Building and using a Schmitt Trigger Oscillator
InspirationAlan Wolke / W2AEW has a youtube channel in which he does all manner of informational posts about electronics, RF and Ham Radio. He did a great video showing how to do TDR measurements by using an oscilloscope and a Schmitt Trigger Oscillator which he built up from some scraps on his bench. I decided to build one for my own use.
Build and Test
I had most everything in my scrap box. I elected to use a CR-2032 (3V) button battery, seen on the left, since it would keep the whole thing small. I just had to order some Schmitt Trigger Oscillators from Digi-Key.
Upon re-watching his video, he used a 47nF cap. I used a 4.7nF cap. Oops. Well, the oscillator frequency came out about 24 khz. That's still completely serviceable. I opted not to bother fixing it.
The second issue (which I don't understand) is that the rise time is not as fast as Alan observed. I'm getting a rise time of about 9 ns. He was getting about 2ns. I used the same 72AC14 Schmitt Trigger DIP that he was using. I'm not sure why it might be slower. Still, even at 9ns, it's perfectly fine, and still much better than the cheap signal generator on my desk (24ns) which is also much bulkier.
I observed 173ns. We are using Beldin 9913 coax, which has a velocity factor of 84%. Using 11.8 inches/second, at 84% that's 9.912 inches/sec. With 173 nanoseconds that yields 1714.78 inches (142.898 feet) round trip. That's about 71.5 feet to the end of the cable.
Kevin / WB2EMS used a proper TDR cable measuring device and came up with 72 feet. So, we're definitely in the right ballpark.
All in all, it worked great and served the purpose. It's small and easy to use. I've popped it in my toolbox with my adapters and other "antenna and coax" tools. Project successful!