Poor Man’s Homebrew TDR With 4cm Resolution – Part 1

The standard equipment for measuring impedance variations is a Time Domain Reflectometer, TDR. TDRs are very effective but resolving small elements requires a wide bandwidth, which implies the TDR will be very expensive. This note explores a £35/$55 alternative based on SDR dongles and noise sources, to see what can and cannot be achieved.

Although there are limitations, initial results are surprisingly good and useful. For example, Figure 1 shows reflections in two different transmission lines with an open-circuit stub 3.1m from the TDR.  The first stub is 19cm long, and the second is 29cm long.


Figure 1: 19cm & 29cm Stubs

The stubs’ differing lengths are clearly distinguishable.

Why do such impedance variations matter? Because with RF circuits and medium/high speed digital circuits, connections must be uniform-impedance correctly terminated transmission lines. Impedance discontinuities in RF circuits causes peaks and troughs in the frequency response, leading to poor performance and/or link failure. Impedance discontinuities in digital circuits cause signal integrity problems, leading to marginal operation and/or pattern-sensitive errors.

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Happy days: a new-old 1.5GHz Scope Probe

For a while my best probes have been HP10020A 1.5GHz passive probes, but only the 10:1 variant. They are really pleasant to use because they are light, robust and cheap (unlike active probes), have very convenient spear tips, and most importantly, don’t distort the signal (unlike the common 10:1 high impedance probes). For the latter reason, these poorly named “low impedance Z0” probes have been a preferred way of looking at high-speed signals. For more information on their characteristics, see my scope probe reference material.

The only problem has been that I only have two, and so have been reluctant to use them in case they were damaged.

Well, that’s changed thanks to Ebay and an Australian vendor: I was able to buy a new-old HP10020A with the 1:1, 5:1, 10:1, 20:1, 100:1 tips, and all accessories in a case remarkably cheaply. That’s the definition of a good transaction: both parties are pleased.

Happy days.

Images of Late-70s Burr-Brown Thick-film Hybrid ICs

In the late 70s Burr-Brown made some of the most advanced analogue ICs on the open market. Technology limitations prevented the full integration of many components, so the “thick-film hybrid” IC was used instead. As a publicity gimmick Burr-Brown distributed a calendar of the some of the ICs internals.

For no reason other than the pictures are pretty in themselves, I kept them. Now I’ve finally got round to crudely digitising them. Here they are…

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RTL SDR Dongles: Anomalous Frequency Domain Response

I have been experimenting with a cheap-and-cheerful SDR receiver containing an R802T2 front end and RTL2832U demodulator. My ultimate aim is to make a low-cost network and spectrum analyser, but before that it is necessary to understand the receiver’s strengths and limitations.

In an ideal world the receiver’s power-vs-frequency response would be flat, but inevitably that won’t be the case with such a low-cost device: it will need calibration. That calibration has revealed some strange quirks in the R802T2 RF front end frequency response, discussed below. While the anomalies don’t kill the idea of using the SDR dongle as a spectrum/network analyser, I would feel happier if their cause (and preferably cure) was understood.

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Scope Probe Accessory Improves Signal Fidelity

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.

With the unmodified ground lead, 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. With the homebrew spear ground the ringing is removed, revealing the undistorted waveform.

There is a little ringing at 270MHz, 0.5% overshoot, 4% peak-peak amplitude and <5ns duration. Some 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.

The updated results with a higher frequency oscilloscope and higher frequency probe are even more impressive.

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Creating PCBs With Surface Mount Components

I have been creating circuits for mumble years, using wirewrap, IDC and standard PCB technology with plate-thru hole (PTH) components. I’ve recently been “forced” to use surface mount devices (SMD), and was concerned that it would be too difficult for an amateur using only equipment available at home.

I was wrong; it was easy.

On the off-chance it inspires others, here’s what I’ve used, what worked and what didn’t. The tl;dr version is DesignSparkPCB, DirtyPCBs, OSH Stencils, 179C SnPb paste, magnifying visor, *8 lenses, reflow sand in skillet, soldering iron.

Design Layout View

Design Layout View

Also see my techniques for designing, making and assembling homebrew PCBs, and reference material.

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Assembling PCBs With Surface Mount Components

Summary: it does work and isn’t too painful. Naturally I’m evolving methods and techniques as my skill improves.

This outlines how I placed solder paste, placed components, saw small components, soldered and reworked small PCBs. I didn’t want to be constrained by the time it takes to apply solder paste, position components, and reflow in the HackSpace oven, so I looked for ways I could do the whole process at home.

Also see my techniques for designing and making homebrew PCBs, and reference material.

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Manufacturing PCBs With Surface Mount Components

Summary: cheap ‘n cheerful, good enough for double-sided experiments, but not good enough for 4-layer impedance-controlled PCBs.

The laser toner technique and very-low-cost PCB manufacturers are good enough for the low-speed digital signals found in Arduino-class circuits. They are not suitable for large boards, or for containing medium speed digital signals capable of bit-rates up to 1Gb/ or 2Gb/s. A requirement for 50Ω or 100Ω differential impedance lines on >=4 layer PCBs implies the PCB cross-section “stack” must be tightly specified in terms the prepreg’s thickness and \epsilon _r .

This post outlines experiences for small experimental boards.

Also see my techniques for designing and assembling homebrew PCBs, and reference material.

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Designing PCBs With Surface Mount Components

Summary: all the design tools work well.

This describes the tools I used and a constantly-evolving personal style.

I’ve used the free-as-in-beer DesignSparkPCB commercial product from RS running in wine. I didn’t choose Eagle because of its irritating limitations (size, pins etc). I didn’t try KiCAD, and have no opinion about it.

Also see my techniques for making and assembling homebrew PCBs, and reference material.

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PLA Scope Probe Accessory Improves Signal Fidelity

This page describes an earlier version, and is retained for historical interest.

See this new page for details of the theory, the performance and manufacturing.

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.

Continue reading