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Author Topic: Oscilloscope digital Low-pass filter - It saves the day but causes delay  (Read 508 times)

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Offline Kiriakos GR

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Indeed, Oscilloscope digital Low-pass filter, this saves the day but causes delay.  :)

This period of time I have started a new series of tests, of Low current and ACA measurements at 50~400Hz.
By connecting TA189 probe at my GDS-2102A, I did notice that AC current measurement this was full of noise.

This time I am not using any current coil amplifier, as boost of current measurement.
This time I have to deal with Oscilloscope Noise floor along of AC/DC probe noise floor.

The only way making my current probe to function properly with my oscilloscope, this was by activating it digital Low-pass filter.
Acquisition Setup  -> FILTER
Now I have ACA measuring range since 10mA up to 30A.  8)

But I did sensed, that there is some time lag, when measured current this changes from a known value to another known value.
For example, from 55mA the load changes at 80mA = instant change, but from the side of the oscilloscope, the delivered measurement this becomes accurate after of three seconds of time.   

---------------------------------------------------------------------------------------------------------------
I did searched for relative documentation,  and I did find an quick description from the side of Rohde&Schwarz
Lowpass filters reduce the speed of the signal inside the instrument and cause a delay of the signal on the screen.

Rohde&Schwarz even compiled at table, this delivering ms delay VS selected low-pass filter frequency.
Examples:
1 kHz  /  4.347 ms
2 kHz  / 2.173 ms     
5 kHz  /  869.3 μs
10 kHz / 434.7 μs

What is very interesting, this is the observation, that at multiples of 2 ( 2, 4, 6, 8, 10) the digital filter causes a smaller delay at the oscilloscope electronics.  ;)

My Oscilloscope this is (GW Instek) GDS-2102A, I am unaware of this digital filter, actual caused delay per Low-Pass filter setting.
But electronics theory of operation, this is not much different between oscilloscope vendors.

For more than ten years, I were involved at product reviews, and never thought as feature for be bench-marked, the speed of digital Low-pass filter, between reputable vendors as is Tektronix  VS Rohde & Schwarz  VS GW Instek.     
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Offline Kiriakos GR

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Re: Oscilloscope digital Low-pass filter - It saves the day but causes delay
« Reply #1 on: February 11, 2026, 10:31:28 AM »
At ITTSB there is also a similar article, with application examples.
Sensitivity of the current probe and oscilloscope combination too low

Either way, the combination of theory facts along of actual generated experiences, by using your equipment, this is all the assistance that you may get.




       
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