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Voltage controlled band pass filte

m_t_c

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Hi all

I need to design a band pass filter of 14 KHz bandwidth (3dB) such that its centre frequency can be varied from 80 KHz to 900 KHz by some means e.g. tuning voltage. Is there any commercial IC available for such application? If not, how can i design voltage controlled band pass filter using discrete components?
 

barry

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Hi all

I need to design a band pass filter of 14 KHz bandwidth (3dB) such that its centre frequency can be varied from 80 KHz to 900 KHz by some means e.g. tuning voltage. Is there any commercial IC available for such application? If not, how can i design voltage controlled band pass filter using discrete components?
How many poles? Maybe you can use a switched capacitor filter like this: https://www.ti.com/lit/ds/symlink/mf10-n.pdf

or try an internet search; there’s a lot of stuff out there
 

danadakk

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The MF10 is not suitable for OP design requirements @ 900 Khz.

OP : Do you want cosnata Q or consta BW in the filter ? Maximally flat or
linear phase ? Ripple ? You need to post a more complete description.

1659836459717.png



You can use ladder structures and varicap diodes to tune in that freq range.
Order of filter dependent on your specs. Varicaps available with 25:1 ratio.

Also frequency control accuracy you need to stipulate. As well as linearity of
F versus V.....

DSP certainly doable for this as well.

A good starting point - https://rf-tools.com/lc-filter/

Regards, Dana.
 

Warpspeed

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Another approach would be hetrodyning up to a much higher frequency than 900 Khz and using an off the shelf fixed filter of some kind that has the required performance.

Suppose you could find a hypothetical 20 Mhz surface acoustic wave filter that had a nice flat 14Khz bandwidth with suitably steep skirts.

You mix your incoming 80Khz to 900Khz with a voltage controlled oscillator that works between 20.08Mhz and 20.9Mhz. Feed that through the filter, then mix the output from the filter back down with the exact same frequency you used to convert upwards. That gets you back down to the original base band frequency.

Its just a case of finding a suitable filter.
And the centre frequency of the filter is not that important.
--- Updated ---

Just found these filters after a very superficial search.
10.7Mhz centre frequency, 15Khz bandwidth at -3db, and the in band ripple is pretty good.
https://www.fcd-tech.com/rf-microwave/crystal-filters/monolithic-crystal-filter-mcf-10-7mhz-series

Also on flea bay:
https://www.ebay.com.au/itm/295129455784?hash=item44b7161ca8:g:eP8AAOSwa3hiHm3d
https://www.ebay.com.au/itm/114159646257?hash=item1a9471f631:g:w-sAAOSwGCBefGUq
 
Last edited:

FvM

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I think, we should hear the requirements first. Filter order, dynamic range, tuning accuracy, could stepped tuning be a solution?

As for varicap solution, 1:11 frequency range corresponds to 1:121 C variation, not feasible. Clock tuned SC can handle the range, but no commercial ICs for 900 kHz as far as I'm aware of. gm-C active filter tuned by gm variation can be a solution, accuracy and dynamic range is however limited.
 

Warpspeed

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Superhetrodyne receivers were first developed to solve this exact same problem.
Keep the filter fixed at one frequency and optimize it.
Then frequency convert up and then back down again.
It also solves the 1:11 frequency span problem.
 

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