TDR circuit modification idea

You might know my Time Domain Reflectometer (TDR) signal source circuit published on many years ago. It has worked well for me many times.


Some years ago I made a modification to my own TDR unit. This simple modification allows me to use the same box also as a signal source with a wide frequency range (kHz to almost 30 MHz) and controllable output impedance. This kind of square wave signal source is useful for all kinds of testing.

The TDR circuit shown above can be modified to a square wave signal source by modifying the oscillator part of the circuit (R1, R2, D1, one gate of IC1 and capacitor C1..C5). This oscillator is pretty normal square wave oscillator circuit with just D1 and R2 as extra. So if you leave out D1 from the circuit you get square wave signals. If the D1 is just removed the oscillator outputs square wave at frequency range from few kHz to few hundred kHz (frequency controlled by R1 and capacitor).  If you replace the D1 with a short circuit you get higher frequency from hundred kHz to almost 30 MHz (frequency controlled by R1+R2 in parallel and the capacitor). If you leave the D1 as it is, the circuit works as TDR signal source.

The modification needed to add all this new functionality and still keep old things working is to add one three position changeover switch (onA-off-onB) to the circuit. Just wire it in such way that you get all the D1 as it is (=TDR), D1 open circuit (=low frequency) and D1 short circuit (=high frequency) settings.


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  6. Rick Amorgos says:

    This is very exciting! I found your TDR circuit on line and am very interested in applying this in a moisture sensor application. I believe your circuit could be a good starting point for this purpose. Instead of using a “Cable Under Test” I will replace that with two metal probes which will act as a transmission line. The probes will be inserted into the material under test. I expect the reflected wave to be delayed proportionally to the amount of moisture in the material. Have you had any experience applying this circuit to this application? I appreciate any feedback you might have. I suspect I might need to use a higher frequency but I’m not sure.

    • tomi says:

      I have no experience applying this circuit to moisture measuring application.

      I have used the TDR circuit for different cable and electronics component measuring applications only.

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  14. Tomi Engdahl says:

    If you need much faster signals than my TDR circuit can provide read this article:

    Sub-10ps pulse generators: technology, performance, and applications–technology–performance–and-applications?cid=EDNToday

    Pulse generators boasting rise times in the sub-10 ps regime have a wide range of important applications in electronics and semiconductor design, manufacturing, and test. While shortest possible rise (or fall) times remain the banner specification, there are many other factors that contribute to the performance of a sub-10 ps pulse generator. Two architectures are commonly employed for these high performance devices. The choice of architecture has a strong impact on the speed, quality, and flexibility of the edge produced. This article will present an overview of sub-10 ps pulse generator architectures, important performance considerations, and conclude with a brief discussion of key applications.

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  38. Tomi Engdahl says:

    Quick & dirty cable length & impedance measurement—dirty-cable-length—impedance-measurement

    Most analog folks are familiar with the use of a pulse generator and a scope to measure cable reflections and impedance.

    If you know the cable length, you can calculate the round trip time. In most cables, the pulse travels at a velocity factor of about 66% of the speed of light in a vacuum (300 meters/µs),

    If you do not know the cable length, you now know how to measure it. Play with the pulse period and time base until you can see both the launched pulse and the reflected pulse. The time between them is the round trip time of 0.1 m/ns, or 10ns per metre of cable length.

    Impedance measurement is simply a matter of placing a small trimpot of about 200Ω across the far end of the cable and adjusting it to minimize the amplitude of the reflected pulse

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  41. Tomi Engdahl says:

    TDR response tells a story

    Some of the most versatile and intuitive tools in a signal integrity engineer’s arsenal are TDR (time domain reflectometry) and TDT (time domain transmission). Time domain analysis gives an insight into how electronic signals propagate through different dielectric media through a conductor whose geometric dimensions also influence these responses.

    As a signal propagates through PCB vias, connectors, interfaces, ground-plane interruptions, etc., TDR and TDT are commonly used to identify and quantify these environments. Let’s look at how one can associate different environments to TDR responses.

    It must be noted that, when it comes to interpreting complex TDR responses, there is no substitute for experience. With that said, fundamental TDR responses to simple environments provide the building blocks for interpreting complex TDR responses.

    From these classic TDR responses, you can begin to decipher a complex TDR response, a section at a time. You should know the general layout of the structure being measured to identify landmarks such as connector interfaces.

  42. Tomi Engdahl says:

    One paper on TDR measurements:


    1 Scope
    This method describes the test procedures
    required to measure the characteristic impedance of flat

    The laboratory standard is
    connected to the TDR generator output, and the cable with
    unknown Z0 is connected to the end of the laboratory standard.

    The far end of the cable may be left unterminated, or it may be terminated
    with a precision resistor to verify the laboratory standard.

  43. Tomi Engdahl says:

    Easily measure diode capacitance and reverse recovery

    The other day Linda from Purchasing came to me with a problem: Lou from Operations needed to source a replacement for a shorted diode on a switching power supply. The darned thing was marked with a strange part number that no amount of Googling could decipher.

    There was a recognizable logo marking, but that manufacturer could not provide a data sheet. The part number was from a previously acquired company and was unique to a specific customer. We were on our own.

    Fortunately there was a second identical unit in for repair, and Lou was able to provide me with a good diode of the same type. Now all I had to do was figure out what it was. A standard rectifier diode? A zener? Schottky? Reverse voltage breakdown rating? Junction capacitance? Recovery time?

    The required PIV rating could be resolved by initially substituting a test diode with a high voltage rating, and scoping later.

    This left only the unknown junction capacitance, Cj, and reverse recovery time, Trr. This is the time that a diode remains conducting when suddenly switched from forward to reverse biased. I had to figure out a way to measure these parameters. Not with exotic equipment; just enough to get in the ballpark – in other words, a function generator with a falltime of 40ns, and a 100MHz scope, which was all I had to work with.

    The test setup was easy: Drive the diode-under-test (DUT) with a 5V pulse – DC offset set mostly negative to bias the diode on only during positive peaks. Scope both sides of the DUT and trigger on the turn-off edge. Varying the DC voltage offset controls the DUT forward voltage and conduction current. Measure the DUT conduction current as the voltage dropped across its series 50Ω resistor.

    Based on the above measurements, the UF4004 or UF4007 would be a good choice to substitute for the unknown fast-recovery DUT.

  44. Tomi Engdahl says:

    Differential/Single-Ended High Frequency Probe

    Testing for small discontinuities on non-planar PCBs is now possible on a 40GHz VNA or very fast TDR because of the durable and repeatable probe design. Twinax and coaxial cable can be probed directly.

  45. Tomi Engdahl says:

    Create a stimulus-response system with an AWG and digitizer
    Arthur Pini , Greg Tate & Oliver Rovini -March 02, 2016

    Many devices and systems don’t require a stimulus signal for you to test them. But, when it comes to amplifiers, filters, transmitters, receivers, digital interfaces, or any system that requires an input signal, you’ll have to generate it and measure the output. Modular digitizers and arbitrary waveform generators (AWGs) let you test one or more devices that need input signals. Combining the two products in one system provides very cost effective and efficient way to meet an extensive range of automated test requirements.

    AWG’s let you generate almost any waveform, analog or digital. Consider a simple test for determining the frequency characteristics of an amplifier or filter. The test requires a signal source with a bandwidth greater than that of the device under test.

    Some circuits require diodes with matched I-V curves. While you can make these measurements with a curve tracer, you can also make automated measurements using an AWG and digitizer

    Because an AWG can generate almost any waveform, you can perform communications tests by generating modulated signals. Amplitude, phase, and frequency modulation can be achieved as was demonstrated in the first example on frequency response testing where a linear frequency swept sinewave was used.

  46. Tomi Engdahl says:

    Some other pulse circuit ideas:

    Simple nanosecond-width pulse generator provides high performance

    If you need to produce extremely fast pulses in response to an input and trigger, such as for sampling applications, the predictably programmable short-time-interval generator has broad uses. The circuit of Figure 1, built around a quad high-speed comparator and a high-speed gate, has settable 0- to 10 ns output width with 520 ps, 5V transitions. Pulse width varies less than 100 ps with 5V supply variations of 65%. The minimum input-trigger width is 30 ns, and input-output delay is 18 ns.

    Home> Test-and-measurement Design Center > Design Idea
    Pulse generator has low top-side aberrations

    Impulse-response and rise-time testing often require a fast-rise-time source with a high degree of pulse purity. These parameters are difficult to achieve simultaneously, particularly at subnanosecond speeds.

  47. Tomi Engdahl says:

    This video shows TDR in use to test underground power cables:

    Cable Injection

    ComEd uses cable injection fluid to extend the life of a distribution cable. Host utility: Commonwealth Edison

  48. Tomi Engdahl says:

    Here are some links to circuits to generate very fast pulses:

    The Fastest Rise Time In The West: Making A Truly Quick Pulse Edge

    Testing the Speed-of-Light Conspiracy

    The apparatus starts off with a very quickly pulsed IR LED, a lens, and a beam-splitter. One half of the beam takes a shortcut, and the other bounces off a mirror that is farther away.

  49. Tomi Engdahl says:

    #88: Cheap and simple TDR using an oscilloscope and 74AC14 Schmitt Trigger Inverter


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