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The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl Design >> Analog Design >> Gm-C / Active RC / S-C https://designers-guide.org/forum/YaBB.pl?num=1255597803 Message started by Mayank on Oct 15th, 2009, 2:10am |
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Title: Gm-C / Active RC / S-C Post by Mayank on Oct 15th, 2009, 2:10am Hi all, I have done quite an extensive reading on design of analog fitlers. But anyways, before starting the design, i wanted to take a counsel... Application : Analog Base-Band for WLAN RF Intended Use : For Digital Video Broadcasting Filter Cut-off : 80 MHz Major Concern : Power Savvy Design. Others Concerns : Low-Noise, Area Savvy. Which Topology should i go for -- Active RC/ Gm-C / S-C ?? regards, Mayank. |
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Title: Re: Gm-C / Active RC / S-C Post by aaron_do on Oct 15th, 2009, 4:57am Hi, just to get the discussion going, I think switched-cap filtering is out since it requires anti-aliasing and a high clk frequency. With that kind of cutoff frequency, you would need a pretty high UGB for an active-RC filter. For example, for a loop gain of 20 dB at 80 MHz, you would need a UGB of 800 MHz. You also need to consider the out-of-band linearity and the highest interferer that you want to filter. For gm-C your linearity will not be as good as the active-RC, but you should be able to get low power consumption. The linearity will be most important for the first stage of course... Feel free to correct me if i'm wrong. BTW what topology did you think was the most suitable? cheers, Aaron |
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Title: Re: Gm-C / Active RC / S-C Post by buddypoor on Oct 15th, 2009, 5:00am Hello, Mayank ! I suppose you need a LOWPASS, right ? For my opinion, RC-active as well as S/C are not very well suited to your requirements (80 MHz). As a result , only the gm-C techniques remains as an alternative, which in addition is suited for IC technology. |
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Title: Re: Gm-C / Active RC / S-C Post by Mayank on Oct 15th, 2009, 5:16am Hi all, I am sorry i forgot to mention -- I am talking about a Low-Pass Filter. @ aaron_do : I already did the math for Active-RC filter. But the main fight, as you said and as i experienced in 65nm, was high BW Product. High Speed Clock for S-C is not a problem, I can get it from RF LC-VCO---Did some math for it also,but not the whole architecture. My Choice :--- I want to go for Gm-C coz of Power Optimization and (OTA vs Opamp advantage) BUT am a bit skeptical how it will turn out in the end in terms of linearity and calibration for Cut-Off Frequency Tuning. I was currently studying some papers on linearized Gm Cells and high speed Gm cells (such as Nauta Cell). How should i proceed with the Architecture for Gm-C ?? Any further reading material on Higher Order Gm-C Filter Design would be helpful. @ Buddy : Yeah, i am sorry not to mention it's a Low-Pass Filter. |
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Title: Re: Gm-C / Active RC / S-C Post by buddypoor on Oct 15th, 2009, 6:27am Mayank wrote on Oct 15th, 2009, 5:16am:
Mayank, itīs a rather challenging task to choose an architecture which best serves all needs and requirements. Mainly, the answer depends on the required filter order which directly is connected with complexity. For orders of four or higher I would suggest a topology which is based on a passive ladder structure (leap frog structure). This topology has the advantage of being least sensible with respect to passive element tolerances. |
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Title: Re: Gm-C / Active RC / S-C Post by Mayank on Oct 15th, 2009, 8:26am Hi, Thanx Berti for your advice...I was thinking on the same lines...Actually, i did the math for Active RC implementation of 4th Order Chebyshev LC Ladder. I think most Probably i will be needing a 4th Order Cheby OR else a 5th Order ButterWorth Filter... Question : When i designed Active RC Impl. of 4th Order Cheby LC Ladder, I designed a fully optimized (for noise/sensitivity) Ladder, where each L & C value were different, resulting in different R & C values. Can we design a chebyshev LC Ladder where all L & C values are identical respectively ?? --Mayank. |
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Title: Re: Gm-C / Active RC / S-C Post by Mayank on Oct 15th, 2009, 8:30am Hi, 1 more question : The Primary Resonator Block PRB Approach used in FLF(Follow-the-Leader Feedback) Topology is useful because i want tuning capability for Cut-Off Frequency.... Can i do something of that sort in LeapFrog ?? I also have to keep in mind the calibration circuitry required for cut-off tuning. That's the reason i wanted equal L & C ladders....Pls help. regards, Mayank. |
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Title: Re: Gm-C / Active RC / S-C Post by buddypoor on Oct 16th, 2009, 12:40am Mayank wrote on Oct 15th, 2009, 8:30am:
1.) No, I donīt think that the leapfrog architecture allows easy tuning of the cut-off. In this respect , PRB is more flexible. But I would say that for a lowpass of (only) 4th order a classical cascade approach would also be appropriate. Donīt you think so ? And it requires less amplifiers. 2.) To your former question: [i]Question : ................... Can we design a chebyshev LC Ladder where all L & C values are identical respectively ??[/i] There are some filter programs available (partly for free via internet) which can be used to tune LC laders and to modify parts values. Regards LvW |
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Title: Re: Gm-C / Active RC / S-C Post by loose-electron on Oct 16th, 2009, 5:39pm IMHO at this bandwidth its going to be gmC, that said, the problem really has 2 big parts - Splane definition of the filter and transistor level definition of the gmC sytem itself. I have done the Splane in behaviorals in Cadence (Vlog AMS), and Matlab (Simulink), and Excel (!! using phasor methods) The you got to define the gm stage, and get the linearity right, and then define the filter stages themselves. I prefer cascaded bi-quads, easier to understand whats going on. Uses 4 gm stages. Dont forget to define how you are going to tune the thing. Adjusting either Cap size or gm value. If the linearity is an issue, I prefer switching the capacitor values, (digital switching) and running a tuning adjust cycle fairly often. (thermal variance) On that, I like to adjust the -6dB point of the filter, because it is easy tune with an input signal, and compare it in amplitude with a Vsignal/2 version. What else? Hmmmm.. may want to go one order higher on the filter (5 pole instead of 3 for example) because the rejection characteristics may not be quite as good as the ideal math model. My thoughts and "learning experiences" as they say. :D jerry |
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Title: Re: Gm-C / Active RC / S-C Post by Mayank on Oct 21st, 2009, 7:58am Hi folks, I was on a vacation...Festival of Diwali in India...Now am Back..Saw your replies.. @ buddypoor : Thanx for ur reply...But in Biquad Cascading...each RLC network is separated by active gain stages...resulting in larger uncorrelated variation in poles of each stage with component variation...That's The reason i dnt wanna use it... BottomLine -- Linearity can be best attained in Ladder Simulated structures--LeapFrog serves good..So wanna go with Leapfrog...You are right...Tuning in LeapFrog is tough...but any ideas on how on efficient calibration in Ladder structures ?? @ Jerry : I like your style, Jerry...Gotta know you better .... 1. So Gm-C is Final...But i saw you quoting on some other topic that high o/p resistance of Gm Cells increase noise of the cell...Now that seriously scared me Sir[u seem quite senior].... :( My Noise Specs are tight too...esp on the RX side... 2. Quote:
3. In accordance with my Specs, I arrived at either a 4th order Cheby OR 5th order ButterWorth...Do i need to go for even higher orders considering the non-ideality effects as you indicated ?? |
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Title: Re: Gm-C / Active RC / S-C Post by loose-electron on Oct 21st, 2009, 9:39am "seemed quite senior" - LOL! Grumpy old dude eh? Well Here: http://www.designers-guide.org/experts.html Bottom of the page, and that's a pretty recent picture (last summer) Noise is an issue, no doubt, and the only way you are going to know is to put it together and model it, and run some noise analysis. As for the tuning? 1. Create a sinusoid wave (heavily BW limit a square wave, it doesnt have to be perfect.) The sinusoid gets created at a frequency that the filter has as its properly tuned -6dB point. (Vsig) 2. Put Vsig into the filter to get an output (Vsigfilt) 3. Take Vsig and voltage divide it with some resistors. (Vsig/2) 4. Compare the amplitude of Vsigfilt and Vsig/2 (you need some peak detection and averaging circuits to create DC equivalents.) 5. Adjust your "tuning knob" until Vsigfilt = Vsig/2 and you have tuned your filter. |
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Title: Re: Gm-C / Active RC / S-C Post by Mayank on Oct 22nd, 2009, 8:01am Hi Jerry, Naah man !! :P dint mean an old dude...Just respect...saw the pic...looks pretty cool man.sailboating..shall i mail u sometime if thats nt a problem ?? its good to know great designers... As for Tuning, Chebyshev shows 1 dB PassBand Ripple. So isnt the 1dB point the filter cut-off Point ?? I thought i would tune the 1dB point for it..i wanted to know a bit more about noise analysis of filter...Wanted to arrive at a noise spec for Gm-Cell for designing the OTA..How can i do that ?? thanx, Mayank. |
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Title: Re: Gm-C / Active RC / S-C Post by loose-electron on Oct 22nd, 2009, 8:54am If you tune the -6dB point its a lot simpler. Everything shifts together, and doing V/2 is easy. Yeah, I take email, no big deal. Busy as h@ll right now, and coming to this forum is my coffee break entertainment. jerry (at) effectiveelectrons(dot)com |
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Title: Re: Gm-C / Active RC / S-C Post by duncandu on Oct 24th, 2009, 9:48am I don't think SC is suitable here, because you must put another high-frequency anti-aliasing filter prior to it, also with the problems of clock feedthrough and high-speed clock generation. It seems Gm-C would be more suitable for ~80MHz application rather than active-RC, since it is open-loop system which has high-speed potential. Of course, as we all know, it is tricky to solve the linearity problem and parasitic load. However, it is worthy mentioning you can really design an active-RC filter for such a high frequency with a good compromise between power consumption, speed and linearity, as long as you know how to cleverly design the operational amplifier in it. If you are an experienced designer who knows the details of operational amplifiers, just choose active-RC. |
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