Noise & Interference

Created: April 2020Last Upated: 9/16/2020

Introduction

Electronics noise can impact operation stability and performance of any electrical product. The distinction between interference and noise is that interference is artificial noise (radio frequency jammer) while noise can be natural (thermal noise) or man made.

One needs to understand the basics of noise source, its impact, and correlation with external factors in order to guideline one to debug and find the root cause of an issue.

We will go over commonly seen noise and interference issue hardware system design and testing.

Common Electronic Noise

Analog noise

Thermal noise: this real world noise increases linearly with temperature also refer as 4KB /Johnson–Nyquist noise.

    • This noise is critical in radio receiver and degrade signal quality by raising noise floor.

Flicker noise/1/f/pink noise: this noise's power density is dominant in low frequency and is inverse proportional to frequency.

    • Pink noise is caused by fluctuation process inherent to electronic materials used in resistor, transistors, etc.

    • The unique property of pink noise energy is that its energy is the same for each octave frequency band (i.e 125 to 250Hz has the same noise energy as that of 8KHz to 16KHz band)

Shot noise

    • This is cause by random movement of charge carrier during current conduction. It's a phenomenon of quantum physics. This noise become apparent in small current and low light setting; for instance, sensitive sensors such as camera imager sensor under low light has randomly distributed white dots seen on the final image.

      • Shot noise exhibit flat power spectrum density

      • Shot noise is independent of temperature

      • Shot noise is characterized by shot noise current and translates to shot noise voltage.

Digital Noise

Quantization noise = this is the digital noise/error that is found during analog to digital conversion (ADC)

    • This noise limits the precision level of the sampled data hence reducing signal to noise ratio.

Interference

Common Mode noise

  • This noise is the common electronic radiation noise. This noise creates an equal offset voltage both signal transmission path and its return path. The effect of offset voltage (mostly ac) on both path is that signal and return current does not cancel and now must return to another another ground via parasitic capacitance forming a larger current loop. This alternate current return loop gives unwanted noise radiation. Remember large current loop is essentially an effective antenna.

    • E.g, for a signal ended transmission, both signal path and ground return path has the same induced offset voltage.

    • E.g, for a differential transmission, both positive signal and negative signal has the same induced offset voltage.

  • it can originate from natural noises such as light flickers and coupled on the trace.

  • it can be generated due to phase misalignment of differential signal transmission

  • it can be generated due to ground bonce on both signal transmission and return.

  • It can be generated through ac main coupling (60/50 Hz) from wall outlet to the operating electronics.

Differential Mode Noise

This is the common noise seen produced by power supplies. Voltage ripples are a type of differential mode noises.

Differential means that the noise current has equal and opposite polarity on transmit path and return path. (i.e current goes from transmit path and return from the ground path).

Intersymbol Interference (ISI)

ISI is a signal distortion seen on the digital communication receivers. The interference is cause by overlapping previous digital symbol pulse with the new digital symbol pulse. This overlap is due to channel parasitic capacitance that holds residual waveform from previous pulse that extends over to the next symbol's timeslot.


Noise Mitigation

    • Thermal Noise

      • Use thermal pad and other solutions to limit the maximum IC package temperature.

      • Use smaller resistance parts.

      • Use wider copper place to limit temperature rise of the conductor on PCB.

    • Quantization noise

      • Oversampling the analog signal spreads the noise power ( a fixed value based ADC minimum resolution voltage) to a wider spectrum resulting in lowering the noise floor and achieving higher signal to noise ratio.

      • Ditering/noise shaping is a process of adding random analog noise to the signal before digitization/quantization that reduces noise floor due to quantization noise.

        • It should be used when signal of interest is:

          • periodic

          • low amplitude (sightly over minimum ADC resolution)

          • low frequency and slow moving signals.

Summary and Conclusion

  • Three dominant noise, thermal, pink, and quantization noise in electronics.

  • Reduce operation temperature of the product reduce dominant analog noise

  • Quantization noise limit resolution of analog to digital conversion.

Both analog and digital noises affect electronic system performance. As a hardware system engineer, understanding the source and impact of noise on the electronic system helps one design in the correct noise mitigation solution.

Further Reading: