#88: Cheap and simple TDR using an oscilloscope and 74AC14 Schmitt Trigger Inverter
Why Electrical Engineering Stack Exchange recommends it
Electronics Stack Exchange answerers point people to it when they need a simple fast-edge pulse generator or a DIY TDR for testing coax and transmission lines.
Recommended by 3 people across 3 different months, 2013 to 2026.
What it is
A w2aew (Alan Wolke) bench video showing how to build a cheap, fast-edge pulse source from a 74AC14 Schmitt trigger inverter chip, with one gate as an oscillator driving five others in parallel. He uses it with an oscilloscope as a time domain reflectometer (TDR) to measure coax length and impedance more precisely than a slow function generator allows.
What people said
Recommends Biduleohm · 2014-10 · Electrical Engineering Stack Exchange · on “How should I test a coaxial cable?”
You can measure a lot of things (impedance, velocity factor, distance to short-circuit, distance to open-circuit, ...) with a TDR (Time Domain Reflectometer) as shown and explained in multiple tutorials. For example: Cheap and simple TDR using an oscilloscope and 74AC14 Schmitt Trigger Inverter [this video] How to measure coax velocity factor VF and impedance Z [LINK] "TDR" or Time Domain Reflectometer, build and use this circuit [LINK] Determining Velocity Factor of coaxial cable [LINK] Understanding DTF or Distance To Fault, using a TDR [LINK] Determining Coax Impedance with a TDR [LINK]
Recommends Tim Williams · 2023-09 · Electrical Engineering Stack Exchange · on “Generate Square Wave with Fast Rise Time for Testing”
Typical approach is to build a multivibrator with standard logic, and just use really fast logic to get the desired edge rate. Bus drivers can be used for higher output into a transmission line (while abusing ratings less), or multiple gates can be connected in parallel, at some compromise in performance (the propagation delays won't be perfectly matched, smearing out the edge slightly). Here are examples from another community: Yet another fast edge pulse generator - Leo Bodnar | EEVblog Forum [LINK] Compact 74AC14 pulse generator PCB - rfmerrill | EEVblog Forum [LINK] W2AEW #88: Cheap and simple TDR using an oscilloscope and 74AC14 Schmitt Trigger Inverter | YouTube [this video] ECL/CML/LVDS ga
Recommends periblepsis · 2026-01 · Electrical Engineering Stack Exchange · on “How can I build a simple pulse generator to demonstrate transmission lines”
Thanks, Mattman! I just mentioned the 74AC14's in a comment yesterday. I've used them to make a cheap TDR, following #88 Cheap and simple TDR using an oscilloscope and 74AC14 Schmitt Trigger Inverter [this video] and #90 Measure Capacitors and Inductors with an Oscilloscope and some basic parts [LINK] ! +1 to your answer. I recently purchased 1000 for my hobbyist lifetime supply *(along with a variety of other AC parts.)* Nice to see what you've done!
Cites JYelton · 2013-08 · Electrical Engineering Stack Exchange · on “What can reduce overshoot and ringing on a simple square wave pulse generator?”
I've built a simple RC and Schmitt-trigger-based square wave pulse generator. On the breadboard, it has some obvious unwanted qualities due to jumper length, the breadboard itself, etc. Schematic and breadboard version: And the waveform output: In particular, the rising edge of the square wave has a substantial amount of overshoot (about 200mV over 500mV peak) and ringing. It is easy to make it worse, by physically touching R1. See edits for correct info. In looking for solutions I've ran into terms like snubbers and dampening for RF circuits and things beyond my hobbyist pay grade. Anindo suggests in an answer to a related question [LINK] that one should use a 50Ω resistor for a load. I am
Cites JYelton · 2013-08 · Electrical Engineering Stack Exchange · on “Is this measured rise time inaccurate because of limited oscilloscope bandwidth?”
I don't have an LCR meter and wanted to learn how to use a fast edge pulse generator and an oscilloscope to do capacitance measurements as an academic exercise. In W2AEW's videos, he builds a simple fast edge pulse generator using a 74AC14 [LINK] hex Schmitt trigger (inverting) and demonstrates how to use it for capacitance and inductance measurements [LINK] as well as a time-domain reflectometer [this video] . I've built this up on a breadboard, but am getting some different results. I am measuring the input and output of the first Schmitt trigger with a 100MHz digital oscilloscope, with the same value components (6.8kΩ resistor; 0.047μF capacitor) at 5V DC. First, the frequency is about 2.5 kHz
Cites periblepsis · 2024-10 · Electrical Engineering Stack Exchange · on “How am I supposed to drive 50-Ohm terminated "CMOS inputs" (of AD9545 Evaluation Board)”
Consider using the schmitt inverters in the 74AC14. This video on making a TDR [this video] provides some details. Not promising a fit, just suggesting something to consider.
Cites Mattman944 · 2026-01 · Electrical Engineering Stack Exchange · on “How can I build a simple pulse generator to demonstrate transmission lines”
I am going to answer my own question. Feel free to offer alternatives. I won't accept my answer. When I was a junior engineer, a senior engineer demonstrated transmission lines in the lab with a pulse generator, a scope, and a few BNC cables. I learned more playing with real circuits than I learned in school. Later, I would repeat the demonstration for other engineers. A typical home lab won't have a pulse generator with a fast rise time (my function generator has a rise time of 20 ns). Fortunately, it is not hard to build one. This circuit has a measured rise time of 3 ns (could be limited by the scope, it may be a little faster). And if you build a pulse generator that can create pulses sh