Ripple on Logic pulse trains , what is causing this?

[danny davis]

I'm getting rippling on my logic HIGH , look at the ripple on the logic pulse trains

Is this normal or what can cause this type of problem

Artifacts between Pulses , they are a lighter green inbetween the LOW LOGIC State, what is this called?

You can see the Artifacts discharging in the 3rd and 4th pictures, what is this called and are these artifacts?






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Ripple On Logic HIGH , What causes this, and can some circuit have this as being normal?




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The LOGIC high pulse has bending only on the LOGIC HIGH, what causes this bending? it happens every other pulse in a pattern

 

[danny davis]

DC logic HIGH is not a straight level voltage, it has regular shark fine pulses , sometype of ripple riding ontop
What can cause this? what is the name is this problem?

 

[Audioguru]

Look up the "noise margin" on the logic ICs and you will see that it is at least 10 times higher than the tiny amount of ripple. No problem.
The ripple is because the power supply and its wiring is not absolutely perfect but its voltage drops a little when it is stressed by the load.

 

[danny davis]

The ripple is because the power supply and its wiring is not absolutely perfect but its voltage drops a little when it is stressed by the load.

It's not coming from the power supply, i put my O-scope probe on the output of the power supply and I don't see any ripply or ripple waveforms coming in on the VCC or VEE to the IC logic chips or Op amps.

So it's not coming from the power supply, where else can it come from?

So Logic have ripple , how is this OK?

How much Noise can Logic have before it becomes a problem?

What is the first pictures, is this artifacts or what?

 

[Audioguru]

The wiring from the power supply to the ICs has inductance. The inductance allows the supply voltage at each IC to jump up and down a little unless each IC has its own 0.1uF ceramic capacitor very close to the supply and ground pins of the IC. Logic also causes the logic high voltage to have some ripple which is normal.

Look on the datasheet for the IC you see ripple on (you did not say if it is TTL or Cmos) to see that its noise margin is at least 10 times more than the ripple you have so you have nothing to worry about.

In the first pictures the shaded green is caused by having very high frequency pulses (shaded) inside lower frequency pulses (solid outline). The focus of your 'scope is poor so you cannot see the high frequency.

 

[danny davis]

In the first pictures the shaded green is caused by having very high frequency pulses (shaded) inside lower frequency pulses (solid outline).

What is this type of logic pulse train called when you have high frequency pulses shaded inside lower frequency pulses solid?

What kind of logic circuits do this please?

Look on the datasheet for the IC you see ripple on (you did not say if it is TTL or Cmos) to see that its noise margin is at least 10 times more than the ripple you have so you have nothing to worry about.

Are TTL or CMOS logic more sensitive to noise and ripple?

What I mean is , having noise and ripple on the Logic pulses can cause what kind of problems or errors?

The inductance allows the supply voltage at each IC to jump up and down a little unless each IC has its own 0.1uF ceramic capacitor very close to the supply and ground pins of the IC. Logic also causes the logic high voltage to have some ripple which is normal.

So without having those 0.1uF caps from VCC to ground , Caps in parallel on the power supply rails, WITHOUT them this is what causes the ripple and noise?

 

[pratomer]

There may be a problem of frequency, many times when the frequency is too low then there can be some ripples seen as there is always some charging and discharging of capacitance there in the circuit

 

[danny davis]

There may be a problem of frequency, many times when the frequency is too low then there can be some ripples seen as there is always some charging and discharging of capacitance there in the circuit

The Frequency is 1.8 HZ circuit board product model#1 , 1.8hz = 547 milliseconds pulse period

The Frequency is 1.3 HZ circuit board product model#2 , 1.3hz = 750 milliseconds pulse period

Why does ripple occur when the LOGIC frequency is low?

ripples seen as there is always some charging and discharging of capacitance there in the circuit

Is the circuits capacitance the VCC power supply traces?

 

[Audioguru]

There may be a problem of frequency, many times when the frequency is too low then there can be some ripples seen as there is always some charging and discharging of capacitance there in the circuit

It is a digital logic circuit and it probably does not have any capacitors to charge and discharge.
The ripple amplitude is tiny and does not affect the digital circuit.

 

[danny davis]

The ripple amplitude is tiny and does not affect the digital circuit.

Can ripples cause the LOGIC inputs to premature turn on a logic gate, reset pin or enable pin?

When can Ripple in digital logic circuit becomes a problem? or what kind of problems can it cause?

Is it True that a low frequency Logic pulse train is normal to have ripple? and why is there ripple on low frequency logic pulses?

 

[Audioguru]

Again, the ripple amplitude is tiny and does not affect the digital circuit. It looks like the ripple causes the +5V output to drop to +4.9V.
1) Is the supply voltage +5V?
2) Are the logic ICs CD4xxx Cmos?
3) Do the datasheets for CD4xxx Cmos logic say that the minimum logic high input is +3.5V?
4) Does the ripple cause the digital output to drop to less than +3.5V?

 

[danny davis]

1) Is the supply voltage +5V?

25 volts

2) Are the logic ICs CD4xxx Cmos?

It's Op amps and transistors as switches , not logic TTL or CMOS

A PNP transistor as a switch works in reverse, the base needs a HIGH to output a LOW

I had a problem today with a PNP transistor as a switch, the PNP transistor tested good in diode mode but the Beta was bad

It's output voltage was 0.6 volts on the collector, a GOOD PNP transistor would output +12 volts , the emitter had the VCC +12 on it, but the bad PNP would output 0.6 volts on the collector but the GOOD PNP transistor would output +12 volts

In diode checker mode, it tested good but the Beta was bad I'm guessing? the output voltage was 0.6 volts, what is wrong with the PNP transistor?

3) Do the datasheets for CD4xxx Cmos logic say that the minimum logic high input is +3.5V?
4) Does the ripple cause the digital output to drop to less than +3.5V?

Why would ripple cause the digital output to drop? the ripple doesn't cause the digital output to drop, it just RIDES ontop , its superimposed

Ripple is not a dropping resistor

 

[Audioguru]

I know the voltage spec's for TTL and Cmos logic which is what I thought you have. But I do not know the voltage spec's for the mix-mash of opamps and transistors that are doing their own logic in your circuit.

You had a bad PNP transistor with low beta. When it was turned on its collector did not go as high as +12V, instead it went only to only +0.6V. What is wrong?? What is beta? What does a transistor do or not do when its beta is low? (look it up).

Your supply is +25V. The ripple shown on your scope photos show it dropping to +24.5V and rising to +25V. But we do not know the threshold voltages of this logic circuit.

 

[danny davis]

You had a bad PNP transistor with low beta. When it was turned on its collector did not go as high as +12V, instead it went only to only +0.6V. What is wrong?? What is beta? What does a transistor do or not do when its beta is low? (look it up).

Yea i know the beta is low, but there is no shorts or open junctions, how does a transistors beta go low like this? it outputs 0.6 volts and not +12 volts

Have you come across something like this? what is this called when a transistor is like this?

Your supply is +25V. The ripple shown on your scope photos show it dropping to +24.5V and rising to +25V. But we do not know the threshold voltages of this logic circuit.

How do you need to know the threshold voltages?

It's not logic circuits, it's DC differential op amps comparators my manager said

 

[Audioguru]

The maximum reverse voltage for a transistor's emitter-base junction is about 5V because more reverse voltage will cause avalanche breakdown which overheats the junction (usually this junction has zero bias or forward bias with very low heating) which reduces its beta. Maybe a soldering iron that is too hot or is not used properly will reduce beta.

I never design a transistor circuit where an emitter-base junction has avalanche breakdown and my soldering iron is temperature-controlled so I have never seen a transistor with low beta.

Your 'scope photos showed digital logic signals, not analog opamp and transistor signals. If you do not know the threshold voltages then you are just wildly guessing.

 

[danny davis]

Your 'scope photos showed digital logic signals, not analog opamp and transistor signals. If you do not know the threshold voltages then you are just wildly guessing.

They aren't logic signals, they are op amps as comparators which is DC differential op amps, and transistors as switches

Why do you need to know the threshold voltages, i don't get it, tell me why? guessing of what?

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usually this junction has zero bias or forward bias with very low heating) which reduces its beta.

What else can Reduce the beta of a transistor? different temperature environments? aging? getting old? stressing it out how so?

 

[FvM]

Where's the point in discussing waveforms of an unknown circuit with unsaid purpose? Unless there are any indications of circuit malfunction, I would expect it's just normal operation.