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Voltage controlled band pass filte

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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?
 

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m_t_c said:
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?
Click to expand...
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
 
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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.
 

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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
 
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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.
 
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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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danadakk said:
The MF10 is not suitable for OP design requirements @ 900 Khz.
Click to expand...
Will MF10 work for frequency ranges up to 500 KHz?
danadakk said:
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.
Click to expand...
A constant BW of 14 KHz is required in filter with linear phase.
 

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I read post #10 as a hint that an analog constant BW variable frequency filter can be build with multiple variable gain elements (VCA, OTA, analog multipliers) if Q is tuned along with f. As for the original posters requirements, we still don't know more than -3 dB BW and the keyword linear phase being mentioned. The latter is only significant for higher order filters that are not easy to build with variable frequency.

The usual reason for not giving a clear specification is that you don't have it (yet). Can we at least know the filter application?
 

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This architecture, state variable, can be easily tuned by replacing integrator R's
with multiplying DACs (MDAC) or programmable pots. By replacing R6, R7 with
a MDAC that can be tracked to maintain a bessel damping factor. A tad complicated
over doing this in code/DSP approach. As the damping factor not a simple linear
relationship to R6. R7. Would be a simple table lookups with interpolation......
a possibility. But to FVMs point forum needs more information about use and
specs.


1661596459704.png


Of course the approach now has a latency due to table lookups, but may be inconsequential
due to settling time, probably BPF response dominant.

Makes one wonder if JFETs would be practical in altering the integrators....but then matching
issues creep up, and shaping the damping with a JFET tracking......

Or use of transconductance OA's, state variable here :




Regards, Dana.
 
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The remarks about difficult tuning of filter Q in the quoted article don't apply to "state variable" filters in general. They are due to the simplified three OP topology. There are better (4 OP) topologies with constant filter gain independent of Q setting.

As for FETs as variable gain elements, a circuit with acceptable tolerance needs paired reference FETs for generation of the gate control voltage.

I agree that Bessel filter prototype would be a candidate for linear phase filter. However a 1st order (single complex pole pair or "biquad") filter of certain Q can represent Bessel, Butterworth or Chebyshev. The difference becomes significant when assembling a higher order filter.
--- Updated ---

There's an old Edaboard thread about JFETs as variable resistors, unfortunately the appended schematics and diagrams are lost. https://www.edaboard.com/threads/using-jfets-as-variable-resistors.129234
 
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I saw it was ON but all the distributors, Mouser, Digi, reporting its obsolete. I see ON
is showing inventory, and that those distributors have inventory.....the web never lies.....

Regards, Dana.
.
.
 
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