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

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

You might try using the schmitt trigger chip instead of an inverter.
Schmitt triggers are made for the purpose you are using the 7404 for. Can't
remember the 74XX number off hand, but a look through a Digi-Key catalog or
Mouser catalog will reveal it. Good luck with the project...

Scott
N0EDV
Tech by day, University student by night.
Fly by day, fly by night ;)

"Paul Burridge" wrote in message
...

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."

You might try using the schmitt trigger chip instead of an inverter.
Schmitt triggers are made for the purpose you are using the 7404 for. Can't
remember the 74XX number off hand, but a look through a Digi-Key catalog or
Mouser catalog will reveal it. Good luck with the project...

Scott
N0EDV
Tech by day, University student by night.
Fly by day, fly by night ;)

"Paul Burridge" wrote in message
...

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."

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.

Paul-

Others have given good advice. Ringing is due to reactance, whether it is on
the power supply side or the load side of the chip.

Consider that the path between the 74HC04 and the following stage is a kind of
transmission line that has inductance and capacitance. It needs to be
terminated. I'm not sure how or if you can calculate the value of a
termination resistor, but you should be able to do it experimentally.

But, how do you know when you have reached your goal if the measuring
instrument introduces reactance? Perhaps measuring beyond the following stage
will provide sufficient isolation, in addition to using short leads as others
have suggested.

One other thought - would a plain 74C04 work at 8 MHz? If so, its slower speed
might reduce ringing.

73, Fred, K4DII

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.

Paul-

Others have given good advice. Ringing is due to reactance, whether it is on
the power supply side or the load side of the chip.

Consider that the path between the 74HC04 and the following stage is a kind of
transmission line that has inductance and capacitance. It needs to be
terminated. I'm not sure how or if you can calculate the value of a
termination resistor, but you should be able to do it experimentally.

But, how do you know when you have reached your goal if the measuring
instrument introduces reactance? Perhaps measuring beyond the following stage
will provide sufficient isolation, in addition to using short leads as others
have suggested.

One other thought - would a plain 74C04 work at 8 MHz? If so, its slower speed
might reduce ringing.

73, Fred, K4DII

On 18 Oct 2003 14:43:27 GMT, (Fred McKenzie) wrote:

Others have given good advice. Ringing is due to reactance, whether it is on
the power supply side or the load side of the chip

Thanks for that, Fred.

Consider that the path between the 74HC04 and the following stage is a kind of
transmission line that has inductance and capacitance. It needs to be
terminated. I'm not sure how or if you can calculate the value of a
termination resistor, but you should be able to do it experimentally.

I'm not sure what you mean by "terminated" actually. Do you mean it
must have its load connected when the measurement is carried out? If
so, this load which could be replaced by your suggestion of a
termination resistor, would simply constitute the input impedence of
the next stage, would it now? Hold on... are you suggesting that you
replace Zin of the next stage with its equivalent resistance in order
to eliminate any reactance present in that next stage? Is that the
idea? I'm afraid you'll have to indulge me a bit here as electronics
isn't really my field.

But, how do you know when you have reached your goal if the measuring
instrument introduces reactance? Perhaps measuring beyond the following stage
will provide sufficient isolation, in addition to using short leads as others
have suggested.

The short prods inplace of the flying ground clip certainly reduced
the ringing quite considerably. I'd be interested to know why a long
ground lead is a Bad Thing...

One other thought - would a plain 74C04 work at 8 MHz? If so, its slower speed
might reduce ringing.

Speed's quite critical to this application, unfortunately!
--

"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."