rfmagic wrote on Dec 30^th, 2010, 1:19pm:


At first, I wouln't call S+K and MFB a "Biquad" since both cannot create a biquadratic transfer function - but that's not too important.
In particular:
* S+K is rather sensitive to parts tolerances, but less sensitive to opamps GBW.
* Just the opposite is true for MFB
* The TT-biquad also is not suited for rather high frequencies (because of sensitivity to the GBW). More than that, it uses more than one opamp (as both other alternatives, S+K, MFB).
*One additional hint: The cascade approach (several stages in series) is not the best from the sensitivity point of view. What about active realization of a passive ladder structure (FDNR technique)?

Thanks for your reply...

Buddypoor,

is it possible to design a filter with such a minimal sensitivity to parts and GBW so that it will eliminate the need for calibration?. the tolerance for CMOS 0.18u technology is pretty large (+/- 15%).

I saw some designs that actually use TT stages in cascade  for WLAN zero IF frequencies (<20MHz BW) can you think of a possible advantage of the TT over other configurations even though it uses 2 opamps?.

Which configuration has the best noise performance?

Thanks in advance

I

rfmagic wrote on Dec 31^st, 2010, 4:59am:


Yeah, the biquads are much simpler to understand. But for high orders you get vary high Q-value for some of the biquads and you need low-resistors to get the noise performance. I think it is worth the extra theoretical effort of the leapfrog. I found some simple presentation how to make it work.

http://amesp02.tamu.edu/~sanchez/458-Leapfrog.PDF

/ssahl

rfmagic wrote on Jan 2^nd, 2011, 10:12am:

Hi,

what I meant my high Q circuits is the Q-value of the pole that the active filter is implementing, not the Q value of the components itself.

For the biquad implementation all poles needs to be placed in a specific biquad. Placing the ones with high Q early will make the filter to peak internally. This generates large voltage swing. To handle that you probably need to lower the gain to that point. Keeping the SNR you need to lower the resistor values and increasing the capacitor values, which cost you current in the opamps.

If you try out the leapfrog structure it looks like there is less peaking internally in the filter. (I have no theoretical proof for this but it is my experiance.) This lower peaking means that you can keep a larger signal swing in the filter and therefor keeping the SNR high without going for low resistor values.

I hope this explaination makes sense.

/ssahl

buddypoor,

That's a very good idea, following your input and ssahl inputs, I started to wonder how to compare the different alternatives quickly, and without making this project to complicated. I am working with ADS and the "Filter Design Guide" only synthesises lumped elements ladder networks so I suppose that this is not the right tool as it still leaves me too much work to transform the passive ladder into an active one.

It will be very nice to use a program that can generate diffrent Active filter architectures in order to compare their performance. Is the program that you recommend (filter_solutions) can do that?

buddypoor wrote on Jan 3^rd, 2011, 7:15am:

Hi,

I have found a simple program from Linear by which you can get the poles and zeros from a filter. I have found it useful.

http://www.linear.com/designtools/software/filtercad.jsp

Hi ssahl, I know the program from LTC - and, indeed, it is very useful to find the pole parameter for different approximations.
However, it gives parts values only for the design of filter modules from LTC. Thus, it cannot help by designing other structures.