- Joined
- Jan 22, 2008
- Messages
- 52,394
- Helped
- 14,748
- Reputation
- 29,778
- Reaction score
- 14,091
- Trophy points
- 1,393
- Location
- Bochum, Germany
- Activity points
- 297,975
To start with a simple point, the specification doesn't particularly suggest a cut-off frequency of 40 kHz. You should better specify the maximum signal frequency and acceptable affection.Ok, this is the reason. The signals are generated by a DAC at 100kHz S/s, and they are synthesized, not sampled. Therefore there is no energy over Nyquist.
However I see after the filter the signal mirrored, i.e. if I output 23kHz I see energy at the mirrored frequency 100-23kHz. I want to eliminate this.
The filter that I described looks insufficient. I am trying to make it sharper by adding more stages after. I have to make sure I do not change the phase till 25-30kHz. Since signals are generated at 100kHz the best frequency for the cutoff is 40kHz.
Good point. I fully agree.A point I forgot to mention. A probably much simpler way to implement an almost ideal digital-analog conversion with 100 kHz sampling rate is to use an oversampling audio DAC. It has a perfect built-in digital linear phase interpolation filter and only requires a simple RC filter for the oversampling rate (e.g. x64).
Hi,
Good point. I fully agree.
Additionally they are very cheap and very low distortion.
Klaus
Hi,
Just go to a dustributor and do a search for "audio dac".
Like this: https://uk.farnell.com/w/c/semicond...ers-dac/prl/results?st=Audio dac&sort=P_PRICE
Then go through the results and check the datasheets.
One if the cheapest is: PCM5100APWR
Even at 8x oversampling it has an impressive reconstruction filter.
Klaus
Is "an oversampling audio DAC" the same as a switched capacitor lowpass filter IC like a Maxim MAX295 Butterworth 8th-order? Years ago I used a National Semi switched capacitor IC to make an extremely low distortion audio frequency generator with it and the original signal was a digitally stepped sinewave.
The MAX295 switched capacitor lowpass filter IC has an internal clock, or one external clock can drive hundreds of them because it is low power Cmos. It is filtered by using oversampling.
So - you are allowed to modify the existing filter with respect to cutoff and pole-Q ?
In that case - where is the problem? Modify the existing stage according to 4-pole parameters and add a second stage.
Sorry - but in post #31 I can read:
Still looking for a tool that allows designing a filter and modifying first stage
So - you have caused the misunderstanding.
According to your link, and also the one I have used originally:
http://sim.okawa-denshi.jp/en/OPseikiLowkeisan.htm
the filter I posted is correct, in fact the 39k, 39k, 100p, 100p give a cutoff at 40kHz.
However, I simulated this filter with LTSPICE and it seems that the cutoff is at 27kHz, while at 40kHz I have exactly -6dB.
I do not understand why.
Then I used this calculator
http://www.calculatoredge.com/electronics/sk low pass.htm
which, with 39k and 39k, provides 144p and 72p as capacitor values. With these LTSPICE is happy and I have -3dB at 40kHz.
Very useful clarification, congratulations!
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?