Voltage controlled band pass filter

Thread Starter

naumankalia

Joined Feb 19, 2016
43
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?

Thanks & Regards
 

LvW

Joined Jun 13, 2013
1,561
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?

Thanks & Regards
Just to avoid misunderstandings: The midfrequency should be varible between 80kHz and 900kHz and - at the same time - the bandwidth should remain the same (14kHz)? Rather hard requirements! This would mean that the Q value must increase with the midfrequency.
What about the midband gain? I suppose it should remain constant?

I remember one bandpass topology which should be able to fulfill all these requirements - however, tuning is accomplished using one resistor which is NOT grounded. This circuit is a second-order bandpass in GIC topology ("Fliege bandpass") .
This filter stage can be interpreted as a simple R-L-C bandpass (LC in parallel), where the inductance L is realized as an active inductor (classical GIC block with two opamps as introduced by Antoniou). Using an opamp output as output of the whole filter the midband gain is Am=2 over the whole tuning range.
 
Last edited:

Ian0

Joined Aug 7, 2020
6,312
Years ago I used an LMF100 switched-capacitor filter IC to make a distortion analyzer. Others are available today.
A variable-frequency clock for the filter is easy.
Except that the bandwidth of the LMF100 only extends to 100kHz and I don’t see many switched capacitor filters with bandwidths extending into the MHz region
80kHz to 900kHz is an unusual requirement, suggesting AM radio, but that would require 4.5kHz bandwidth. Perhaps the TS could enlighten us as to the application.
I would suggest something tuned with a varactor at those frequencies.
 

Thread Starter

naumankalia

Joined Feb 19, 2016
43
80kHz to 900kHz is an unusual requirement, suggesting AM radio, but that would require 4.5kHz bandwidth. Perhaps the TS could enlighten us as to the application.
I would suggest something tuned with a varactor at those frequencies.
I want to design a common preamplifier with voltage controlled band pass filter that can be utilised with multiple acoustic transducers ranging from 80 KHz to 900 KHz. Any reference design using varactor for said application please?
 

LvW

Joined Jun 13, 2013
1,561
I want to design a common preamplifier with voltage controlled band pass filter that can be utilised with multiple acoustic transducers ranging from 80 KHz to 900 KHz. Any reference design using varactor for said application please?
When you want help, it would be a good thing if you would answer the additional questions (Q-factor, gain).
 

Ian0

Joined Aug 7, 2020
6,312
What is the purpose of the preamp? Do you require the signal for further processing or are you just measuring its amplitude.
Presumably the transducers are piezoelectric ultrasonic devices. They tend to be have a resonant frequency and quite a high Q so won't give much signal outside their own bandwidth, so I wonder how necessary the filter really is.
 

Audioguru again

Joined Oct 21, 2019
5,183
There is a system of audio-modulated ultrasonics. When the ultrasonic beam is aimed at an object or at a group of people then the object or group causes interference which produces the previously modulated sounds to occur there.
 

Ramussons

Joined May 3, 2013
1,316
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?

Thanks & Regards
Have you looked into the possibility of Superheterodyning? The pass bandwidth will be the same over the operating frequency range.
 

Thread Starter

naumankalia

Joined Feb 19, 2016
43
When you want help, it would be a good thing if you would answer the additional questions (Q-factor, gain).
Preamplifier will have a variable gain with gain variation of 80 dB. I think Q factor will depend upon the voltage controlled centre frequency of band pass filter and will range probably from 6 to 36 for centre frequency from 80 KHz to 500 KHz
 
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