Simple BPF VS Cavity Filter

Discussion in 'Wireless & RF Design' started by Fovakis, Oct 3, 2013.

  1. Fovakis

    Thread Starter New Member

    May 14, 2013
    23
    0
    Hi to all,

    i have a SISO system with microwaves devices. The 16-QAM information is about 20KHz , upconverted at 30 MHz and then at 2462MHz.At the receiver the same procedure, downconverting. After the first LNA i have a BPF fc=2430MHz which has large Bandwidth so it does not protect me from interferense signals. It has 200MHz BW. Although, i have a BER about 10-4 maybe 10-5 if the channels is clear (more often at evening).I did this setup in a Laboratory and behind us there are a lot of with WiFis transmitting all day etc...

    i did a purchase of Cavity Filters with center freq at 2462MHz and BW=25MHz. i replace the BPF with the cavity and........the result is worse!!![​IMG][​IMG] i have 10-3 maybe 10-4 BER Can you please tell me some ideas to fix this problem?

    http://i1284.photobucket.com/albums/a578/fovos1/receiver_zps552b2e4a.png
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,138
    1,789
    It is not clear to me exactly what is going on, but it sounds like two things are possible.
    1. Most of the interference is within 25 MHz of fc
    2. The insertion loss of the cavity is greater which means less signal for the receiver
    What to do is to understand the spectrum of the interference and try to improve the signal and attenuate the noise. How you do that is not immediately evident for the usual reasons. Words are a poor substitute for schematics and pictures.
     
  3. Fovakis

    Thread Starter New Member

    May 14, 2013
    23
    0
    Hi and thanks for your reply.So,

    1.if the most f interference was with 25MHz of fc at both filters there would be problem.

    2.The insertion loss of cavity is really greater but only 1 to 1.5dB. I don't think is so critical

    I am thinking about group delay. The simple Bpf has huge bw so small delay. The cavity has the opossite. My signal is 16QAM so i have information in phase so maybe that's why. (?)

    Also i don't know what is going on with the matching of the filters (?)
     
  4. Papabravo

    Expert

    Feb 24, 2006
    10,138
    1,789
    Do you have a VNA that will allow you to characterize the filter and the cavities. If you can do that, the necessity for a matching network will be apparent. These are just shots in the dark without being there to see the setup.
     
  5. Fovakis

    Thread Starter New Member

    May 14, 2013
    23
    0
    i have a network analyser not a vector :O
     
  6. Papabravo

    Expert

    Feb 24, 2006
    10,138
    1,789
    Can you measure the s-parameters as a function of frequency for both the BPF and the cavities?
     
  7. Fovakis

    Thread Starter New Member

    May 14, 2013
    23
    0
    yes! i will do that tomorrow ;)
     
Loading...