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

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

On 19 Oct 2003 03:44:51 GMT, (Fred McKenzie) wrote:

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.)

Thanks, Fred. I'll make a mental note of that lot.
--

"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 19 Oct 2003 03:44:51 GMT, (Fred McKenzie) wrote:

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.)

Thanks, Fred. I'll make a mental note of that lot.
--

"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."

An oscilloscope probe has capacitance between the probe tip and ground
connection. Any inductance in series with this path makes a resoant
circuit that rings when hit by a fast transition. I highly recommend
reading Tek's "ABCs of Probes", which you can get at

http://www.tek.com/Measurement/App_N...60W_6053_7.pdf

It mentions the phenomenon on p. 10, but it would be very worthwhile to
read the whole thing.

Roy Lewallen, W7EL

Paul Burridge wrote:
. . .
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...
. . .

An oscilloscope probe has capacitance between the probe tip and ground
connection. Any inductance in series with this path makes a resoant
circuit that rings when hit by a fast transition. I highly recommend
reading Tek's "ABCs of Probes", which you can get at

http://www.tek.com/Measurement/App_N...60W_6053_7.pdf

It mentions the phenomenon on p. 10, but it would be very worthwhile to
read the whole thing.

Roy Lewallen, W7EL

Paul Burridge wrote:
. . .
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...
. . .

On Mon, 20 Oct 2003 02:27:18 -0700, Roy Lewallen
wrote:

An oscilloscope probe has capacitance between the probe tip and ground
connection. Any inductance in series with this path makes a resoant
circuit that rings when hit by a fast transition. I highly recommend
reading Tek's "ABCs of Probes", which you can get at

http://www.tek.com/Measurement/App_N...60W_6053_7.pdf

It mentions the phenomenon on p. 10, but it would be very worthwhile to
read the whole thing.

Many thanks, Roy. I will. I really need to get better acquainted with
these finer points.
--

"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 Mon, 20 Oct 2003 02:27:18 -0700, Roy Lewallen
wrote:

An oscilloscope probe has capacitance between the probe tip and ground
connection. Any inductance in series with this path makes a resoant
circuit that rings when hit by a fast transition. I highly recommend
reading Tek's "ABCs of Probes", which you can get at

http://www.tek.com/Measurement/App_N...60W_6053_7.pdf

It mentions the phenomenon on p. 10, but it would be very worthwhile to
read the whole thing.

Many thanks, Roy. I will. I really need to get better acquainted with
these finer points.
--

"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."