Simple and Cheap TDR Pulse Generator

As many people in the community I really enjoy watching W2AEW’s YouTube channel. Almost a decade ago he posted a video showing a cheap and easy to build pulse generator for time domain reflectometry (TDR), consisting of only a simple hex inverter with schmitt-trigger inputs 74AC14 and a few passive components. The schematic looks something like this:

Schematic of the schmitt-trigger oscillator plus output driver

I won’t bother explaining this simple, but clever circuit, since Alan did as great of a job as always. If you haven’t done so yet, you can watch the video here:

Anyway, I decided to build a small board and compare the results with Alan’s. After the board was sitting in the drawer for half a year, I finally assembled the board using a Texas Instruments 74AC14.

For the test I used my Siglent SDS2000X Plus scope (500MHz, 800ps rise time typical @ 50 Ohm) which has to stretch its legs for this one: I measured a respectable rise time of about 960ps. In his video, Alan measured something like 2ns of rise time for his circuit using DIP packaged hex inverter. Both his and my variant show a noticable ringing.

The fastest rise time my scope ever had to measure

By the way: My circuit oscillates at about 2.9 kHz with a duty cycle of just below 55%. I wouldn’t mind a higher frequency. This would be easily achieved by reducing the product of C1 or R1, but I didn’t bother.

The oscillator frequency varies with R1 and C1 as well as the hysteresis thresholds of the Schmitt-trigger input of the 74AC14. The jitter of this oscillator is quite substantial.

I think one could achieve faster rise times with the same circuit, but using a couple of 74LVC1G04 inverters in the output stage instead. I don’t keep those in stock though 😉




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