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What causes notches in a sinewave and how to make them?

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walters

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Notches in a sinewave

What causes Notches in a sinewave?

With No tone controls i run a sinewave function generator through a circuit, amplifier or stages and i get notches above or below the zero crossing points on the oscilloscope

What causes these notches and how can i make more of them or less of them?
 

Re: Notches in a sinewave

This should never happen - if both the generator and amplifier are operating correct.
 

Re: Notches in a sinewave

Its not in the tone circuit cause i bypass it

The sinewaveform has notches in the positive and negative cycles of the sine waveform

But how do u get these notches?
 

Re: Notches in a sinewave

do you mean crossover distortion?
 

Re: Notches in a sinewave

no cause the crossover distortion is at the zero crossing points


The notches are above or below the zero crossing points


The positive cycle leading part with have a notch and the falling part will have a notch at a different degree

How to put notches on a degree of a cycle?

example : sine waveform is 360 clean

Notch#1 is at 40 degrees its look like a bump
Notch#1 is at 70 degrees it looks like a bump

But how do u get theses bumps and how do u put them on a degree of the cycle too?
 

Re: Notches in a sinewave

Hi,
If you are getting notches at similar points on all sections of each sine wave, it may that your function generator synthesizes the sign wave and to save the number of samples, it may be using straight line segment approximation at zero crossings. If you expand the waveform, you may find similar discontinuities at other points of the waveform also.
If on the other hand you observe only such humps when you connect to your crcuit, it may be that you have some switching loads loading the input at these angles.
Regards,
Laktronics
 

Re: Notches in a sinewave

no its not coming from the function generator at all

Its the stages/circuit creating these notches

But what causes these notches ?
 

Re: Notches in a sinewave

I understand, that these observations have been made on an audio amplifier within normal operating parameters, e. g. resonable signal frequency, no overload, output load as required (could be meaningful with valve amplifier). If this is true, something must be defective, either generator, amplifier or oscilloscope, cause you can't have such artefacts on a sinewave without producing terrible, hearable distortions.

But my assumption could be wrong: There may be input overload, inappropriate load impedance etc. If this isn't the case, the fault location should be searched for.
 

Re: Notches in a sinewave

Why would a output load give notches ?

Why would input overload give notches ?
 

Re: Notches in a sinewave

Hi,
Are you sure your function generator is not of a synthesizer type, if it is so, the notches that you see may be due to ringing introduced by your circuit at the step changes of the wave form, which will be predominant at higher slopes of the waveform. If you see the input waveform expanded on time scale on the scope, you can observe step changes at these points.
However, if it is not so, do you have discreet switching loads at these angles, which can introduce notches due to ringing introduced at sudden load changes.
A third possibility is usage of switching power supply and the notches can be due to spikes picked up at the edges of the switching square wave.
Regards,
Laktronics
 

Notches in a sinewave

I assume the circuit is an audio power amplifier, yes?

What happens to the shape of the notches when you vary the input frequency?
How about when you vary the input amplitude?
Can you show us a snapshot or drawing of the notches?

You haven't shown us the circuit, so I can only make some wild guesses. It could be crossover distortion or slew rate limiting or even instability (very high frequency oscillation), combined with some phase shift that moves the distortion to a different part of the sinewave.
 

Re: Notches in a sinewave

Hi,
You can also monitor the amplifier points without any input, in AC mode, increase the gain setting of the oscilloscope, you may find glitches at regular intervals on the horizontal zero line. You may find matching square waves in your switching supplies or elsewhere in the circuit. These may get filtered off and may not quite appear at the audio output. If it does affect, you have to shield the high gain stages and use bypass capacitors very near to amplifier ICs.
Regards,
Laktronics
 

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