Paul Burridge wrote:

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?

There are socket with integrated caps:

http://www.mill-max.com/images/products/pdf/021.pdf

--
Frank Buß,
http://www.frank-buss.de, http://www.it4-systems.de

Paul Burridge wrote:

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?

There are socket with integrated caps:

http://www.mill-max.com/images/products/pdf/021.pdf

--
Frank Buß,
http://www.frank-buss.de, http://www.it4-systems.de

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."

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

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

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."

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

Paul-

The next stage has some kind of input impedance, whether it is a logic gate or
an analog amplifier. I'm saying that you may be able to adjust that impedance
for minimum ringing.

If it is a logic gate, can you add a pull-up resistor across the input? If it
is an amplifier, can you add a resistor across the input without upsetting the
bias? Trying several values, you may be able to make an improvement.

Why is a long ground lead a bad thing? Because wire has inductance. Above
some frequency it can no longer be considered a short circuit. The longer the
wire, the lower the frequency. Your ringing contains frequencies that are
multiples of 8 MHz.

In addition to keeping scope probe wires short, have you compensated the probe?
Many scopes have a square wave calibration source available for checking
calibration. Another thing you can use it for, is to adjust the probe for
minimum overshoot and rounding of the waveform. (The adjustment allows you to
make the scope's probe-to-input capacitance ratio the same as its resistance
ratio.)

Fred