[time-nuts] Subject: Re: Working with SMT parts (Bob Albert)

Bob Albert bob91343 at yahoo.com
Thu Aug 18 02:07:55 EDT 2016

Jim, that's the exact one I built!  I rechecked and the rise time is about 2 ns, not longer as I previously reported.  Not only do I have a new piece of test gear but I have gotten some SMT experience.  Previously I have removed chips but this is the first time I have installed one.  I didn't use the liquid solder because I didn't have any and it doesn't keep very long.  I used ordinary solder with a small iron and it worked well, although I didn't have as much control as I wished.
I also have a SNA that can be used for some of the same measurements but this thing is simple.  I checked both with my digital HP scope and my analog Tek and got the same result.
I checked a three foot piece of coax, both open at the far end and shorted there, and got the textbook waveforms.  I love it when the basics check out.

    On Wednesday, August 17, 2016 11:00 PM, Jim Stone <tictocnut at gmail.com> wrote:

 Hi again,
The cable impedance measurement was in a different video (linked below) but
it can use the same HC14 TDR as in the first video. The concept is to just
put a pot on the end of the cable and adjust it until the reflection goes
away. Simple!


--Jim Stone

Hi Bob,
Take a look at this nice simple little one chip "TDR" and lesson on how to
use it to measure cable length and cable impedance. It uses a single AC14
(can use a DIP so no SMD needed) and gives a nice crisp 2 or 3 ns rise. The
AC family is nice fast logic and has been used in many Time-Nuts
application like cheap zero-cross detectors using the AC04.


Regards --Jim Stone

Bob Albert wrote:

 Well I got my adapter boards from China and managed to build my first SMT
project, a square wave generator for TDR use.  And it works!  The IC seems
to run hot so I used my IR temperature measurement device and it checks out
at about 37C, acceptable.  I can now drive a square wave at about 3 kHz
into 50 Ohms.  The rise time isn't very short but I must have not used the
best part for the generator.  Still, I can do some TDR experimenting as
long as the line isn't too short.
Thanks to all for the ideas and encouragement.  I didn't use a microscope,
mostly just a magnifier.  My tiniest soldering iron is a bit large but it
did the job.
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