Output bounce, can this be fixed?

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an

Personal proposal. You can follow a resistance after the 74HC14 output and connected to ground with a capacitor and adjust the values. Maybe it works.

Thank you, I will try that. :-)

I have done this many times with no problems. Sounds like you may not have a VCC to ground bypass capacitor on the HC14. You could also try a 47 Ohm resistor in series with the output, or you may be overdriving the device. Bias up the input with a 15K to ground and a 33K to VCC; then capacitively couple the input. Signal should be about 3 V p-p. You are kind of pushing the frequency for an hc14. A 74AC14 will work better.

Tam

Thanks Tam. I grabbed a 470 Ohm resistor laying nearby and stuck it on the output and it got rid of the ringing. It's not real pretty still, but it looks allot more like a digital signal now than an analog one. ;-)

hes using breadboard btw ... also what scope probe are you using? is it a 100mhz 10:1 type ?

if you look at figure 11 ..

it shows the effect of a scope probe on a signal and it looks pretty much identical to the waveforms you show. the scope probe input basically looks like a tuned circuit at some frequency, wich is dependant on the 1/4 wavelength of the lead wich is probably 50mhz or so. the 470 ohm resistor effectivly damps this to some extent, but not vvery much.

another trick is that many scope probes have a 450 resistor in series with the input, if you then put the scope probe on the X1 position and the scope input to

50ohms then you have a neat 10:1 divider with a purly resistive input.

I dont know if they designer had this in mind ive never seen it mentioned, just found it worked surprisingly well by acident.

Colin =^.^=

The 470 Ohms is going to give you band limiting. Try a smaller value. Did you bypass the 74HC14 VCC line?

Tam

You probably need to be more careful about grounding your scope probe to the Vss power pin of the chip you are looking at, or some 0V (ground) connection very close to this point.

Junior engineers make this mistake all the time, and I once had to persuade a customer who was using a Cambridge Instruments electron beam tester that he had to ground his circuit to the electron microscope inside the vacuum chamber where his circuit was being tested - he was very unhappy with the ringing he was seeing, and dead embarassed when it went away when I hooked up the earthing lead. We were installing the machine - a prototype - at Thompson-EFCIS in Grenoble, and the customer was very nervous.

-- Bill Sloman, Nijmegen

This was alluded to in an earlier response, but are you sure that the ringing isn't an artifact of your scope?

Putting a resistor in series with the signal will slow the edge rate making it both more 'scopable' and limiting the bandwidth (won't be as as square).

Tam is correct, make sure the chip is properly decoupled.

Also, do you have a good solid ground plane or are you running bread board or serpentine traces? Without a solid ground reference, you will get a lot of bounce that will screw things up.

Since you mention tghe 'scope, I hope he is grounding the probe at pin 7 of the '14. Bypass cap means pin 7 to 14 of the 74HC14. Not the other end of the board. We used series Rs between 22 and 51 Ohms to get rid of ringing. Extreme case was something like 100 Ohms to meet FCC Part15.

Tam

I did that and it did improve the signal some, but not allot.

That's what I do. I straddle the IC with them.

I'm only a hobbyist, but I've "scoped" quite a few signals. It's been mostly low frequency stuff and I admit that I'm sure I could learn plenty about using one. But I've put probes on PIC pins that were unloaded and the ground was far from the PIC Vss pin, and I still never saw ringing like that on one.

I just looked at the specs and it looks like the typical rise time on a PIC I/O pin is twice as long as the 74HC part. Slowing the rise/fall times down a little by loading the output pin eliminates most of the ringing. I guess I just haven't been playing with fast enough parts. ;-) I did notice that my old probes (that have thicker cables) don't show the effect quite as much.

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Can

I've the same setup as you and plugged a 74HC14 in. Saw no problems. See

15nS rise and fall with no overshoots. Gate fed from a fast Wavetek function generator. Couldn't get what you show whatever I put in. 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?.

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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?

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

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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A 0.1 uF ceramic in parallel with the 10 uF might help.

Thanks John, I've done plenty of PIC projects running at 8 and 10MHz with no issues like this at all. I really don't understand the hatrid of these solderless boards. If I was building it dead-bug style with unclipped leads, nobody would care. Or on top of a solid copper clad board with islands cut out, would be acceptable thru low VHF. But don't use a proto board, oh no too much capacitance. ;-)

I've never had to go to any great lengths to see signals in this frequency range without a 50% overshoot.

The times that I've seen decoupling make a big difference on these solderless breadboards have been relatively few. I do put them in out of habbit though. The major exception is the some of the nanowatt PICs (16F88s come to mind) not wanting to start. Add a cap straddling the proc power pins and viola, it starts every time. In that case I believe the cap is acting more like a close battery for switching surges than anything else.

I really suspect that the whole thing is resonant at about 40MHz and plinking the chip at approx 1/4 of that frequency is hilighting that. Right now the oscillator and the IC get their power straight from a battery. I don't have a low ohmage series resistor going to the oscillator or buffer stages, perhaps this is part of the problem.

At first I thought the bias resistors I used on the IC input pin were messing up the oscillator waveform. It turns out that the problem seems to be from the IC switching. (I posted a shot of it over on ABSE). Do you think the IC switching is mucking up the oscillator signal by causing a sudden change in impedance and reflecting back into the oscillator buffer, or something happening on Vcc that is looping around the power rail and getting into the oscillator and buffer?

Thanks for everything. :-)

Right

Beyond a few megs (MHz, Mc) I don't bother with any extra series supply impedance. The connecting wires and traces have quality built in inductance so I just add a capacitor locally and it forms a free LC low pass filter. My working mental pic' is of -all- connecting wires having a notional '1uH' and I design accordingly.

to

Yes. The nasty little jiggles are normal. The incoming signal takes a capacitive feedback hit every time the gate switches through it's threshold region. It's those input fets fed from the gate's input pin. Can be ameliorated (todays word) somewhat, by adding say 470ohms in series with the gate but we're now in a trade off situation, as the gate switching starts to slow down due to the input fets Miller capacitance taking longer to charge/discharge via the extra inline resistance. If I want a sinewave to feed other areas and keep it 'clean' then for any digital squaring up I now use an LMV7219 5V comparator. john

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I've always loved the proto-boards, but I must confess I haven't done much above audio-type frequencies. But I have done pad-per-hole with point-to- point wire-wrap-type wire, to at least 4 MHZ, with no observable problems.

But then again, one of my mottos is, "You can't overcapacitate!" (referring to power busses, of course.)

Cheers! Rich

I did a lot of WireWrap in the '70s and early '80s. I'd much rather WW than use breadboards (yech!). Not so much for speed (though WW isn't all that bad) than reliability. Intermittents are just no fun.

4 MHz is easy. I've done to 80MHz (ECL) on WireWrap (divided down pretty quickly). High speed TTL at 25-50MHz was also quite possible. Short wires and daisy chaining are important, as is...

Yep. Power was more of a problem than signals. *VERY* short power connections to the planes is crucial. I used WW boards that had power/ground on top/bottom with power stripes between the rows of pins. To connect a pin to power one soldered a spring (almost big enough to see) between the pin and plane.

I wouldn't do WW today. Prototype PCBs are too cheap and the turn is too fast to bother these days.

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  Keith

I added the low ohm resistors and even built up another complete circuit. No change. :-( No more oscillations, just a ringing output. I put a resistor and cap to ground to make myself feel better. ;-)

Ouch, I don't like that at all.

Something to try though. It's not like this is microwaves. ;-)

And finally we reach the root of the problem. I should have asked what device to use to start with. I knew it was hopeless to try and use any of the comparators in my junk box. Nice part, no DIP package though. :-( Can you recommed something in a DIP 8 that will switch a 10MHz signal?