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.


  1. Tomi Engdahl says:

    #90: Measure Capacitors and Inductors with an Oscilloscope and some basic parts

    This video shows how to measure the value of unknown capacitors and inductors using your oscilloscope and a simple pulse generator.

  2. Tomi Engdahl says:


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  4. Tomi Engdahl says:

    Send known rise-time edges down transmission lines

    High-speed signals such as microwave carriers and digital data streams lose fidelity as they travel. Losses from PCB traces, connectors, and cables need measurements on those losses, the reult of impedance. For $3500, you can use the J2151A PerfectPulse to send a signal with known rise times through a transmission path. From those pulses, you can characterize the path.

    USB powered and about the size of a flash drive, PerfectPulse generates a stream of square waves with 32 ps rise times at repetition rates of 1 kHz, 10 kHz, 100 kHz, 1 MHz, and 10 MHz.
    The J2151A’s output amplitude is 0 to 500 mV, trimmed to 1% ±2.5 mV.

    The PerfectPulse comes with a 10 GHz power splitter for making time-domain reflectometery (TDR) measurements and a demo board that has PCB traces of known impedance.
    Using a probe such as the Picotest P2100A 50 Ω power-distribution network (PDN) probe or probes from other manufacturers, you can inject the J2151A’s signal into your DUT.

  5. Tomi Engdahl says:

    Pocket-Sized Instrument Packs TDR and Fast-Edge Generator

    This low-cost fast-edge pulse generator provides the means to measure PCB dielectric constants and the impedances of cables and interconnections.

    PerfectPulse Fast Edge Signal Generator from Picotest Corp. It provides square waves with 32-ps rise and fall times and the pulse edges needed to precisely measure things like cable and printed-circuit-board (PCB) transmission-line lengths and impedances, and even the dielectric constant of PCBs and other materials. For all of its capabilities, the PerfectPulse generator is remarkably compact, small enough to fit in a shirt pocket.

    The PerfectPulse signal generator (Fig. 1) generates pulsed output signals at 50 mV and 50 Ω with no overshoot or undershoot—the kind of square waves that are extremely useful for performing time-domain-reflectometry (TDR) and time-domain-transmission (TDT) measurements utilizing a high-speed, real-time oscilloscope with sufficient bandwidth. The signal generator will be shipped with a 10-GHz power splitter so that the dual test signals can be used for high-performance TDR and TDT measurements on PCBs and PCB signal traces, as well as to measure the lengths of cables and signal traces, plus verify the quality of crimps in cables.

    The J2151A PerfectPulse signal generator can be used for oscilloscope probe calibration. The compact signal generator is compatible with all 50-Ω probes, for convenient signal injection and noninvasive stability measurements on circuits.

    The one-port P2100A test probe (Fig. 2) supports connections to 1.5 GHz with almost no capacitive loading: less than 1-pF capacitance, and typically only 420-fF capacitive loading.

  6. Jeff Hurckes says:

    So, I’ve been looking over this project and have a question. Is the TDR circuit a single shot, or does it send repeating pulses? If it is repeating, could it be designed to be a triggerable single shot? I’d like to use this with a Fluke ScopeMeter in testing 75 ohm coax.

    • Tomi Engdahl says:

      This circuit at is designed to generate repeating pulses.
      The time between pulses depends on the pulse length. The time between pulses is approximately 200 times the pulse length.
      Thank you for your feedback.

      I don’t have ready made plans for triggerable single pulse sending application.
      I think should be doable in way or another…

  7. Tomi Engdahl says:

    Time-Domain Techniques for De-embedding and Impedance Peeling

    De-embedding is a common problem in making signal integrity measurements because often, the interconnection between the measurement instrument and the device under test (DUT) requires fixtures, cables, and/or probes. While usually it is not too much of a problem to calibrate the instrument to the end of the cables, which present a coaxial connector as the instrument port, the removal of what is between the instrument port and the desired reference plane of the DUT can prove problematic.

    Adapter De-embedding

    Many de-embedding applications involve the assumption that there is a two-port device (adapter) connected between one or more ports of the measurement instrument and the ports of the DUT

    Time Gating

    De-embedding is not a panacea. Usually, when connecting between instrument ports and the DUT, the desire is to have as transparent a connection as possible. This means a characteristic impedance as close to 50Ω as possible, and as little loss as possible. Otherwise, even if the s-parameters of the intervening device are known, it degrades measurement performance, even when employing proper de-embedding algorithms.

    Even if the quality of the connection is good, there’s always the element of time. Even a well matched, low-loss adapter will have some amount of electrical length (i.e., it will take some amount of time for the waves to propagate through the adapter). In this situation, a very simple form of de-embedding, called time gating, can be used.

    Impedance Peeling

    While time gating is a very simple form of de-embedding used mostly to account for time delay (or electrical length), an improvement can be made by measuring and accounting for the actual impedance of the line, even as the impedance changes over time. This can be performed automatically and is called impedance peeling.


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