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Fast Edge Generator


I've been designing a JTAG level shifter to extend the 3V3 capability of my Bus Blaster up to 5 for a keyboard I've been reverse engineering lately. Since I've decided to go with passive MOSFET-based voltage translators (LSF0204), signal integrity is a factor of concern. In order to perform some basic checks on the signal degradation through this level shifter, I decided to dig out an old PCB I had designed based around the oscillator that Alan W2AEW shows in some of his YouTube videos for TDR (time-domain reflectometry).

Designed around the 74AC14/74ACT14, the buffered square-wave oscillator Alan shows, displays very fast edges. This makes it especially suited to TDR among other things. Alan's videos on the oscillator (and really any videos on his channel) are definitely worth the watch for the electronics or radio enthusiast. Check out his YouTube channel.

Digging out an old design

I decided that in order to characterise the distortion that the LSF0204 introduces to my JTAG signals, it would be a good idea to dig out an old PCB I had designed in August 2019 a little over two years ago. I had designed the circuit and routed a PCB in Kicad, based around a CR2032 cell for power, but never had the board fabricated. Fast-forwarding to 2021, I decided to replace the CR2032 holder with a Molex KK type header, choose a smaller package for the 74ACT14, and reroute the board for size. The result is a nice little stick, not much wider than the through-hole BNC connector at the output.

Top side:

Top side of the board, 3D render

Bottom side:

Bottom side of the board, 3D render

I had originally planned for a nice self-contained unit with a 3D printed case and CR2032 coin cell to power, but decided against this for ease of powering from different voltages for different applications. In theory, the circuit should be able to operate anywhere from approximately 2 volts up to a little over 5, giving more flexibility (and runtime) than a coin cell while losing the inherent isolation which battery operation affords.

The boards are currently being fabricated with OSHPark, and should be here in New Zealand within a month or so, given current postal delays. Updates to come, but hopefully it's mostly uneventful, and all I post are some nice scope traces ☺.