RadioBanter - View Single Post - Ringing on square wave: how to eliminate?

October 18th 03, 04:44 PM

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!
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