measurement/4458644/Build-your-own-oscilloscope-probes-for-power-measurements–part-1-?utm_content=buffer9cb72&utm_medium=social&utm_source=twitter.com&utm_campaign=buffer

    Modern power supplies are edging upward in operational frequency. The benefits include a reduction in size and weight, plus an increase in energy density. For these designs, engineers are migrating to high-frequency power switch and rectifier technologies. The traditional planar or trench MOSFET switches with rise/fall times 30 nsec to 60 nsec are giving way to power switches such as superjunction MOSFETs, GaN MOSFETs, SiC MOSFETs and SiC Schottky rectifiers that switch in less than 5 nsec.
    To view such fast transitions, you typically need an oscilloscope with at least 1 GHz bandwidth. Unfortunately, most commercially available voltage and current probes are woefully inadequate at these high frequencies. The average oscilloscope probe has a bandwidth of less than 300 MHz. Current probes can have bandwidths of 60 MHz to 100 MHz or less. Furthermore, high-frequency voltage probes often cost over $12,000 and slightly better current probes start at $4,000. For power engineers who work for mid-sized companies, there is only one path: build your own probes.

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