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Ringing on square wave: how to eliminate?
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/ |
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 11:28:06 +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 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:
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 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 |
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 |
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"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 |
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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 |
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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." |
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