Periodic oscillations in the output signal

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Discussion Overview

The discussion revolves around the issue of periodic oscillations observed in the output signal of a voltage amplifier when subjected to small square wave input steps. Participants are exploring potential causes and solutions for these oscillations, which appear after the signal reaches a plateau. The scope includes technical explanations and exploratory reasoning regarding amplifier behavior and circuit design.

Discussion Character

  • Technical explanation
  • Exploratory
  • Debate/contested

Main Points Raised

  • Some participants suggest that the observed oscillations may be indicative of ringing in the circuit.
  • Others propose that the oscillations could be related to powerline ripple, particularly given the frequency of around 60 Hz.
  • Questions are raised about the amplifier's specifications, including bandwidth, input impedance, and whether the input is AC or DC coupled.
  • Participants inquire about the configuration of the capacitor and resistor in relation to the amplifier and load.
  • There is a discussion about the decay characteristics of the oscillations, with some seeking clarification on whether the amplitude remains constant or diminishes over time.
  • Concerns are expressed regarding the potential impact of removing the capacitor, which leads to high-frequency noise with spikes.
  • Further questions are posed regarding the voltage gain of the amplifier and the relationship between resistance and oscillation amplitude.

Areas of Agreement / Disagreement

Participants generally agree that the oscillations may be related to ringing or powerline interference, but there is no consensus on the exact cause or solution. Multiple competing views remain regarding the nature of the oscillations and their relationship to circuit parameters.

Contextual Notes

Limitations include the absence of a circuit diagram, which could provide clarity on the configuration, and the need for further information about the amplifier's performance characteristics to fully understand the oscillation behavior.

Who May Find This Useful

Individuals interested in amplifier design, signal processing, and troubleshooting electronic circuits may find this discussion relevant.

prodna
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Dear All,

We have an voltage amplifier. The input is given in small square wave steps to check rise time and noise level of an output signal. The rise time looks good, but we see periodic (noise)oscillations after reaching the plateau. We measure signal voltage across capacitor (10 micro farad) and 1k in series. Can anyone suggest how to get ride of this noise of periodic oscillations in the plateau. Thank you.

prodna
 
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We need some more information in order to help.
1. Is the amplifier home built or a commercial one?
2. What is the bandwidth of the amplifier?
3. What is the input impedance of the amplifier and is the input ac or dc coupled?
4. What is the rise time of the pulses and their duration?
5. How are the series 10 uF cap and 1 K resistor connected to the amp? Are they in series with a load or are they the load (connected between the output and ground)?
6. If they aren't the load, what is the load?
7. About what is the frequency of the oscillations?
8. At the beginning of the flat part of the pulse, does the waveform of the oscillation rise above the pulse first or fall below it first?
9. What is the output impedance of the amplifier?
10. If you have tried any fixes, what were they?
11. A circuit diagram or schematic always helps.

Oh, and welcome to Physics Forums.
 
Just from reading the OP it sounds like ringing.
 
skeptic2 said:
We need some more information in order to help.
1. Is the amplifier home built or a commercial one?
2. What is the bandwidth of the amplifier?
3. What is the input impedance of the amplifier and is the input ac or dc coupled?
4. What is the rise time of the pulses and their duration?
5. How are the series 10 uF cap and 1 K resistor connected to the amp? Are they in series with a load or are they the load (connected between the output and ground)?
6. If they aren't the load, what is the load?
7. About what is the frequency of the oscillations?
8. At the beginning of the flat part of the pulse, does the waveform of the oscillation rise above the pulse first or fall below it first?
9. What is the output impedance of the amplifier?
10. If you have tried any fixes, what were they?
11. A circuit diagram or schematic always helps.

Oh, and welcome to Physics Forums.
Hi here are the answers to the questions, hoping to see reply.

1. Amplifier: Commercial
2. Bandwidth: -3 dB
3. Input impedance: 10 KΩ and DC coupled
4. Time: the time to reach plateau is 200ms
5. 1K resistor is associated with circuit and with load, the load connected to the output socket (BNC). The capacitor is connected at BNC and GND.
6. The output is 75V.
7. The frequency of the oscillation is around 60-65 Hz
8. At the beginning of the flat part of the pulse- the waveform of the oscillation is raise above the pulse first.
9. Output impedance: 150Ω.
10. We are trying to change the value of capacitor and resistor and to see the output.
11. No circuit diagram, since it is commercial one

Thank you
 
Averagesupernova said:
Just from reading the OP it sounds like ringing.
ye it sounds like ringing with some frequency
 
I agree it sounds like ringing however there is the possibility it may be some powerline ripple if your powerline frequency is 60 Hz. Is your input circuit shielded? Do the oscillations decay or is the amplitude constant?

For bandwidth I was interested in frequency bandwidth at the -3 dB points.

Do I understand correctly that your risetime is 200 milliseconds? What happens if you disconnect the 10 uF cap?
 
I too agree that the frequency(60Hz) may come from powerline, but am not very much sure.

Circuit is shielded.

The oscillations decay is constant.

Yes the raise time is 200ms.

Removal of capacitor causes high frequency noise with spikes.
 
I do not understand "The oscillations decay is constant." Does this mean the amplitude is constant or that the amplitude decays (diminishes).

Is the input risetime also 200 ms. I'm wondering if the risetime is bandwidth limited.

How much is the voltage gain of the amp? (What are the input and output amplitudes of the signal?)

What is the amplitude of the oscillation? Is it minor compared to the pulses or does it seem significant?

If you decease the resistance that the amplifier output sees, does that increase or decrease the amplitude of the oscillation?
 
More questions:

Where do you think the high frequency noise and spikes are coming from?

Can you reduce the value of the capacitor an reduce the oscillations without increasing the noise?
 

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