5 Reducing Random Noise on a Signal
Thursday, 12 July 2018
9:38 AM
If the signal you are applying to the DSO is noisy, you can set up the DSO to reduce the noise on the waveform. There are two methods to reduce noise – bandwidth limiting and averaging.
Bandwidth Limiting
This method applies the incoming signal to a lowpass filter before it is sampled by the DSO. This method works only when the noise has very high frequency content. The bandwidth limiter “cuts off” frequencies above 20 MHz.
Bandwidth limiting will only help if the signal period is less than about 1 MHz.
Connect Channel 2 of the DSO to the Sync Out of the FG (it’s on rear of the unit, use the extra BNC to 4mm adapter).

Change the FG waveform to a sinusoid. Set 50.0 s/div. Reduce the amplitude to a minimum. Press the FG’s 20 dB ATTENUATOR button to apply 20 dB of attenuation (i.e. the output is reduced by a factor of 10).

Change the DSO vertical scale so that the peaks of the sinusoid are visible. It should be a noisy sinusoid.

Press 1. Press the BW Limit softkey. The noise is increased because bandwidth limiting is off, and we are “letting through more noise”.

Turn bandwidth limiting on by pressing the BW Limit softkey again.
Averaging
The second method of reducing noise works when noise is present below the cutoff frequency of the bandwidth limit filter. First, you stabilize the displayed waveform by removing the noise from the trigger path. Second, you reduce the noise on the displayed waveform by averaging the samples.
Averaging can only be used to clean up a signal if the noise is “uncorrelated”
Press Edge and then 1 so that the DSO triggers off Channel 1.

Press Mode/Coupling.

Remove the noise from the trigger path by turning on either Noise Rej or HF Reject (choose the one that results in a stable trigger). Noise Rej adds additional “hysteresis” to the trigger circuitry. HF Reject adds a 50 kHz lowpass filter in the trigger path to remove high frequency components from the trigger waveform.

Press Acquire, then press the Averaging softkey.

Turn the Entry knob      to select the number of averages that best eliminates the noise from the displayed waveform. The higher the number of averages, the slower the displayed waveform responds to waveform changes. Set # Avgs to 64.

Change the FG wave shape to triangle, then square, then back to sinusoid to see the effect of averaging.

Turn off the FG’s 20 dB ATTENUATOR button.