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Kai Klaas
11/21/05 06:27
Read: 960 times

#103939 - Anti-aliasing filter will help
Responding to: David Smith's previous message
David said:
Something that caught me out once with FIRs: If you sample at precise 50Hz in order to cancel mains hum, the mains varies +/-1Hz (sometimes worse). You end up with a 'beat' signal of 0-1Hz on the output! This is virtually impossible to get rid of.

Frieder mentioned to use a sample rate which is multiple of 50Hz, means 100Hz for instance.
You are totally right, suppressing of 50Hz hum by the use of 50Hz sample rate is impossible! Assume for instance that you take a sample at the maximum of 50Hz hum sine, then due to a choosen smaple rate of 50Hz, next sample will again be maximum voltage of 50Hz hum sine. And all the other samples too!
Totally different performance when sampling 100Hz though, two succeeding samples will always vanish, because they show same level but of opposite sign. 50Hz is optimally suppressed, provided number of samples taken into consideration with averaging is even.

Unfortunately, hum mostly contains not only 50Hz sine, but also lots of harmonics, especially 100Hz, when a rectification takes place somewhere. Then, sampling at 100Hz, can make the situation even more troublesome. Yes, here choosing a sample rate which is a multiple of 50Hz is not at all a good idea! Then, a sample rate is to be choosen, which's harmonics must not coincide with the with harmonics of hum. So, a sample rate of about 60Hz is indeed a good choice, first coincidation will be at 300Hz.

Because it's mostly the harmonics of 50Hz hum which can make trouble with the averaging methode, the only remedy in most cases is to choose a proper anti-aliasing filter at input of sampler. So, a high order low pass filter (Bessel) with a corner frequency of let me say 10...20Hz in combination with a sample rate of 100Hz would allow a perfect suppression of 50Hz hum and all higher harmonics.


List of 16 messages in thread
Weight Scale Avarging method      Thakkar Bhaskar      11/12/05 02:20      
   Filter providing two time constants      Kai Klaas      11/12/05 07:31      
      median filter, simulate algo on PC      Frieder Ferlemann      11/12/05 07:47      
         Periodic interference can make them fail      Kai Klaas      11/15/05 08:32      
            Acknowlegded.      Frieder Ferlemann      11/16/05 08:24      
               Agreed      Kai Klaas      11/16/05 09:24      
                  50Hz sampling      David Smith      11/17/05 03:54      
                     Anti-aliasing filter will help      Kai Klaas      11/21/05 06:27      
                        what this is all about      Erik Malund      11/21/05 06:37      
   Moving average window.      Graham Cole      11/14/05 01:50      
      Average on condition      Kai Klaas      11/15/05 07:29      
         is that not SOP (standard operating proc      Erik Malund      11/15/05 07:41      
            Hallelujah!      Kai Klaas      11/15/05 07:55      
   meaningless      Erik Malund      11/14/05 06:34      
   What are you using that one for in a wei      Erik Malund      11/14/05 09:14      
   hello from chhatraliamd for weight scale      Chhatralia Mansukhbhai Dh      12/27/05 01:01      

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