Just another spectrum analyzer thread. Programmable filters

Discussion in 'The Projects Forum' started by MatheusLPS, Dec 3, 2013.

  1. MatheusLPS

    Thread Starter Member

    Jul 16, 2011
    34
    0
    Hello everyone

    Reading some application notes today , I came across something that exists for some time but had never crossed my mind.

    So , we have discussed here on the forum about spectrum analyzers. Several colleagues have already given hints solutions to prevent the construction of analog filters for an analyzer with a 12, 16 or even 32 bands, because the schematic would be huge.

    So far , the cleaner idea so far that I found isto use the MSGEQ. Although very cheap. I need to import this chip because here in my country I can not find it.

    I already read about a FFT but it is a hard to understand how this works and the processing power needed to run it.

    Thinking about this subject (spectrum analyzers) I found this today :

    http://ww1.microchip.com/downloads/en/AppNotes/00737c.pdf

    and

    http://www.onsemi.com/pub_link/Collateral/AND8413-D.PDF

    I thought to myself that in this case I could create a variable bandpass filter using digital potentiometers. Probably it has been done in some corner of the internet. But I found the idea interesting.

    I made a block diagram like this:

    http://i.imgur.com/SxNKGA9.jpg

    The text on the paper is in Portuguese and says it: Will use 1 micro controller or a LM3915.

    I researched and found an IC almost ready for it. With him, just by changing resistor values, we ​​can create a filter up to the 8th order. Would be the MAX264 and MAX265 's . Look about them here: http://datasheets.maximintegrated.com/en/ds/MAX263-MAX268.pdf

    The problem is that MAX is not found it easily on my country too . So at first, I would use some common operational amplifier and would change the pass band with the help of a micro-controller .

    The output of the filter would be injected into the input of an LM3915 or in same uC AD controlling the digital potentiometers . If only uses the uC, the same CI would be responsible for the switching matrix of LEDs or LED bars. For each passband filter, a column would be active, showing the amplitude of that frequency range.

    With this, we could have a very clean circuit. And the software would be so simple.

    I thought of using only positive sound source. Eliminate the negative part of the audio signal with a resistive divider .

    My concern is the timing. If I send to the digital pot that I want it to change his value, I do not know how much time the filter will require to settle.

    Considering that I am planning to do 32 passbands, probably the settle time will be too slow and I will will not make any quality measurement.

    Any thoughts?

    bye
     
  2. MatheusLPS

    Thread Starter Member

    Jul 16, 2011
    34
    0
    Anyone?

    bye
     
  3. LvW

    Active Member

    Jun 13, 2013
    674
    100
    MatheusLPS - as far as I could understand your planning you want to build a spectrum analyzer based on a tunable bandpass filter, correct?
    This is the BASIC principle to visualize the frequency components within a certain frequency band.
    Now you could ask why commercial instruments do NOT use this simple principle, but instead the principle of superposition.
    Here are some comments from my side:
    You need a wide-range tunable bandpass, which fulfills the following requirements:
    1.) Very small bandwidth (for sufficient resolution)
    2.) constant bandwidth over the tuning range
    3.) rather short settling time for the filter.

    In particular, No 2 and No.3 are very hard to fulfill if - at the same time - No. 1 has to me met. Reading the referenced data sheet on filters you will notice that during filter tuning the quality factor (Fo/bandwidth) can be kept constant but not the bandwidth.
    Therefore, a spectrum analyzer unit based on the described principle will have a very limited application range.
     
  4. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    You have not said anything about the frequency range for this school project of yours. That is a very important design criteria
     
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