Electronic noise filtering strategy in hemodynamic research

NOISE FILTERING IN RESEARCH
FINDING FUNDAMENTAL FREQUENCY AND
FILTERING THE NOISE USING SOFTWARE
By Filip Konecny

VARIETY OF NOISE CAN BE INJECTED DURING IN ANIMAL PVL
RECORDING
On the image above, injected noise interfering with recorded data, channels 2,3 and 4 have this unwanted signal
disturbance.
Channel 2 is used as an example (grey selection) of how to find source of noise and how to use given software to
mitigate it.
Ch1
Ch2
Ch3
Ch4

BEFORE YOU DECIDE TO FILTER OF AN UNWANTED
ELECTRICAL (INTERFERING) NOISE
Different types of noise are generated by different devices and (or) different processes in electronics; it is an
unwanted disturbance of an electrical signal
BASICS: Lab equipment use an electrical energy to operate, however, part of that energy "escapes" in an uncontrolled way from the
lab equipment. When the energy reaches other electronic equipment through one of its "escape routes“, it might create deficiencies
in operation of other equipment; researcher might be facing an electromagnetic interference problem.
Unwanted noise usually comes from internal or external sources
Most manufacturers mitigate internal parasitic noise when designing their equipment. Electrical interference is generated within
a design or circuit * (3 basic internal noises are generated by e.g., used transistor as Transit noise, conductor as Trigger noise
or by rapid, random movement of electrons in a conductor Thermal noise).
External noise source comes mostly from by man made systems in the animal lab usually comes from light bulbs (also consider
microscope light bulb in case of microsurgeries), fluorescent lights, heating equipment, ECG leads, rectal probes, surgical
tables, electrical door openers, network cables, ungrounded power bars, computers, variety of pumps, microcontrollers etc.

BEFORE YOU DECIDE TO FILTER OF AN UNWANTED
ELECTRICAL (INTERFERING) NOISE
When signal interference occurs in the animal lab, before you start to filter the interfering noise, please try to
switch off the non-essential equipment, while observing recorded channels. You can also remove ECG
electrodes, one by one and if possible, make notes whether noise is receding/ partially receding or completely
disappearing during the process.
Later, you can try to temporary unplug the recording equipment from the common power bar and plug it into a
wall outlet e.g., you can also try to shield wires, limit grounding loops by not attaching other equipment to the
same power ground or build a small Faraday cage (a grounded metal screen surrounding a piece of
equipment), this might be an extreme measure, if the noise can not be mitigated.

BACKGROUND KNOWLEDGE ON NOISE, NOISE REDUCTION
TECHNIQUES
To secure data quality, it is up to the end user and research scientists to first find and limit the parasitic noise injected into data, before
relying on filtering functions. Slides that are presented might give basic guidance of how to find noise frequency in PVL data (on the slides
below, data processing is done using software).
Please note: During filtering of raw data signal, some information carried in the waveform(s) might be lost, best is to
physically limit the incoming noise by methods listed/mentioned earlier. Smoothing of the waveform might be considered
before filtering the signal.
Q) How to reduce this unwanted electric signal disturbance after all elimination techniques has not been successful?
A) Most cases in PVL research by creating bandpass filter or band-rejection filters to eliminate a specific noise frequency.
Q) How we fit band pass rejection filter to noise and its frequency?
A) By running the output through a bandpass filter at certain frequency to amplify the desired signal without amplifying the noise. The noise
will be lost at the output of the filter.
Noise power is measured in watts or decibels (dB) relative to a standard power

IN THE ANIMAL LAB
Noise needs to be physically isolated, this should be the first step of any troubleshooting
Please do not yet troubleshoot using data filtering or channel smoothing before physically
isolating the interfering noise
Grounding specific input/output associated with noise comes from it/them.
Moving data acquisition unit or control PVL unit onto a non-metal surface
Next steps would show how to find the noise frequency

STEPS TO SELECT LOW BAND PASS FILTER BASED ON THE
NOISE FREQUENCY
1. Go to window>spectrum and it will display in dB while selected in red in Hz
Use Spectrum to determine the
frequency components of noise.
This information can sometimes help
discover the source of the noise.

NOISE FREQUENCY
2. Select channel 2

NOISE FREQUENCY
3. Select data from channel 2
which will be displayed as dark red area

NOISE FREQUENCY
4. Fundamental frequency and the rest of frequencies, explore with cursor

STEPS TO SELECT LOW BAND PASS FILTER BASED
ON THE NOISE FREQUENCY
5. Fundamental frequency in this animal was (1.953 Hz) that is 1.953*60sec =117 bpm

6. Using cursor select the area of noise (10.742Hz)

7. Fundamental frequency in this swine was (1.953 Hz) that is 1.953*60sec that was +/- 117 bpm as 1.953
Hz will have to be zoomed at to be selected even more precisely

CREATE CHANNEL 5 >SETUP>CHANNEL SETTING

CREATE CHANNEL 5 AND FROM THAT CHANNEL; THEN
BOTTOM ARROW SELECTS DIGITAL FILTER

DIGITAL FILTER SOURCE CHANNEL 2, LOW PASS
AND 10HZ (IF UNSURE WHY, PLEASE SEE STEP 6) AND HIT OK

CHANNEL 5 LOW BAND PASS 10HZ FILTERED
AND ON XY SELECT 1 VS. 5

YOU CAN ALSO PRE-SET THIS CHANNEL IF YOU EXPECT TO HAVE CERTAIN,
KNOWN TO YOU, NOISE BY E.G. SETTING CHANNEL 5 WITH 10HZ LOW BAND
PASS FILTER

Electronic noise filtering strategy in hemodynamic research

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