Testing Multi-level Bullets & Latex

All too often signal integrity aberrations are visible in digital signals displayed on an oscilloscope. Fortunately signal fidelity can be significantly improved by a simple homebrew 3D-printed accessory that can be retrofitted to any standard scope probe. The “workhorse” scope probe usually supplied with scopes is the high-impedance 10:1 passive scope probe. While very useful in many circumstances, like all scope probes it has its limitations and can introduce distortions into the displayed waveform. This post complements the scope probe reference material and simply:

  • concentrates on one problem that is frequently encountered in medium speed digital circuits
    • illustrates commercial accessories used by high-end passive probes to avoid the problem
      • describes a simple homebrew 3D-printed accessory
  • demonstrates improved signal fidelity

The Problem and Fidelity Improvement

The examples below use an HP10074C scope probe (150MHz, risetime <2.33ns, 15pF, 6inch/15cm ground lead) and Tektronix TDS340 100MHz digital scope. Without the modification, a fast (<1ns) digital edge incorrectly appears to “ring” with a half-period of 5.5ns (91MHz), 20% overshoot, 33% peak-peak amplitude, and 10s of nanoseconds duration:

Standard 15cm Ground Lead

HP10074 with Standard 15cm Ground Lead

With the homebrew accessory the ringing is removed, revealing the undistorted waveform.

Homebrew Bayonet Ground

HP10074 with Homebrew Spear Ground

There is a little ringing at 270MHz, 0.5% overshoot, 4% peak-peak amplitude and pre-transition ringing is visible (even using a 1.5GHz HP10020A probe), but it is not visible on an equivalent analogue scope. Hence that ringing is probably an artefact of the scope itself.

Theory and Commercial Solutions

The cause of the ringing is the interaction between the probe tip’s capacitance and the ground lead’s inductance. An LC circuit will ring at its resonant frequency of f _0 = \frac{1}{2 \pi \sqrt{LC}} . Wires have an inductance of ~0.8nH/mm, so the normal 6 inch (15cm) ground lead and croc clip’s inductance is around 150nH. The probe’s tip capacitance is 15pF. Hence the theoretical resonant frequency is ~106MHz, which corresponds acceptably with the observed value (91MHz). To increase signal fidelity, the objective is to increase f _0 above the probe’s or scope’s bandwidth, so that the ringing won’t distort the trace so much. This can be done by reducing the capacitance or inductance.

The only practical way to reduce the capacitance is to use a different probe. There are some high-quality high impedance probes with slightly lower capacitance, e.g. the 6.5pF HP10431A, but the usual technique is to use an expensive, fragile active probe, or a low impedance Z0 probe. Now there are many advantages to Z0 probes, but it is probably easier to make a homebrew Z0 probe than it is to buy one commercially; see scope probe references for examples.

Fortunately there are three straightforward methods for reducing the inductance applicable to any high impedance 10:1 probe.

The first technique is to use the small wire spring clips supplied with the probe (if they haven’t been lost!), but they need a ground connection at the exactly the right distance. That is often possible for analogue circuits without a soldermask, but is often difficult with digital circuits.

The second technique is to solder a testpoint into the circuit, either a commercial product or more likely homebrew wire coils or paper clips. This is justifiably a favoured technique when breadboarding analogue circuits, but is more problematic with densely populated digital circuits

The third technique is supplied with some high-end probes: a short pivoting spear or blade which allows a variable distance between the probe tip and the ground. Examples are HP10431A probes (6.5pF ~500MHz), HP10020A probes (<0.7pF, 1.5GHz), Keysight N2878A probe accessory (2pF, 1.5GHz). This note describes a homebrew version of that technique.

HP10431

HP10431

HP10020

HP10020

n2878ablade

Keysight N2878A Blade

Homebrew 3D-Printed Ground Accessory

The 3D-printed probe ground accessory shown below was inspired by the HP10020A. Although specifically designed for an HP10074 probe, it also fits similar available probes, e.g. an Elditest GE1521.

HP10074 Assembly

HP10074 Assembly

There are four parts to the accessory:

  • the body, designed in OpenSCAD, fabbed by Shapeways in their strong and flexible plastic nylon material
  • the spear, made from 0.8mm piano wire
  • the ground connection, etched from 0.2mm phosphor bronze, with the fingers cut using a jeweller’s fretsaw
  • the hinge, a 1.7mm/1.5mm spring pin in a 1.6mm hole
Prob Tip Body Exterior View

Accessory Body, Exterior View

Cross-section, With Probe, Phosphor Bronze Ground

Cross-section, With Probe, Phosphor Bronze Ground

The phosphor bronze ground connection fits into the slot and protrudes into the central cavity. It is then deformed so that when the probe is inserted it rubs against the probe’s ground sleeve. Finally the piano wire spear is inserted into the slot against the phosphor bronze, and held in place by the spring pin.

Assembly Component Parts

Assembly Component Parts

Fabrication

Two versions of the body have been fabricated, the nylon version described in this post, and an earlier PLA version made on a local RepRap 3D printer. The experience gained with the PLA version was a useful proof-of-concept, but the professionally produced nylon body is significantly better:

  • the Shapeways sintering process means the dimensions are much more accurate; no post-manufacture filing was necessary
  • the nylon does not scratch the probe body
  • there is a much more satisfactory grip between the body and the probe. The body contacts the probe along the length of the four internal nubs. The distance between the nubs is 0.1mm smaller than the probe body’s diameter. The natural elasticity of the nylon allows the body to deform slightly when the probe is inserted, so that the interference fit is not too tight.

Acknowledgements

Thanks to Russell Dicken and Ian Stratford for help with the RepRap, and Bristol Hackspace for the use of their RepRap.

There are many types of scope probe, each with their own advantages and disadvantages. You need to be able to choose the right type of probe for the job in hand, and use it correctly. If you don’t, at best you will waste time chasing after strange unexpected effects. At worst you will damage the circuit, oscilloscope, or yourself.

Fortunately there is a wealth of information available; these links are a starting point.

Formal

  • basic introduction: Analog Devices’ High-Speed Time-Domain Measurements—Practical Tips for Improvement is a useful first introduction containing basic theory and practice.
  • choosing a probe: Tektronix’ Probe Fundamentals defines the characteristics of a wide range of probes in depth. It guides you through choosing the right type of probe, and illustrates what you’ll see if you use probes inappropriately.
  • effective use: Tektronix’ ABCs of Probes Primer is a good all-round discussion of how to choose a probe and use it effectively and safely. Some of the interesting information, including measurements, is scattered through the document. Tektronix XYZs of Oscilloscopes Primer is the equivalent for scopes. (Fortunately it isn’t necessary to enter personal information, just press the button)
  • fundamental theory: Tektronix’ Oscilloscope Probe Circuits is a very detailed description of the theory and practice of probes and their construction. It is from 1969, but the physics hasn’t changed – even if it has been forgotten or is no longer taught!
  • high speed digital: Howard Johnson’s Probing High Speed Digital Designs is widely cited introduction to the merits of FET probes and low impedance Z0 probes. Everything else on his site is worth reading and understanding. Recommended.
  • practical effects: Jim William’s justly famous AN-47 High Speed Amplifier Techniques shows how to correctly and incorrectly use probes, a good balance between theory and practice, and many photographs and many measurements. Highly recommended.

Homebrew / DIY

  • simulation has limitations, but it enables you get a feel for the relative magnitudes of various effects:
    • Doug Ford’s The Secret World of Scope Probes is an informal simulation-based description of the high impedance 10:1 probes’ performance.
      • simulating 10:1 high impedance probes requires Spice models of lossy transmission lines. There’s a library of model parameters, but note that some Spice engines require the conductance, g, to be zero. That’s probably not a significant limitation – unless you are interested in modelling vintage POTS cables that have been in the ground too long and the waxed paper insulation has become saturated with water.
  • low impedance Z0 probes:
  • active probes:
    • Daniel Kramnik constructed an active probe, 1GHz Active Differential Probe. The hoped-for 1GHz ended up as a 300MHz, thus illustrating how performance requires a deep understanding and attention to detail.
    • David Jewsbury constructed another active probe Poor Man’s 1GHz Active Probe, and claims a 1.5GHz bandwidth but doesn’t present measurements
  • standard 10:1 high impedance probes:
    • these workhorse probes can introduce aberrations above ~80MHz, which are often visible as ringing in medium speed digital signals. A 3D printed accessory can be retrofitted to any standard 10:1 high-impedance probe to significantly reduce the signal integrity distortion
    • alternative ways of connecting a probe are to solder a testpoint into the circuit, either a commercial product or more likely homebrew wire coils or paper clips. This is justifiably a favoured technique when breadboarding analogue circuits, but is more problematic with densely populated digital circuits

Praxis

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