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Re: Output bounce, can this be fixed?
| | List |
| Subject: | Re: Output bounce, can this be fixed? |
| Poster: | john jardine |
| Date: | Fri, 23 Mar 2007 15:27:43 -0000 |
| Related Postings: | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 |
"Anthony Fremont" wrote in message
news:13076uosaghvv05@news.supernews.com...
>
> My rise times were about half that according to the scope. Once I slowed
> them down a bit, they didn't overshoot like that. You tried feeding a DC
> coupled ~10MHz sine wave into it?
>
> > Tried square, triangle and mid biased small sines. Always the same
> > clean suare out.
> > What you show (with it's 40MHz resonance) looks distinctly like a gate
> > feeding a few feet of normal coax connected directly to a scope.
> > Sure that probe's working?.
>
> I tried an older probe that I've had for a long time and got pretty much
the
> same thing, massive ringing and overshoot. I put it on my old Hitachi
> V650-F analog scope and viewed the same results. I had trouble with the
> chip wanting to break into oscillation, so I guess I have just the right
> combination of circuit capactance/inductance/frequency to create resonance
> problems. When I have some time to tinker around, I will do some testing
> using a 10MHz crystal oscillator and a 74HC14 on a another breadboard and
> see if I see the same thing. BTW, I tied all unused inputs to Vss or Vdd.
>
The gate works OK with a DC coupled 10Mc sine input. Even works down to a
1Vpp sine input but for this small a signal the sine needs to be sitting
about an accurate midrail of 2.5V. (Can even get down to 0.8Vpp input if
using a square wave!.)
Other than what most non users casually suspect, those protoboard things are
NOT a problem. The things are good to at least 50Mc.
To clear another point, I also added 2 foot of extra wire to the scope probe
ground lead and only got 1/2V overshoot (20Mc ring) , so that's not the
problem either.
My chip has a 10uF 'lytic across it's supply pins so 'decoupling' is not as
problematic as many people would like to believe.
Then directly connected a 1mtr 75ohm coax test lead to the gate and still
only saw about 0.8V overshoots (15Mc ring).
As you note, it's the chip-on-the-edge-of-oscillation that's the root cause
of the problem. The key probably sits with the reason for that whopping
great 12Vpp output.
--
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