midband gain in tow thomas
lily1981216 said:
I select the structure, because I want to design a frequency tunned filter. And I saw some papers about frequency tuned filter select the tow-thomas filter and introduce the structure that the frequency could be tuned just by modifying the the cap value. In fact, I don't know some other structures more suitable to the frequency tuned filter with the maximum cutoff frequency above 1MHz.
Hi lily19811216,
may I start with a general remark: To select one filter circuitry (out of perhaps 20...30 alternatives) is a very difficult and tricky task which always results in a compromize. This compromize depends strongly on the requirements and the operating conditions. Therefore, it would have been better from the beginning to say something about your basic requirements:
Tuning capability and frequency "above" (above???) 1 MHz. (By the way: 1 MHz or even more may be a problem for Tow-Thomas, which is rather sensitive to phase errors which accumulate due to 3 opamps in series !)
But that´s not sufficient.
Therefore my questions:
1.) Low pass of which order ? (I hope: n=2, otherwise tuning is a problem).
2.) Tuning across which range ? (1% for final adjustments or 20....50 % ?)
3.) What about pole Q ? Constant across this range ? Or do you require the bandwidth to be constant ? Or does it not matter at all?
4.) What about the filter gain ? Constant during tuning ?
5.) Which part to be changed during tuning ? R or C ? Any preference ?
Only one part ? (Because in many cases two parts in parallel are to be tuned) .
I think, you now can agree that filter design is a real "design task" and cannot be done by a filter program. Such a software can only calculate parts values - at the end of the design process ! The first 98% of the job are up to you.
Regards
LvW
Added after some minutes: Sorry, cancel "bandwidth" in point 3.) (I had a bandpass in my mind)