Hi

I never designed such complex filtering stuff - new for me. I'm just toying with DDS signal generation. I tried to study some materials about what type of filter to choose - and I choosed Bessel, because of the phase linearity (I want to try also arb waveforms, not only pure sine).

I learned, that it is good to have the sampling frequency tripple the max output frequency. So the output filter's passband shoud be f_{s}/3. But I can't decide, what steepnes (order) of the filter I shoud use. I found some recommendation that spurious component rejection ratio should be 70dBc for a good wavegen. I don't understand exactly what this requirement says (it is the ratio of fundamental harmonic to the level of the whole output signal?)

I also found, that 7th order bessel filter of f_{c} = f_{s}/4 shoud do it enough. Is it true?

So i tried to learn how to design sucha passive network filter. I've learned how to work with frequency normalized filter design tables, such as this one. I calculated some values for a filter, just a try, and they have all fit almost exactly what some software told me it shoud be. (I've used AADE filter design tool - old and rusty piece of software, which tried to install some rubbish into my computer).

I have 20Msps multiplying 12bit DAC with current ouput. For the purpose of testing and development only, I am going to use TS613 (common ADSL driver fast high output current OPAmp, 80MHz BW ) opamp both for the I/V and for the output. I am not sure if this is good idea, but I've got a lot of these salvaged from scrap, so I'll give it a try. This design is only for testing and education only, separate final version will be made after.

There is also a second big unknown and that is what impedance should that filter be designed at? After the I/V stage I will have very low output impedance, so designing the filter at down to 50ohms should be possible, but I'd rather wouldn't do that, so is it ok to go higher, lets say 100 to 300 ohms, to lower the load on the opamp as much as possible, to reduce power consumption and maybe also reduce unnecessary distortion?

First step I think will be I make the filter circuit alone and have a measure of its response, to verify the design. After having some concrete values to work with.

How much critical are the component values? The only readily available for me is the E12 value line, both at capacitance and inductance. So how much accurate should the parts be? Am I right that with increasing filter order, I should keep the values accurate as much as possible? So I should for example 1.438nF make from two capacitors, like 1.2n + 220p (= 1.42nF). Or even more accurate?

Is it good enough to use 0805 surface mount inductors? Top end characteristics of this educational design are not required. Caps could be combined in parralel to easily get desired values, but the inductors are not as much accurate, I think about 10%? Is that enough? I think there not many ways making inductors more accurate then this.

Summary:

For sine and arb generating DDS, 7th order Bessel low pass at f_{c (-3dB)} = f_{s} / 4 should be enough. Or shouldn't? Input and output impedances in the range from 100 to 300 ohms.

And I observed some interesting fact (maybe not much interesting for more experienced designers) with an odd order lowpass filter, beginning and ending with a capacitor, having the same Z_{src} and Z_{load} - so having an "symetrical looking" filter, why are the values of components not symmetrical? I used some filter calculating tools and some of them actually made those LC networks symmetrical from both ends. But there was none note about what approximation the filter is. So the question is, does the filter component values symetry happens only with filter of Q=1? (which is not Bessel obviously) And then is the bessel passive filter network symetrical, if the component values are not? I think then it is also not.

Thank you for your kind help,

And I want to apologize for my behaviour in my last thread.

Y