I have a square wave signal whose rise and fall times are too big to feed into another 74xx-series logic chip, such as the 74HC175 flip-flop, which requires rise and fall times in the nanosecond range.
I've tried feeding the square wave signal through a 74HCT14 schmitt trigger inverter, like this:
I've built the circuit on a bread board with components that were just lying around – so it's a bit messy:
Now, rise and fall times doesn't seem to improve when i feed the signal through the schmitt trigger (yellow: input, blue: output):
As can be seen, rise and fall times are roughly the same.
Also, I find it a bit strange that in the two last pictures the output signal begins transitioning before the input signal has passed Vt+.
The data sheet can be found here: http://www.ti.com/lit/ds/symlink/sn74hc14.pdf
Best Answer
It looks like you've got the oscilloscope displaying some uber-expanded portion of a capture- it's only changing slope every two horizontal divisions .. so anything faster than that all looks the same.
The first photo shows it changing slope every division, rather than two divisions in the bottom two photos, but in each case it's 20\$\mu s\$ per division, which implies your effective sample rate is only 50ksps, as @RJR comments.
If you want to see 74HC rise and fall times, 1Gsps would be more appropriate.