Hello, I would like to ask one question ^^

in Gm-c filter design, we must make sure the phase margin of intergrator is equal 90 degree. if phase is lower than 90, the Q factor will reduce. If phase is larger than 90. the Q will increase.

I wonder the phase error only can change Q factor? Is there any other disadvantage of phase error ?

thank you very much ^^

thank you very much !  I would like to discuss more ^^

I attach the filter and formular ^^

I am designing 2nd-order gm-c bandpass filter now. I want to design the center frequency of bandpass filter is 860MHz, and the bandwidth is 20MHz. According to the transfer function, center frequency, Q factor value, I have some questions:

(1) after calculation, i set the gm3,gm4,c1,c2 to get the center frequency 860MHz, but it is different from the theroy.  Also. if we want to change the center frequency, which value should we change? Gm3,Gm4 or C1, C2.  and when I change anyone, the phase of integrator will be change, not 90 degree anymore

(2) use B=Gm2/C1, i set the bandwidth of the filter. But it is also not exactly same. I guess because of parasitic capacitance, is that true?

(3) From the formular, after we choose the very high value Gm3. Gm4, and very small value Gm2, we can get very high frequency and very narrow bandwidth. But in practical, when I desing, it is not ture

Sorry to ask so many questions

I am designing a RF tracking filter now. So i need to tune the center frequency from 50MHz to 860MHz, and keep narrow bandwidth about 20MHz.  when i tune the circuit, i can not make sure the specification.

Thank you very much ^^

have a nice day ^^

Buddypoor, thank you very much for your valuable advice, I am really appreciate it ^^

I follow ur advice and check my OTA

1) the OTA parameters (bandwidth=2GHz, output charateristics=2K)

2)about the bandwidth:
When I tuning the center frequency, I tune Gm3, Gm4.  I didn't use Gm2 and C1 as the BW is required to be constant. But the bandwidth still change  I guess because of the paracitic capacitance, they are difficult to control, isn't it?  So it is very difficult to me to keep the constant 15MHz bandwidth.

3)about the central frequency
From the formula we can see that, if we make the Gm3 and Gm4 very large, we can get high frequency. But when I simulate, I make very large Gm3, Gm4 (about 15mS), but the frequency still not as high as I expect  I guess because of phase error or paracitic capacitance

4) about phase error:
since i should tune my tracking filter from 50MHz to 860MHz,  I use parallel Gm3 and Gm4 (2Gm,4Gm,8Gm.. as shown in attachment) to tune the center frequency. Sure thing the phase can not be 90 degree for every case. Should I compensate the phase every time? If that, the tuning will become very complex:(

I am so sorry to ask u so many questions again thank you very much ^^

have a nice day ^^

thank u very much ^^  sorry to ask u so many questions ^^  i think u have many experiences to design filter, am I right ? ^^

A good guideline - the parasitic poles of any of the system parts and pieces need to be at least a decade away from the controlled filter poles that you are trying to create.

So - If you want a gmC filter with 100MHz rolloff, your OTA (or gm stages) need to nave negligible phase response up to 1GHz over all process corners. That 20X figure mentioned above is probably pretty close.