TDR circuit kit available

My Time Domain Reflectometer (TDR) circuit is now available in kit form. Far Circuits has made a TDR kit that consists of circuit board and components. The Far Circuits version is a slightly modified circuits, the modification is that the circuit board has a 5V regulator IC in it. With that regulator the circuit power supply can be in 8-15V DC range (maybe eve higher). I have received a sample kit but I have not yet built my kit.



  1. Rick Amorgos says:

    Can you suggest any modifications for me to use this as a moisture sensor? I would like to use your circuit with a moisture probe (replaces the cable under test) and a scope.

  2. Elijah Phiri says:

    How can I get already assembled unit

  3. Elijah Phiri says:

    How can I get an already assembled unit for soil moisture measurements?

  4. george nash says:

    I want a kit thanks

  5. online pcb quote says:

    This excellent website definitely has all of the information I wanted about this subject and didn??t know who to ask.

  6. Ken says:

    I can’t seem to find a price for theTDR kit.
    Does the kit come all the parts I need and complete instructions



  7. inam says:

    hey…m a student…mt project is to design TDR….i have design a pulse generator getting reflections from fault as well but i need a complete circuit detail…will u plz help me in this regard

  8. desherbeurs thermiques says:

    An intriguing discussion is definitely worth comment.
    I do think that you need to publish more on this
    topic, it might not be a taboo subject but typically
    people do not speak about these subjects. To the next! All the best!!

  9. David says:

    I just received my kit from Far Circuits. It contained everything except the 7805 regulator that is marked on the board. Dunno if this is simply because Tomi’s original didn’t use one, but it’s probably too much work to hassle Fred for the “missing” bit.

    • tomi says:

      You are right that my original circuit did not have 7805 regulator, because I powered it from 4.5V battery power (first 4.5V battery and later three 1.5V AA batteries).

      If I remember right (long time ago so I am not completely sure), the kit I received had the regulator IC in it..

  10. Jon says:

    Is the TDR kit still available please and could you post to the UK? If so, what payment (PayPal?) do you accept? Thanks

    • Tomi Engdahl says:

      The kit is supplied by and they list it on their web site.
      So order through from

      The page says:
      “Orders accepted only by surface mail or FAX. NO orders , or via e-mail. All orders are to be prepaid by check, money order, or PAYPAL. To order please indicate the ship to address, home phone number, quantity of boards, magazine article title, publication, issue date, and any other information that would be helpful in identifying the board.”

  11. Al Yelvington says:

    I used an early version of this circuit when I was a Coast Guard field service tech. It was shared through a joint Navy and Coast Guard pub targeted at technicians, and it used a 9V battery without a regulator. The rise time was good, and it was great for shooting cables when I didn’t have my real Tektronix TDR with me. I’m going to see if I can update this design to surface mount parts, and bump up the output pulse amplitude.

    • Tomi Engdahl says:

      Thank you for your feedback.

      By the way what was the version of 74xx14 IC used in your design that run directly from 9V battery?
      According to the datasheet ( ) the recommended maximum supply voltage for 74AC14 is 6.0V and absolute maximum is said to be 7.0V. So running that IC at 9V would not sound right?

      The same voltage range applies also to 74HCT14 and 74HC14 etc…

      I am interested on what was used on circuit that runs from 9V battery without regulator.

  12. Tomi Engdahl says:

    More expensive TDR instruments:

    TDRs measure reflections to 10 GHz and 15 GHz

    Whenever you launch a high-frequency signal into a PCB trace, connector, or cable, you worry about signal integrity. Impedance mismatches cause reflections as a function of frequency, which degrade signals. Therefore, you need to measure how much of an incident signal will reflect to the source. While vector-network analyzers can measure S-parameters that characterize reflected energy, they can be overkill in some applications. That’s where a time-domain reflectometer (TDR) can fill the bill.

    TDRs are often sold as extensions to oscilloscopes, but they can be separate instruments, That’s where the T3SP10D (10 GHz) and T3SP15D (15 GHz) TDRs from Teledyne LeCroy come in. The instruments connect to a PC, making them portable. Indeed, you can use them in the field because they can run on battery power. Teledyne LeCroy calls the TDRs “Teledyne Test Tools (T3),” the first instruments of the brand that will complement its oscilloscopes.

    TDRs send a pulse to the target and measure the reflected signal, then calculate S11 (see S-Parameter Backgrounder), time-domain measurements (time of reflection), or produce Smith Charts based on the rise time of the pulse.

    T3SP Time Domain Reflectometers

  13. uncategorized says:

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