Hi BradAre you referring to an LC low pass filter?
Choose L so it passes almost entire current at cutoff frequency to your load. Its choke action starts to take effect at a frequency 1.5 to 2 times that of cutoff frequency.
Choose C to create combined LC resonant frequency 1.5 to 2 times your cutoff frequency. At the same time C shall correct for power factor error at the cutoff frequency.
The L:C ratio is high for small current, low for large current.
Yes, that´s the point. This makes the filter meaningless.( i realize while I write this comment that this second filtering might not be needed because I focus on the first harmonic in Bin1 who cares what happens in the other bins.)
As already said: the FFT output has a bandwidth of (about?) the FFT_frequency_step size. This means the "40Hz" tap will show frequencies from 20Hz ... 60Hz. But it has "fading" character.Sequentially filter these sample results in a digital filter that I haven't got yet and do a FFT to see if I have a peak value at my bin1 (40Hz). If so the line frequency is that value.
Sadly not perfect.My Aliasing filter has a second order fc of 1000hz and my minimum fs is 2564. That is more then 2xfc=2x1000Hz and Nyquist told me that this should work .
Hi,
there´s a problem.
a digital filter doesn´t work like anlog filters. Analog filters have a fixed cutoff frequency dependin on the part values.
But a digital filter depends on it´s parameters ... and it always depends on sampling frequency.
This may cause problems here, because you change the sampling frequency.
T depend on his also has influence on the tap frequencies of the FFT. They allsampling frequency.
As already said: I see no benefit in the digital filter before the FFT. What´s your idea behind it?
later in txt you write:
Yes, that´s the point. This makes the filter meaningless.
As already said: the FFT output has a bandwidth of (about?) the FFT_frequency_step size. This means the "40Hz" tap will show frequencies from 20Hz ... 60Hz. But it has "fading" character.
If there is a 45Hz signal, then the 40Hz tap will show the highest peak, but the 80Hz tap will also show some signal. At 60Hz bout taps (40Hz and 80Hz) will show about the same amplitude.
Sadly not perfect.
Let´s say you have a 2nd order 1000Hz butterworth LPF as anti aliasing filter.. and fs = 2564Hz.
Then the nyquist limit is fs/2 =1282Hz.
At this frequency the attenuation of your anti-aliasing filter is about 6dB. The signal is still there. But just with half of the amplitide.
Let´s say there is an input of 2000Hz with 1V RMS amplitude. The LPF will attenuate it to about -11dB this is about 280mV RMS. And this surely will cause an alias frequency.
2000Hz is mirrored at 1282Hz .. resulting in 564Hz with 280mV RMS.
I hope I don´t discourage you with that news... Go on with your idea.
Klaus
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?