74HC14 schmitt inverter issue

[psubond]

I am trying to get more familiar with using a 74HC14, as an experiment i have an ldr set up in a voltage divider configuration and the center tap goes to the input of the 74HC14 input 1.  I have all of the other inputs tied to ground.  everything is soldered together on a perf board.  I have a .1uF, .01uF, .001uF, .0001uF cap on the power pin (probably overkill).  Input voltage is 5v.  The voltage divider and the IC are both powered off the same rail

The IC triggers as expected when light is shined on/removed from LDR.  Below is the waveform I am getting, it is very consistent from test to test.  What I don't understand is why the signal is dropping to -2.4 or so when triggered then staying below 0.  The rising edge has an overshoot to about 6v but is smoother. Any information/guidance would be appreciated.



https://drive.google.com/open?id=1oG6Eh2Gwj9bf4IXGBJ9i5LoJoLvwybN6

[wraper]

1. Measuring with long ground connection. For best results you should use short ground and signal connection directly from IC.
2. Probe compensation probably is not properly adjusted.

[psubond]

Thanks, will give that a try!

[mvs]

What I don't understand is why the signal is dropping to -2.4 or so when triggered then staying below 0.

74HC14 edge is typ. 7ns fast (140MHz), you need a quite fast scope to see real signal.

[iMo]

I have a .1uF, .01uF, .001uF, .0001uF cap on the power pin (probably overkill).

The recommended ratio for the decoupling capacitors is 1:100 (for demanding applications).

Like
100u || 1u || 10n
10u || 100n || 1n
1u || 10n || 100p

In your case 2 capacitors would be ok.

[psubond]

What I don't understand is why the signal is dropping to -2.4 or so when triggered then staying below 0.

74HC14 edge is typ. 7ns fast (140MHz), you need a quite fast scope to see real signal.

my scope is a 200Mhz scope, I don't have a function generator that can push it that fast to see if it can actually do it though.

[rstofer]

To see a 7 ns rise time requires a scope with around 50 MHz of bandwidth.

B = 0.35 / t_rise = 0.35 / 7E-9 = 50,000,000 Hz = 50 MHz  Not a lot, actually

https://interferencetechnology.com/rise-time-vs-bandwidth-and-applications/

You can build a somewhat reasonable square wave generator from the spare inverters:

http://www.talkingelectronics.com/pay/BEC-2/Page49.html

A 555 timers is probably a better source for a square wave.

[mvs]

What I don't understand is why the signal is dropping to -2.4 or so when triggered then staying below 0.

74HC14 edge is typ. 7ns fast (140MHz), you need a quite fast scope to see real signal.

my scope is a 200Mhz scope, I don't have a function generator that can push it that fast to see if it can actually do it though.

Bandwidth is just a metter of signal attenuation in analog part of a DSO. Sampling rate is a bit more important here.
Let assume your DSO has 1GSpS (1 sample per 1ns). You will get less then 10 samples to reconstruct a signal from. Depending on interpolation function your scope is using, you may get ringing on the screen which is actually not in real signal.
Dave has a vid on this topic.