PulseGen

Fast Pulse Generator/Coax Tester

• Uses of the Fast Pulse Generator
  • Start at 15:12

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Features

• Very fast pulse edge (typical 2 nS rise time) and symmetric high/low drive.
• 5mm Terminal Block for power allows for batteries, etc to be easily connected without soldering
  • Power Supply from 2V to 6V range so battery operation is possible eliminating need for power supply, etc
• 50 or 75 Ohm (other values could be made, send us your specs and we can add them)
• BNC connectors on both ends
• Connectors are a stuffing option with either BNC or SMA connectors (SMA is a special order so contact us)
• Four mounting holes - 4-40 holes
• Rounded corners fit more easily into plastic enclosures with inside radius at the corners
• Nuts and washers for BNC mounting included
• Small size (1.8" x 1.3)

What is it?

Fast Edge Pulse Generator for use as a Time Domain Reflectometer (TDR). Used to measure cable length and test continuity/termination of 50 or 75 Ohm coax cables.

Requires a fast oscilloscope for precision measurement. To use it, plug one BNC using male-to-male adapter into a scope and attach a cable to the other BNC connector. Allows measurement of the cable and determination if there is a cable defect (open/short, termination, Etc.).

3D Printed Case

3D Printed case in picture not included - printer files are here.

• Case

• Lid

Why did you make it?

The video, by W2AEW, inspired me to want to make a Pulse Generator for use with an oscilloscope as a Time Domain Reflectometer (TDR). There were some DIY boards available but none that are as good as this design.

What makes it special?

Within a month of first building this board, I had a bad piece of RG-8X coax and it was difficult to diagnose the problem. I used an ohmmeter to verify that the coax was good from a DC perspective and it looked fine while I was testing with the meter. It was connected end to end and there were no opens or shorts that I could see with the meter.

However, I knew the cable was bad because I swapped it out for another cable and the other cable worked just fine. When I went to use the broken cable, I could jiggle the cable and make it work and then it would flake out but it was far from consistent. Intermittent problems are the worst sort of problems, aren't they? As they say - Murphy's Law is a universal constant.

I decided to hook the questionable coax up to this Pulse Generator board and the results were impressive to say the least. When I flexed the cable the waveform went from a nice stepped waveform to nearly flat on my oscilloscope (a little bump which went down quickly). That told me that the problem was on the near end of the cable and that it was a short between the center conductor and the shield. In the end the problem was a short at the PL-259 connector which (once found) was easily repaired.

Factory Acceptance Test (FAT) Procedure

Hardware

• QTPy49
• QT_Py_(RP2040_based)
  • A0 - Analog Input - samples
• Cables
  • USB-C
  • Power cable
  • Analog in (BNC)
• Terminator - dependent on build option
  • 50 Ohms
  • 75 Ohms
• Unit Under Test (UUT)

Test Steps

• Connect Power from STE to UUT
  • Connector is 3 pin Dupont housing with 2 male pins (center pin open)
  • Minus (black) goes to the left, Plus goes to the right (towards C2 capacitor)
• Connect BNC cable from STE to SCOPE connector on UUT
• Connect USB-C to the QT_Py_(RP2040_based)
• Add terminator
• Run Thonny IDE
• Open PulseGen-01.py
• Run (green arrow)
• Results

PulseGen Card Tester V1.0
Measuring Frequency, 512.0 Hz is in expected range
Looking for low...Got Low val 0.594075%
Got HIGH terminated val 49.6907%
Got HIGH un-terminated val 99.7151%
Detected all values

Test Station Software

• PulseGenTest GitHub Repository

Modifications to Use as a 50 Ohm Driver

• The R/C pair for the oscillator can be removed and the card can be used as an embedded 50 Ohm driver
• Detailed instructions for build as 50 Ohm driver
• Test using Test50-01.py

Assembly Sheet

• PulseGen Assembly Sheet