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Thread: High performance high-pass filter design.

  1. #1 High performance high-pass filter design. 
    Forum Freshman jammer's Avatar
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    I am trying to build a high performance high-pass filter that is either surface mount and smaller than 1 cm x 1 cm or is made of strip-line and is smaller than 3.5" x 2.5".

    The required specifications are:
    -60 dB stop-band attenuation
    -less than 2 dB insertion loss
    -stop band is huge 1.75GHz-2.12GHz
    -pass band is 2.2GHz-2.3GHz

    I can't use air loaded strip-line because of manufacturing constraints. I have tried parallel coupled-line filters (broadside and edge), lumped strip-line model composite approach, stub, lumped element and the most promising is the hairpin filter. I have gotten very close with the hairpin model, but no amount of tuning is getting me there.

    Any other ideas out there? Does anyone have experience with filters and constraints like these? I'm running out of ideas to model in Microwave Office.


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  3. #2  
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    There is an amazing book by Bowick called "RF Circuit Design" it is published by Newnes.

    It has good sections on passive circuit designs, and is very much worth the 20 dollars I bought it for. You should pick it up at the library, and follow the directions on band-stop filter creation.

    However, your filter is too specific. You are giving more information than is necessary and are looking for the wrong type of filter.

    There are 4 main types of filter, High Pass, Low Pass, Band Pass, and Band Stop. High pass has a -3dB cutoff frequency, and a "rise time," as does Low Pass. They both cut off half of the spectrum. High pass cuts off below the cutoff frequency, low pass cuts off above it. Band pass cuts off above and below the bandwidth, and band stop cuts off everything within the bandwidth.

    What it sounds like you're trying to make is a band pass filter. Something that will pass only 2.2-2.3 GHz, and has a cutoff frequency at both 2.2 and 2.3, with a -60 dB point at both 2.12, and 2.38 GHz. To do this, you're looking at a high order filter, maybe 8 stages or more. Also, you're going to need to look at your "transmission" lines to make the shortest possible distance between the components, or you may end up with a different characteristic. Also, when you order components, make certain they have small tolerances, or are made for RF applications.


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  4. #3  
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    You are right, I am looking for a bandpass filter, but the low side is the important side. I need 60 dB of rejection in the entire range of 1.75 GHz-2.12 GHz. Above 2.3 GHz I don't care very much because the antenna is not going to be built for frequencies above this.
    I am using the high-pass/band-pass filter for the sole purpose of protecting the receive side from the power amps on the transmit side. I am making a diplexer and I have already designed the low-pass filter. I have tried modeling many of the typical filters as described above without good results. The hardest part being the fact that I have to detect a very weak signal and therefore need low insertion loss.
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  5. #4  
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    It sounds to me like you need a new chain if you're not getting enough signal this way.

    Your chain should be:

    Antenna->PreAmp->BPF->PreAmp->BPF->GAIN->SignalRejection->PostProcessing

    They all should have minimal signal distortion. Your first preamp is used to boost the entire received signal so that you have a better signal to filter. Then you filter out your too highs and lows with the BandPass, I'd say Bessel filter or chebychev here, look at their characteristics and then choose. Bessel is slow, but won't distort, chebychev is fast and has a ripple. After this, you amplify your signals again, this time to bring them to a better processing range. Filter again, this time to remove highs and lows added by the amp, then a gain stage for JUST the signal bandwidth. Then you validate the signal, and reject what you don't want. Finally process digitally to get your messages.

    Once again, you should get RF Circuit Design by Boswick, or a suitable microwave circuit design book. Unless you understand how everything works in the chain, using a silly program isn't going to get you very good results.
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    "Those that would give up essential liberty to obtain a little temporary
    safety deserve neither liberty nor safety."

    -Benjamin Franklin, An Historical Review of Pennsilvanya, 1759
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  6. #5  
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    There are many phase shifters and switches in this setup which are causing the noise problem and I have no control over the Tx/Rx chain other than the diplexer and antenna. I think I am going to use a lower performance filter with a self-diplexing antenna. Now I will have trouble with getting the self-diplexing antenna to have enough bandwidth.
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