Electronics > Beginners
Schmitt disappointment
sdouble:
you could get rise time of 5-6 ns... so 100 MHz harmonics should be considered.
2N3055:
--- Quote from: sdouble on October 01, 2018, 09:27:04 pm ---you could get rise time of 5-6 ns... so 100 MHz harmonics should be considered.
--- End quote ---
You are right..I was going by datasheet value of avg 7ns... I had some chips from some manufacturers going 4ns on rise time without much capacity on output...
Anyways, moral of the story is that edges define frequency spectrum contents, not repetition frequency... And can be scary fast even with plain HC chips and 1 KHz clock..
piguy101:
Could the overshoot be due to improper probe compensation? Most scopes have a 1 kHz output waveform on where you can adjust the trimmer capacitor on the side of the probe in order to show a perfect square wave for a good input.
macboy:
--- Quote from: piguy101 on October 02, 2018, 12:37:46 am ---Could the overshoot be due to improper probe compensation? Most scopes have a 1 kHz output waveform on where you can adjust the trimmer capacitor on the side of the probe in order to show a perfect square wave for a good input.
--- End quote ---
This is possible, but the effect would be lost much above 1 kHz. At 100 kHz, the amplitude would be simply appear to be too high (in the case of undercompensated).
More likely, the overshoot is caused by the fast edge exciting the LC circuit formed by the probe tip capacitance (~ 8 to 20 pF) and the several inch long ground lead (~ 200 nH self-inductance). The resonance falls in the order of 100 MHz, maybe quite a bit lower for cheap probes with high capacitance and longer ground leads. This causes overshoot and/or ringing on the observed edge. The solution is a very short ground lead to push the resonance frequency up beyond the capability of the scope and probe. If your probes didn't come with ground springs you can make your own. A homemade "Low-Z" probe can also be very effective if you have a 50 ohm scope input and the circuit can handle a 500 or 1000 ohm resistive load (a logic gate can).
PerranOak:
Thank you all very much.
It's the same on every chip.
The scope picture with the two traces shows the input (yellow) and the output (blue) overshoot and ringing.
note: the zero base is different for each to get the whole traces on.
The scope picture with one trace is when I completely disconnected the input probe and jumpers and put as short a ground connection as I could.
(I wondered what those springs were for but now I can't find them!)
The final picture is the rats' nest! :-[
So, this is a common problem with fast switching things is it? Is it only an artifact of the scope rather than a "problem" with the circuit?
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