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Design >> Analog Design >> how to calibrate gm-c filter and get the measured frequency response
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Message started by lhlbluesky_lhl on Dec 27^th, 2012, 6:25am

Title: how to calibrate gm-c filter and get the measured frequency response
Post by lhlbluesky_lhl on Dec 27^th, 2012, 6:25am

i have designed a gm-C bandpass filter used in ir receiver module. in my design, C is integrated on-chip (f0=gm/C), so C will change with process variation (may be 20% ~ 30% variation). i want to calibrate the variation of C with process through varing gm (bias current of gm), but how to calibrate this in detail?
besides, in simulation, i can get the frequency response(amplitude vs frequency) of my bandpass filter (ac analysis), but for real chip, how to measure the frequency response of bandpass filter? how to add the stimulus and how to get the frequency response curve?
thanks all.

Title: Re: how to calibrate gm-c filter and get the measured frequency response
Post by Dan Clement on Dec 27^th, 2012, 6:38am

I just finished a gmc bandpass filter design with everything on chip. I used a PLL for auto tuning.

I ran a transient simulation until the PLL was locked. And then I ran AC with the same operating points.

Title: Re: how to calibrate gm-c filter and get the measured frequency response
Post by nrk1 on Dec 28^th, 2012, 6:34am

There are a lot of ways of tuning the time constants. One is to use a PLL as Dan Clement said. You can also use an integrator and adjust things based on the ramp. For measurements, you have to use buffers and calibrate out its response. See the paper below(Fig. 1c, Fig. 8b) and references [3,9].

Nagendra Krishnapura, Abhishek Agrawal, and Sameer Singh, "A High IIP3 Third Order Elliptic Filter with Current Efficient Feedforward Compensated Opamps," IEEE Transactions on Circuits and Systems II-Express Briefs, vol. 58, no. 4, pp. 205-209, April 2011.

Title: Re: how to calibrate gm-c filter and get the measured frequency response
Post by lhlbluesky_lhl on Dec 30^th, 2012, 2:41am

for the PLL way, do you use a switched-capacitor structure for bpf, and tuning the clock frequency of switched-capacitor?

Title: Re: how to calibrate gm-c filter and get the measured frequency response
Post by lhlbluesky_lhl on Dec 30^th, 2012, 2:48am

besides, nrk1, can you give me a copy of the paper you said? i cannot get it from web, thanks.

Title: Re: how to calibrate gm-c filter and get the measured frequency response
Post by nrk1 on Dec 30^th, 2012, 3:58am

lhlbluesky_lhl wrote on Dec 30^th, 2012, 2:48am:

besides, nrk1, can you give me a copy of the paper you said? i cannot get it from web, thanks.

http://www.ee.iitm.ac.in/~nagendra/papers/tcas2-wlanfilter-pap.pdf

Title: Re: how to calibrate gm-c filter and get the measured frequency response
Post by Dan Clement on Dec 30^th, 2012, 12:03pm

I inherited the filter and PLL architecture from someone else so I did not have the luxury of doing it a better way...

Technically it's really more of a DLL...

I did a master slave tuning topology but I don't like the way it works. You can't tune out the transport delay of the filter and end up a systematic error between master and slave.

And since I used the Bram Nauta Gm cell, I had to tune using a regulator. The regulator makes the loop dynamics tricky.

But my simulations indicate a centered tolerance of two percent and that meets the needs for the system.

The filter is not sw cap as it has three programmable center frequencies up to 8MHz.

Title: Re: how to calibrate gm-c filter and get the measured frequency response
Post by lhlbluesky_lhl on Dec 30^th, 2012, 5:25pm

thanks.