Hi chaps,

Heading sums it up. I'm using a 74HC04 to 'square-off' a sine wave,
but it's causing a fair amount of ringing on the output signal's highs
and lows which appears to be finding its way through to the eventual
final stage. How do I best eliminate (or at least drastically reduce)
this nuisance? The square wave frequency is 8Mhz., btw.

Thanks,

p.
--

"Windows [n.], A thirty-two bit extension and GUI shell to a sixteen bit patch
to an eight bit operating system originally coded for a four bit
microprocessor and produced by a two bit company."

Paul Burridge wrote:
Hi chaps,

Heading sums it up. I'm using a 74HC04 to 'square-off' a sine wave,
but it's causing a fair amount of ringing on the output signal's highs
and lows which appears to be finding its way through to the eventual
final stage. How do I best eliminate (or at least drastically reduce)
this nuisance? The square wave frequency is 8Mhz., btw.

Thanks,

p.

If you put a low inductance termination resistance right at the pin with
no wire and a low inductance bypass cap at the power supply and properly
introduce the signal to a scope, is the ringing still there? If it
ain't, the chip ain't causing it. Does the output ring with a square
wave input? Ringing is often more about the
interconnect than the chip...often...not always...
Oscillation, which can look like ringing is often about the input drive
for low dv/dt inputs.
Observed ringing, as opposed to Actual ringing can also be all about
measurement technique.


Can you be more numerical about "fair amount"? Amplitude and frequency
of the ring? Getting rid depends on how much you got and how much you
can stand and what you're driving.
mike

--
Bunch of stuff For Sale and Wanted at the link below.
laptops and parts Test Equipment
4in/400Wout ham linear amp.
Honda CB-125S
400cc Dirt Bike 2003 miles $550
Police Scanner, Color LCD overhead projector
Tek 2465 $800, ham radio, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/

On Wed, 15 Oct 2003 11:28:06 +0100, Paul Burridge
wrote:


Hi chaps,

Heading sums it up. I'm using a 74HC04 to 'square-off' a sine wave,
but it's causing a fair amount of ringing on the output signal's highs
and lows which appears to be finding its way through to the eventual
final stage. How do I best eliminate (or at least drastically reduce)
this nuisance? The square wave frequency is 8Mhz., btw.

Thanks,

p.

Layout, layout and layout.

Decouplers close to chips, short ground leads. Also be quite certain
that the ringing is really there. Ditch the ground lead from the scope
probe and make a very short one by wrapping stiff wire round the earth
ring at the end of the probe and leaving a piece as long as the probe
tip sticking out. Get up close to the chip signal and ground pins and
see if the ringing is gone.

d

_____________________________

http://www.pearce.uk.com

On Wed, 15 Oct 2003 11:47:40 +0100, Don Pearce
wrote:

Layout, layout and layout.

Decouplers close to chips, short ground leads. Also be quite certain
that the ringing is really there. Ditch the ground lead from the scope
probe and make a very short one by wrapping stiff wire round the earth
ring at the end of the probe and leaving a piece as long as the probe
tip sticking out. Get up close to the chip signal and ground pins and
see if the ringing is gone.

Thanks Don. I'll try that suggestion. I've read Mike's reply but I
think he's pretty much saying the same thing as you.
I've ended up with a decoupling cap that straddles diagonally over the
top of the IC. For some reason, on this chip, the supply and gnd pins
are on opposite corners, which is a nuisance, so the cap's leads are
longer than I'd have liked. I suppose this sort of situation doesn't
help?
--

"Windows [n.], A thirty-two bit extension and GUI shell to a sixteen bit patch
to an eight bit operating system originally coded for a four bit
microprocessor and produced by a two bit company."

On Wed, 15 Oct 2003 13:34:45 +0100, Paul Burridge
wrote:

On Wed, 15 Oct 2003 11:47:40 +0100, Don Pearce
wrote:

Layout, layout and layout.

Decouplers close to chips, short ground leads. Also be quite certain
that the ringing is really there. Ditch the ground lead from the scope
probe and make a very short one by wrapping stiff wire round the earth
ring at the end of the probe and leaving a piece as long as the probe
tip sticking out. Get up close to the chip signal and ground pins and
see if the ringing is gone.

Thanks Don. I'll try that suggestion. I've read Mike's reply but I
think he's pretty much saying the same thing as you.
I've ended up with a decoupling cap that straddles diagonally over the
top of the IC. For some reason, on this chip, the supply and gnd pins
are on opposite corners, which is a nuisance, so the cap's leads are
longer than I'd have liked. I suppose this sort of situation doesn't
help?

I was once called down to the production testing by a newly hired tech with a
similar complaint of excessive ringing. Turned out that the scope was grounded
to the circuit by a separate banana lead. The scope probe ground was removed
"because it was is not convenient" although there were plenty of local ground
point provided on the board. There was some residual ringing in his ears
afterwards.

--

Regards,

Boris Mohar

Got Knock? - see:
Viatrack Printed Circuit Designs http://www3.sympatico.ca/borism/
Aurora, Ontario

In article ,
says...
On Wed, 15 Oct 2003 11:47:40 +0100, Don Pearce
wrote:

Layout, layout and layout.

Decouplers close to chips, short ground leads. Also be quite certain
that the ringing is really there. Ditch the ground lead from the scope
probe and make a very short one by wrapping stiff wire round the earth
ring at the end of the probe and leaving a piece as long as the probe
tip sticking out. Get up close to the chip signal and ground pins and
see if the ringing is gone.

Thanks Don. I'll try that suggestion. I've read Mike's reply but I
think he's pretty much saying the same thing as you.
I've ended up with a decoupling cap that straddles diagonally over the
top of the IC. For some reason, on this chip, the supply and gnd pins
are on opposite corners, which is a nuisance, so the cap's leads are
longer than I'd have liked. I suppose this sort of situation doesn't
help?


How about swapping the '04 for a '14, AND make sure it is properly
decoupled?
Maybe the '04 is getting a little funky in the zone between a valid high
and low?

Jim

On Wed, 15 Oct 2003 13:34:45 +0100, Paul Burridge
wrote:

On Wed, 15 Oct 2003 11:47:40 +0100, Don Pearce
wrote:

Layout, layout and layout.

Decouplers close to chips, short ground leads. Also be quite certain
that the ringing is really there. Ditch the ground lead from the scope
probe and make a very short one by wrapping stiff wire round the earth
ring at the end of the probe and leaving a piece as long as the probe
tip sticking out. Get up close to the chip signal and ground pins and
see if the ringing is gone.

Thanks Don. I'll try that suggestion. I've read Mike's reply but I
think he's pretty much saying the same thing as you.
I've ended up with a decoupling cap that straddles diagonally over the
top of the IC. For some reason, on this chip, the supply and gnd pins
are on opposite corners, which is a nuisance, so the cap's leads are
longer than I'd have liked. I suppose this sort of situation doesn't
help?

I was once called down to the production testing by a newly hired tech with a
similar complaint of excessive ringing. Turned out that the scope was grounded
to the circuit by a separate banana lead. The scope probe ground was removed
"because it was is not convenient" although there were plenty of local ground
point provided on the board. There was some residual ringing in his ears
afterwards.

--

Regards,

Boris Mohar

Got Knock? - see:
Viatrack Printed Circuit Designs http://www3.sympatico.ca/borism/
Aurora, Ontario

In article ,
says...
On Wed, 15 Oct 2003 11:47:40 +0100, Don Pearce
wrote:

Layout, layout and layout.

Decouplers close to chips, short ground leads. Also be quite certain
that the ringing is really there. Ditch the ground lead from the scope
probe and make a very short one by wrapping stiff wire round the earth
ring at the end of the probe and leaving a piece as long as the probe
tip sticking out. Get up close to the chip signal and ground pins and
see if the ringing is gone.

Thanks Don. I'll try that suggestion. I've read Mike's reply but I
think he's pretty much saying the same thing as you.
I've ended up with a decoupling cap that straddles diagonally over the
top of the IC. For some reason, on this chip, the supply and gnd pins
are on opposite corners, which is a nuisance, so the cap's leads are
longer than I'd have liked. I suppose this sort of situation doesn't
help?


How about swapping the '04 for a '14, AND make sure it is properly
decoupled?
Maybe the '04 is getting a little funky in the zone between a valid high
and low?

Jim

"Paul Burridge" wrote in message
...
On Wed, 15 Oct 2003 11:47:40 +0100, Don Pearce
wrote:

Layout, layout and layout.

Decouplers close to chips, short ground leads. Also be quite
certain
that the ringing is really there. Ditch the ground lead from the
scope
probe and make a very short one by wrapping stiff wire round the
earth
ring at the end of the probe and leaving a piece as long as the
probe
tip sticking out. Get up close to the chip signal and ground pins
and
see if the ringing is gone.

Thanks Don. I'll try that suggestion. I've read Mike's reply but I
think he's pretty much saying the same thing as you.
I've ended up with a decoupling cap that straddles diagonally over
the
top of the IC. For some reason, on this chip, the supply and gnd
pins
are on opposite corners, which is a nuisance, so the cap's leads are
longer than I'd have liked. I suppose this sort of situation doesn't
help?
--

"Windows [n.], A thirty-two bit extension and GUI shell to a sixteen
bit patch
to an eight bit operating system originally coded for
a four bit
microprocessor and produced by a two bit company."

The 'HC and 'AC families (at least the simpler parts) can be made to
give
_extremely_ clean square wave outputs, but as Don and Mike say, you
have to be careful.

The decoupling cap you mention above sounds like a leaded part, with a
very substantial amount of lead inductance when mounted like that. For
good results, ideally you would have planes for power and ground on
adjacent
layers in the PCB under the chip. That allows you to put the decoupler
at
either end of the chip, and still retain low inductance. Adjacent
planes also
acts as a small but very good at HF capacitor.

Boris makes a good point about the size of the loop between probe tip
and
ground. You can get little socket adapters which you solder to the
board to
minimise this.

Regards
Ian

"Paul Burridge" wrote in message
...
On Wed, 15 Oct 2003 11:47:40 +0100, Don Pearce
wrote:

Layout, layout and layout.

Decouplers close to chips, short ground leads. Also be quite
certain
that the ringing is really there. Ditch the ground lead from the
scope
probe and make a very short one by wrapping stiff wire round the
earth
ring at the end of the probe and leaving a piece as long as the
probe
tip sticking out. Get up close to the chip signal and ground pins
and
see if the ringing is gone.

Thanks Don. I'll try that suggestion. I've read Mike's reply but I
think he's pretty much saying the same thing as you.
I've ended up with a decoupling cap that straddles diagonally over
the
top of the IC. For some reason, on this chip, the supply and gnd
pins
are on opposite corners, which is a nuisance, so the cap's leads are
longer than I'd have liked. I suppose this sort of situation doesn't
help?
--

"Windows [n.], A thirty-two bit extension and GUI shell to a sixteen
bit patch
to an eight bit operating system originally coded for
a four bit
microprocessor and produced by a two bit company."

The 'HC and 'AC families (at least the simpler parts) can be made to
give
_extremely_ clean square wave outputs, but as Don and Mike say, you
have to be careful.

The decoupling cap you mention above sounds like a leaded part, with a
very substantial amount of lead inductance when mounted like that. For
good results, ideally you would have planes for power and ground on
adjacent
layers in the PCB under the chip. That allows you to put the decoupler
at
either end of the chip, and still retain low inductance. Adjacent
planes also
acts as a small but very good at HF capacitor.

Boris makes a good point about the size of the loop between probe tip
and
ground. You can get little socket adapters which you solder to the
board to
minimise this.

Regards
Ian