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Powergrid noise with 25Hz component?

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Karel:
Hi,

I received an ECG recording where all leads show strong (powergrid?)noise.
I have seen this kind of noise before but the base frequency componnet was always 50 Hz.
This recording however, has a strong 25 Hz component and I can't understand where this is coming from.
The recording was made on a remote location and I wasn't present so I don't have much info.
The ADC used is a delta-sigma type with a modulator frequency of 512 Khz and an output data rate of 1 KHz.
The system had a multiple-notch filter active with notches on 50 Hz, 100 Hz, 150 Hz, etc.

Any ideas where this 25 Hz came from?


NiHaoMike:
Might there be some kind of load that's turning on every other cycle? Most likely it would be a solid state controlled heater of some sort.

RoGeorge:
ECGs usually have adaptive filters to remove the 50Hz, something like this



so most probably you see "strong" 25Hz only because the adaptive filter was very good at removing the 50Hz, or maibe those 25Hz are in fact the adaptive filters' artifacts, IDK.

TimFox:

--- Quote from: NiHaoMike on October 28, 2021, 01:26:04 pm ---Might there be some kind of load that's turning on every other cycle? Most likely it would be a solid state controlled heater of some sort.

--- End quote ---

I have seen interference at subharmonics of the power line:  in my case, a 10 Hz frequency with a 60 Hz line due to a soldering station with a zero-crossing switch in its servo connected to the same outlet strip as the device under test.  Resting in its holder, the thermal servo apparently cycled synchronously due to the zero-crossing switch.

Vovk_Z:

--- Quote from: RoGeorge on October 28, 2021, 01:49:32 pm --- or maibe those 25Hz are in fact the adaptive filters' artifacts, IDK.

--- End quote ---
Yes, it depends on how large is a relative level of those 25 Hz components on a TS spectrum. He shoved us absolute levels (uVolts) so it may be anything.

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