Beside the chip that he suggest, the 74AC14 Schmitt trigger, is there any other chip with faster rise times?
I found this one LTC6993-1, but it still has rise time of 1.1us.
Since I am working with cables that are short 100-300mm, I need something even faster. Is there a chip with rise time in low pico second range, that doesn't cost much. Preferably in DIP format or at worst in SOP?
74AC14 : 2ns
LTC6993 : 1.1ns
ATmega328P : ? (I can't find it in the datasheet, it is 3.6ns for SPI)
PO74G14A : 0.8ns
I think that normal rise times are about 10ns. You need a chip for the GHz range. A Schmitt-trigger for GHz is hard to find. The 74AC14 is already very fast.
Do you have an oscilloscope that can measure into the GHz range ?
For such short cables your also going to need a fast oscilloscope.
Why do you need to fault check/measure such short cables?
You could maybe put a fixed length cable between test cable and the pulse generator to negate the need for such short rise times by delaying the reflection, this would need a good, repeatable quality connection on the cables to prevent the joint causing to much reflection.
Yeah, I have a secondhand 1GHz oscilloscope. So I can go pretty fast.
I am working on laser rangefinder idea.
But as well for general learning. I want to learn about reflections in cables etc., like transmission line theory. But most of cables that I have are quite short.
PO74G14A is fast, but pity not available any more.
For future reference, I searched Farnell's catalog for "comparator" and found few interesting ones:
ADCMP573BCPZ-R2 HIGH SPEED COMPARATOR with 150ps rise time, but it is in a $30 range and it is in that awkward LFCSP-16 package with connectors under the IC.
ADCMP552BRQZ in QSOP package, with 750ps rise and cost of about $10.
MAX9601EUP in TSSOP package, with 200 ps rise time, for about $16.0.
Years ago I did a couple TDR designs. One used an 8 bit Flash AD converter with output to fast RAM. No good for your short distances.
The other was a small handheld instrument that monitored the reflections with a threshold detector. You would slowly adjust a potentiometer to lower the threshold until the strongest echo triggered it, which then stopped a counter that was driven by a high speed clock. You read out the distance (the count) on 7 segment displays, very simple. High impedance faults have a positive echo and low impedance have negative echos (I Think, this is from old memory ) I used a couple LEDs to indicate whether it was a short/ground or an open.
The second example might be possible for your needs (haven't put a lot of thought into this, maybe someone else will have better input on using this method for your very short distances.)
Maybe a PECL or LVPECL driver? Something like MAX9321B,
That is one fast chip and differential inputs makes it perfect; one for echo and one for threshold voltage. Trouble is, C is so darn fast. My back-of-a-napkin calculations show a best resolution of about 50 mm if you happen to get a chip with Differential Input-to Output Delay (min) of around 145 ps . ..Maybe... Layout would be critical, of course.
My calculation could be off by an order of magnitude or so because it's past bedtime here.
[edit] Whoops - forgot to add in propagation delay in stopping the counter.
) I used a couple LEDs to indicate whether it was a short/ground or an open.
because it's past bedtime here.