Plot freq vs. dBm on computer

Discussion in 'The Projects Forum' started by PeteB, Oct 10, 2009.

  1. PeteB

    Thread Starter New Member

    Oct 10, 2009
    3
    0
    G'day all,

    I was doing a search on the www (world wide wait) in an attempt to get some information to start a project that I would like to build and I found a link to this forum. Looks like I it paydirt as I've done some browsing of this forum and there is certainly an incredible amount of knowledge behind it. I also did some seraching in the forum, but haven't found anything that appears to help with my intended project (although my search terms may not have been great).

    I work in the field of technical support for office equipment and one of my responsibilities is fax support. Most well engineered fax machines have a setting that allows a technician to adjust tx and rx cable eqalistaion to compensate for the frequency response (or "slope") of the line. The problem I face is most techs do not not have access to sophisticated equipment required to measure line slope, so I want to build a (relatively) simple device that will plot frequency vs. dBm on a computer. The premise for the project follows.

    The project will have a hardware component and a software component. The technician will connect the hardware component across the telephone line and while the fax machine is receiving, the project will plot a graph on the computer of frequency vs. dBm between 300Hz and 3400Hz at 500Hz intervals. The dBm levels at each frequency will be average over the period of the communcaition, which the tech can control to about a minute. Most telecomms systems use a complex impedance, but I'm happy to start by basing the measurments on a 600 Ohm load. Connection between the device and the computer can be serial, parallel or USB.

    Obviously, I need to build the hardware device and create the software. It's been about 20 years since I last designed a circuit, so I guess the text books will have to come out again if need be. As for the software, I've done a bit of C++ and Java (and BASIC!), but it will take me a while (to say the least) to get back into it.

    Yes, I have considered the legal implications of connecting an unapproved device to the telecomms network. I expect the device will be powered from a low voltage (7.5v DC?) plug pack, and I can build in some isolation if need be.

    So, before I go reinventing the wheel, I'm wondering if anyone can point me in the direction of a circuit design that may be adapted and a software program that could be similalry adapted?

    Any and all help gratefully accepted.

    Best regards,
    pete.
    (the newbie).
     
    Last edited: Oct 10, 2009
  2. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    I think this can be done by sound card and a laptop. A sound card has a frequency range from 20 to 20Khz. Most sound cards support a sample rate equal to 44100Hz or more. Sound cards as thumb rule do have AC cupeled inputs but I think they should work fine in your case.
     
  3. PeteB

    Thread Starter New Member

    Oct 10, 2009
    3
    0
    A sound card?!! Your know, it didn't even occur to me.

    Thanks heaps t06afre.

    Now I just need help with the program. Any takers?

    pete.
     
  4. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    I use Labview in my work, and for my hobbyist project. It should be quite simple to whip something up in Labview. At least if you have some experience with Labview. It is not a beginner project. Also sound card differ somewhat from brand to brand regarding input and output level. You may go around this by using a USB sound card from a specific vendor.
     
  5. PeteB

    Thread Starter New Member

    Oct 10, 2009
    3
    0
    I'm well on my way now. Did a quick search of the web for "sound card spectrum analyzer" and found some potential solutions - some even free. I'll cetainly have a look at Labview as well.

    Thanks again t06afre

    pete.
     
  6. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    You could even use the open-source program audacity.
    http://audacity.sourceforge.net/?lang=en
    This is an audio editor with the possibility to do an FT transform.
    With the FT transform you can change the time domain to frequency domain.

    Greetings,
    Bertus
     
Loading...