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Output bounce, can this be fixed?

R

Rich Grise

Jan 1, 1970
0
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 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
 
K

krw

Jan 1, 1970
0
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.

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

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.
 
A

Anthony Fremont

Jan 1, 1970
0
john said:
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.

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. ;-)

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.

Ouch, I don't like that at all.
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.

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

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?
 
J

john jardine

Jan 1, 1970
0
Anthony Fremont said:
[...]
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.

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?

Love to find some DIP items myself (at reasonable cost).
Seem forever nowadays to be soldering specks of coal dust onto DIL adapter
PCBs. I can no longer trust the makers datasheets, hence test their stuff in
isolation. Bring back the DIP!, we don't all design cell phones and ipods or
have immutable space constraints.
 
C

colin

Jan 1, 1970
0
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?

I would reckon the best thing to do would be to use a simple digital
oscillator
say a pierce oscillator built around a single cmos logic inverter,
and buffered with a second one. any cmos inverter would do at 10mhz.

The sinewave can more easily be extracted from the squarewave than the other
way round,
as all it requires is a low pass filter.

There will actually be a fairly good sinewave at the input to the first gate
anyway,
an inductor and/or resistor from this point to a low input capacitance
linear amplifier buffer,
say a simple single jfet amp.

to reduce the coupling of the buffer the pick off point could be taken from
a capacitive divider wich takes the place of the tuning capacitor at the
input to the inverter.

Colin =^.^=
 
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