The best method to reduce noise on signals measured with your oscilloscope is to use a low-noise, high-resolution oscilloscope that provides 12 bits resolution at full bandwidth. But any oscilloscope can have its noise reduced using analog hardware or digital software filters provided that the tradeoff of lower bandwidth in exchange for reduced noise is acceptable.
Hardware filters are usually displayed as a 20 MHz or 200 MHz (or similar) bandwidth limit in the channel menu. These filters tend to have very slow rolloffs, so their noise reduction capability is probably less than that of a digital software filter.
Digital software filters may be math functions, may be high-resolution modes, or may be software filter selections in the channel menu (e.g., Teledyne LeCroy’s Enhanced Resolution (ERes) selection). Mathematically, every halving of the sample rate (and bandwidth) reduces noise by 3 dB (~30%, or 0.5 effective bits). Sometimes the digital software filters interpolate sample points after the mathematical filter operation, but the hardware sample rate has still been reduced.
Be wary of high-resolution modes that promise better performance than what is mathematically possible, or that are the only means of achieving high resolution (and lower noise) in what would otherwise be an 8-bit resolution oscilloscope.
Reference Comparing High Resolution Oscilloscope Design Approaches for more details about tradeoffs made to reduce noise in oscilloscopes. Reference webinar Part 6: How Can I Reduce Noise on Signals Measured With an Oscilloscope? in the 2023 Oscilloscope Coffee Break Webinar Series for other details.
What is “effective number of bits” (ENOB) in oscilloscopes?
Oscilloscope ENOB is derived from measurement of the oscilloscope SINAD as follows:
Oscilloscope ENOB= (SINAD-1.76)/6.02
If the front-end amplifier is not the dominant source of noise in the oscilloscope system, the system ENOB will approach the ENOB of the ADC. It is important to understand that the ADC ENOB is an upper bound on the system performance, but the system performance is the critical performance to understand. Realistically, the oscilloscope (system) ENOB will always be less than the ADC ENOB.
If the applied input signal is not 100% of full-scale amplitude, then the ENOB is derived as follows:
Oscilloscope ENOB= (SINAD-1.76+20 log((FullScale Amplitude)/(Input Amplitude)))/6.02
A “rule-of-thumb” of 6 dB SINAD per effective bit can be inferred from this equation. Thus, improvement of half an effective bit equates to 3 dB (30%) reduction in noise, and improvement of a full effective bit equates to a 6 dB (50%) reduction in noise. Small differences in ENOB mean a lot in terms of vertical (voltage amplitude) noise.
Reference Comparing High Resolution Oscilloscope Design Approaches for more details about various types of noise and why the ADC rated number of bits isn’t fully achieved when deployed in digitizers or oscilloscopes.
Reference Part 2: What Are Oscilloscope ADC Effective Bits and ENOB? in the 2024 Oscilloscope Coffee Break Webinar Series for more details.