2m filtering revisited

Folks

Front end filtering on the FCD continues to be a hot topic for some users. Although the FCD has front end fixed and programmable filtering, particularly at VHF, in the presence of strong out of band signals the front end can become overloaded (as has been discussed before on these pages for example http://www.funcubedongle.com/?page_id=74, http://www.funcubedongle.com/?p=456 and http://www.funcubedongle.com/?p=321).

We have been working on this and with help from the team we are presently looking at using SAW filters which presents a no tune solution. Here are the latest incarnation.

The span is 130MHz to 160MHz, and as you can see the 3dB points are at about 142 and 148MHz. The attenuation to the infamous UK pager frequencies at 138 and 153MHz is 45dB.

This prototype is not pretty! The PCB has been re-laid out and is 1″x1″ to be able to fit inside the feed point of an antenna.

It is powered either from the coax (it is designed to be comapatible with the 5V bias T in the 1.1 version of the FCD) or it can be powered separately.

Howard

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13 Responses to 2m filtering revisited

  1. Guillermo says:

    Hi. Thanks for still thinking in make easy our hobby !.
    Now the questios is: where we could found the SAW filter or the PCB or order the complete Kit or item ?
    Thanks in advance
    Guillermo

    • admin says:

      Hi Guillermo,

      The team is still working on this project. The SAW filter I used here is quite expensive, about €10. There is another one being looked at which is much cheaper.

      Howard

  2. EB5AGV Jose says:

    Hi Howard,

    Thanks a lot!. This will be for sure a nice addition to an already wonderful Project. Count me in for any kind of beta-testing you may think. I have a well-equipped RF lab.

    73 JOSE EB5AGV

  3. Mike says:

    Great! Just what we need.

    Mike

    • Nidge (G0NIG) says:

      From my home using the dongle on a standard VHF/UHF co-linear I have only
      experienced very marginal blocking from local pagers. The local antenna
      mast that carries a 1KW DAB multiplexer and three 100W+ pagers on 137MHz is about a mile away.

      My local radio club is about 1/2 mile away and 1/2mile closer to the mast. Here the Funcube Dongle started to suffer from blocking in the 2m band and
      VHF airband but was still usable. To be honest the amount of receiver degridation encoutered by the Dongle is not a lot different to that which I experience when I connect a widerange handheld receiver to a medium gain
      VHF/UHF antenna in this area.

      I’ve only had the Dongle a few days, it arrived Tuesday morning but my experiences so far have been very encouraging. My mast currently only has a VHF/UHF vertical antenna on it but I’m hoping to get my 19 ele LPDA (100MHz-1300MHz) out of retirement in the next few weeks.

      The proposed filter option will certainly be a welcome addition to the receiver chain regardless of the users situation and I’ll be keeping an eye on any future developement.

      Regards

      Nidge

  4. Michael Roe says:

    Thanks for working on this!

    My FCD can receive Radio Cambridgeshire at round about 96MHz (distorted, because the broadcast FM signal is wider than it can sample) and some very strong intermittent signals around 153MHz (pagers, at a guess), but in brief tests so far I’ve not had a lot of luck with it on the 2m amateur band, which is what I’m really interested in.

    When I get some more time to work on it, I’m planning to try it with better antennas, antenna placement and/or filters, and hoping it’ll work better.

    • admin says:

      Hello Michael

      This can be a number of things, especially if you are using an antenna right on the FCD next to the computer: computers generate lots of nasties!

      Generally if your noise floor rises by tens of dB when you hook up an antenna, there’s something overloading, or blocking, the front end, so it will appear to be insensitive. Either this is local hash from your computer (I have a rather noisy flourescent light in my kitchen for example) or if you are using an external antenna you may have desense caused by pagers or band II FM broadcast for example.

      It might sound counter intuitive, but decreasing the mixer gain actually increases the receiver’s dynamic range for example.

      Certainly if you are using an inside antenna, move it as far away from your computer as you can, and preferably by a window for example.

      Howard

  5. Vy nice, and small, too! is there no need for a similar one on 70 cm band, is not it?
    The vhf problem is eventually for those who have a non bias-T one.
    I’ve bought some day ago a small sturdy VHF bandpass filter by Chase , small but still many times the FCD volume, hi, will test it as soon I’ll receive some SMA teflon surplus cables.
    Howard I’ ve sent you a msg with some personal questions of mine. Did you get it?
    73 es congrats for the nice job! Augusto i2jjr/9h3jr

    • admin says:

      Hello Augusto

      I have some 70cm SAW filters too but they are in a different package.

      You will be able to power externally too if you have no bias tee.

      I haven’t received an email from you for some time, let me know when it was, I’ll check my spam folders.

      Howard

      • Hi Howard,
        sorry, I’ve not seen your answer until today.

        Inbetween I went on the decision to put my FCD for sale at the cost, so the message is not important anymore. I had very silly problems putting sat-following antennas on the roof ….

        I’ve tested the FCD with the Chase filter I wrote about, but still the desensing is present as filter bandpass includes some vy strong signals. I should retune that filter, narrower, but I have no suitable instruments here, so nothing to do.
        The solution , being interested in VHF wideband, could be a tuneable multicell filter and this could also require a preamp, but at this point, as I have neither instrumentation nor a lab place anymore, where to work, it’s all over , sadly.

        In the 430 Mhz band, it behaves very good, it’s nice, using a whip for 70cm on the balcony here at 1st floor, I have received a lot of signals on 433,92 by gate controls and from other gadgets . It’s very sensitive.
        What would have been of my interest were having a quick and continuous sweep mode; maybe it will be implemented next in the QTHID , who knows. The actual scan utility is too slow.

        If you know or hear of somebody interested into my FCD, would be better in I6, I2, or southern HB9, let him know about mine, so I may give it him de visu.

        Oncemore congrats for the fine work you’re doing; I will have a look on the SAW filters here, from time to time, very interesting .

        Have my best regards, Howard.
        73 de Augusto
        i2jjr 9h3jr hb9tza

  6. Joris PE1KTH says:

    Hi Howard,
    I am interested in the trade mark and type number of the two SAW filters you are experimenting with for 145 MHz and 435 Mhz. Helicals are more costly.
    Are you willing to inform me.

    Thanks Joris

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