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low pass filter with corner frequency tunable/trimmable

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gte582w

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for example, 3-bit tuning/trimming, funny thing is that it's required that
the next corner frequency is 1/sqrt(2) times of current corner frequency,
say if the current corner frequency is 1MHz, then next corner freq. should
be tuned 0.707 MHz, and the next will be 0.5 MHz, etc.

Anybody came across such LPF before?

Thanks,
 

the circuit is what I am asking for
 

Required dynamic range, filter order and frequency accuracy are essential parameters to choose a suitable design.
 

Thank you all for your kind replies. But my question is really how to realize such LPF with corner fequency trimmable in that special fashion.
 

You can try using a few resistors in series with pass transistors to select the desired value.
 

I've done an adjustable corner frequency with a switched capacitor filter. Its corner frequency was controlled by the switching frequency. The corner frequency was adjustable between 30Hz and 200Hz. The design was made round MAX295 chip.
 

gte582w said:
Thank you all for your kind replies. But my question is really how to realize such LPF with corner fequency trimmable in that special fashion.
Click to expand...

Here is my recommendation: At first, you should select a filter structure which, in principle, has the capability to change its corner frequency without influencing the pole Q values (I presume that this is a requirement, is it not ?).
Question: What is the required order of your lowpass ?
Then, as a second step, you should think of a special circuitry to trim the frequency in that special fashion.
 

Depending little on the structure of your filter...

If it is a ladder simulation the implementation will be

Active-RC

or

Gm-C

then either the R or Gm is switched in 3.01dB increments or the C or both combined. But typical you have to do for all components of the same type.
 

Yeah, I think I should try Gm-C implemented with bipolar as gm=I/vt and we can vary the I logrithmatically. Thanks, guys.
 

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