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I want to make bandpass filter with its center frequency variable(range 50Hz to 5kHz), bandwidth the smaller the better but not less than 50Hz.The bandwidth to be same across the range should be same. Can anyone suggest me an design. I am looking at implementing the following design with 741 http://www.electronics.dit.ie/staff/ptobin/3chapt07.pdf.

Thanks :)

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What you are asking is very difficult. The problem is that you are asking for a constant bandwidth, rather than a constant Q (that is, frequency divided by bandwidth). A 50 Hz bandwidth at 50 Hz is easy - 50 Hz at 5 KHz is very narrow. The sort of circuits you've been looking at are hard to do at very narrow bandwidths due to sensitivity, where the exact bandwidth gets very sensitive to the exact (less than 1%) values of the components you use.

Furthermore, narrow filters are usually intended to have a sharp cutoff at all frequencies away from the center. A simple filter of the sort you link to is called a second-order filter, and for frequencies much away from center the cutoff is quite gradual. You can get around this by specifying higher-order filters, but each of these must be adjusted at the same time by the right amount, and the separate responses tend to interact to produce weird frequency responses.

You have not specified how you intend to change the center frequency. Do you want to use a knob on a front panel, or a voltage? If the former, if you're willing to stick to a 2nd-order filter (the simplest kind), you can get get ganged pots, where 2 resistors are connected to one shaft, and this may possibly do you, although I doubt it.

If you want electronic control, and especially if you want narrow filter response such as 50 Hz at 5 KHz, I suggest you look into switched capacitor filters. Linear Technology and TI both produce compact solutions. LT has an app note http://cds.linear.com/docs/en/application-note/an40f.pdf which is a good place to start.

Most importantly, do some more research on exactly what filter responses mean. Do you need Butterworth response, Bessel or Chebyshev? What order filter do you need? Until you understand the implications of your requirements, it will be hard for you to select the right filter.

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  • \$\begingroup\$ Sir, I looked at the ICs u suggested, but my project is to do a continuous time filter. Is there some IC that can be used even to some extent if not to the extent i mentioned. \$\endgroup\$ – Aditya Mar 22 '15 at 22:03
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    \$\begingroup\$ @Aditya - You still need to answer some questions. Do you need electronic tuning, or is mechanical (a shaft) OK? What order filter do you need? What type of response is required? If you don't know the answers, please describe your situation as completely as you can. \$\endgroup\$ – WhatRoughBeast Mar 23 '15 at 17:43
  • \$\begingroup\$ It has to be tuned mechanically. I am looking at 4th order or higher butter worth filter( or any filter which has very little ripple in pass band). \$\endgroup\$ – Aditya Mar 27 '15 at 16:21
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I suggest that you build a variable oscillator (LO) 10kHz to 15kHz. Mix your input signal with the LO to feed a bandpass filter centred on 10kHz with whatever fixed bandwidth you design. The output of filter goes to a second mixer also fed by the LO. While the output of this dual conversion arrangement provides what you ask, it may need an extra filter at the input if the "image response" to frequencies over 20kHz is a problem.

http://en.wikipedia.org/wiki/Image_response

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  • \$\begingroup\$ Sir, I knew about this implementation but my project is specifically about filters. \$\endgroup\$ – Aditya Mar 22 '15 at 4:13

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