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Design >> Analog Design >> OTA-C filter for low-IF receiver
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Message started by aaron_do on Jul 17^th, 2006, 11:31pm

Title: OTA-C filter for low-IF receiver
Post by aaron_do on Jul 17^th, 2006, 11:31pm

Hi all,

i'm supposed to look into designing an OTA-C filter for a low-IF receiver (2MHz) in CMOS. So I need a f3db of about 3.5 MHz i guess. Anyway i'm pretty new to analog so can anyone tell me what are the important specifications for this type of application?

For example, what sort of 3dB frequency should I design the OTA circuit for if the filter has f3db=3.5 MHz? I guess since it is an OTA, we normally look at transconductance gain not voltage gain right, so what is typically a good low frequency transconductance assuming I want a low-power (microamp range) OTA? What are the other important parameters? In practical designs how concerned are we with PSRR, CMRR and such...

At the moment i only have access to research papers which tend to leave out a lot of practical circuitry such as CMFB and biasing e.t.c

Thanks for the help,
Aaron

Title: Re: OTA-C filter for low-IF receiver
Post by ACWWong on Jul 18^th, 2006, 2:21am

Hi Aaron,

the system design should clearly give you specifications of your filter, and these specifications will determine whether the design is difficult or easy. Specifications such as filter order & shape (i.e pole/zero positions) are determined from reciever adjacent channel rejection and inband ripple & groupdelay (requirements depends on the data modulation type). Having been given the filter order & shape, then one usually chooses and filter architecture (Ladder, cascaded biquads, SFG etc.) in conjucntion with realization type (opamp RC, gmC etc.)
ANyway, There is alot of stuff on the web about gmC (OTA-C) filters, try circuitsage.com or anything by rolf schaumann for basic information. Also the book by Laker & Sansen is good starter.

In answer to your direct questions:
Where in opampRC designs one uses R*C to produce the timeconstants required for the transfer function, OTA-C uses gm/C. so yes its the value of the OTA's gm which is used. so for a pole/zero frequency~ 3.5MHz, with C's about 10pF we are talking ~220uS. The same timeconstant could be used with C=20pF and 440uS for lower noise but at the cost of area/power.
The bandwidth of the OTA will depend on the Q and frequency of the filter poles you are trying to realise and what and where they are used in your filter architecture. Generally you'll be pretty safe with Q^2*Fc for the OTA bandwidth.
The other important thing for gmC filters is linearity, as often the bottleneck for linearity in buried within the filter architecture (usually where the Q is highest) and often the OTA linearity requirement is much larger than the overall filter linearity spec.

Practical considerations such as CMFB are you state are always inportant (there's nothing worse than an oscillating CM loop!), but PSRR and CMRR are usually ok if one uses differential toplogies.

sorry for my long winded response, as you can see continuous time filter design is a large subject!

cheers
aw

Title: Re: OTA-C filter for low-IF receiver
Post by aaron_do on Jul 18^th, 2006, 9:23pm

Thank you for the reply.

I understand how to get my filter order based on adjacent channel rejection, but the inband ripple is new to me. If i wish to satisfy any modulation should I just go for Butterworth filter? I believe i will have a constant envelope modulation...BPSK or O-QPSK i think.

Also the IF is 2 MHz and the BW is also 2 MHz so Q^2Fc=Fc?....i originally intended to design the OTA bandwidth as 10*F3db so that I can minimize the effects of the parasitic caps wrt the desired caps.

I'll look into your book suggestion...

thanks again for the help,

Aaron

Title: Re: OTA-C filter for low-IF receiver
Post by ACWWong on Jul 19^th, 2006, 4:32am

Hi Aaron,

Passband groupdelay/linearity of phase response will then be important if you aren't too caring about amplitude ripple. The best way to derive what is acceptable is use something like matlab to check the effects... this will ensure you choose a filter response which no only acceptable from the point of view of rejection, but also will not impair in band response.

The Q i'm referring to is the particular pole/zeros Q that make the filter transfer function. I guess my comment was much more relevant to bandpass/stop or very sharp/high order filters when Q of poles can get large. so perhaps the 10*3dB is sufficent in low pole Q situations.

So the OTA bandwidth requirement for say a 2 pole butterworth response at 2MHz might not suffice if you tried to use the same OTA bandwidth for say a 9 pole butterworth at the same frequency. This is because the poles complex poles in a 2 pole butterworth LP are at (in s-plane) -0.707+/-0.707j whereas the highest Q poles in a 9 pole butterworth LP are at -0.174+/-0.985 ie much higher Q.

cheers

aw

Title: Re: OTA-C filter for low-IF receiver
Post by aaron_do on Jul 19^th, 2006, 6:08pm

ok i see what you mean. thanks,

Aaron