Bandpass filter

  • Everything which is blocked can not be transmitted.

    So if you put it after the PA it will eliminate all rf (ok, mostly) outside the bandpass.

    If you put it before the PA all mixing products which can occur in the amplifier will go to the antenna.

    So I would recommend the installation after the PA.

    Hope that makes sense.

  • I agree with the statements above but would like to recommend to add some thoughts:

    if your filter has significant insertion loss and/or limited power capabilities it is better to place it before the final PA.

    Assume you are operating a 200W PA at 50W to keep it very linear for DATV and your filter has an insertion loss of 1dB
    then you probably do not want those additional losses attenuating your precious power ...

    Kind regards


  • Hi, also some thoughts more.

    A hi power filter is more expensive than a low power one. Also if you put a filter before the PA you do not amplify harmonics so you get more useful power. One another more benefit to put it before is that you prevent creation of self oscillations especially if you have a lot of cascaded gain at same frequency. You must take care not to overdrive the PA as it will be harmonics created. If the PA has a power sense meter this is not always measuring real output power !!! In my case the power sense output is display more voltage from one value and more but only harmonics are produced !!! No more power!!! The usual filter that can be at the output of a PA is a low pass one as it has less loss. You need one if your harmonics are more than 40 dB than the carrier. If you have a lot of cascaded gain it is a good practice to put a BPF between every stage. In my case I have about 55 dB cascaded gain (* 500.000 !!!) without self oscillations. Also very very good shielding is required.

    I hope that all this help ...