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Re: [FUNcube] FCD - Optimum gain settings ?

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  • Iz Oos
    Hi Martin,   I use a vertical for 144-432-1296Mhz on the roof or a logper 130-1300Mhz with about 14db of preamp gain to overcome the losses of 30meters
    Message 1 of 8 , Mar 17, 2012
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      Hi Martin,
       
      I use a vertical for 144-432-1296Mhz on the roof or a logper 130-1300Mhz with about 14db of preamp gain to overcome the losses of 30meters RG8XX. I had so much intermod from out of band signals that I could not receive anything at all whatever the gain settings of the Funcube. After using helical filters for 144-432-1296Mhz I hear everything, I mean everything everything!
      The optimal gain setting using the filters are (for me):
      144Mhz: LNA Gain between +10 and +20dB, Mixer Gain 4dB
      432Mhz: LNA Gain +20dB, Mixer Gain 4dB
      1296Mhz: LNA Gain +25dB, Mixer Gain 4dB
       
      the only issue I can now see is intermod from in band signals where front-end filters cannot do much. When I see intermods I set 10db of gain less than I would otherwise. 
      As for 1296Mhz, today I experimented a high pass filter: LNA Gain could easily set +25dB or more, Mixer Gain 4dB. No signs of intermod detected.
      Today also I started to look for 70Mhz band as well, using a SAW filter and a Notch filter against FM broadcasters. Best LNA gain was 10-15db.

      73
      Franco iz2oos

      Da: g8jnj <martin_ehrenfried@...>
      A: funcube@...
      Inviato: Sabato 17 Marzo 2012 19:14
      Oggetto: [FUNcube] FCD - Optimum gain settings ?

       
      Hi All,

      Just got my FCD up and running with SDRRADIO.

      Using a dual band 145/433MHz antenna on the roof. No pre-amps or external filters (at the moment).

      Not much luck with the default gain settings, too much intermod from out of band signals. But after some experimentation I'm now getting good results on most VHF & UHF frequencies. With only very occasional bursts of interference.

      LNA Gain +5dB
      RF Filter 268MHz
      Mixer Gain 12dB
      Mixer Filter 1.9MHz
      IF Gain 1 +6dB
      IF RC Filter 1.0MHz
      IF Gain 2 +3dB
      IF Gain 3 +3dB
      IF Gain 4 0dB
      IF Filter 2.15MHz
      IF Gain 5 +3dB
      IF Gain 6 +3dB
      LNA Enhance Off
      Bias Current 11 V/U Band
      IF Gain Mode Linearity

      Sound card attenuation set to 30dB.

      I played with various combinations of LNA, mixer and IF Gains, but these settings gave the best S/N ratios for UHF sat downlinks in the 250 and 437MHZ bands.

      The downside is that the DC offset is rather high with respect to eanted signals. Making tuning at the centre of the screen unusable.

      I'd be interested to hear of other users experiences, and prefered settings.

      Regards,

      Martin - G8JNJ

      www.g8jnj.webs.com



       
    • g8jnj
      Hi Franco, Thanks for the reply - interesting results. I found that it was better to keep the mixer gain high at +12dB and the LNA gain low at +5dB (+10dB at
      Message 2 of 8 , Mar 18, 2012
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        Hi Franco,

        Thanks for the reply - interesting results.

        I found that it was better to keep the mixer gain high at +12dB and the LNA gain low at +5dB (+10dB at the most).

        I suspect that the LNA is not quite as low noise as it should be. So adding additional LNA gain doesn't do much to improve the overall S/N. But it does increase the risk of introducing more intermodulation products.

        It would be interesting to perform some RX noise measurements with different gain settings.

        My personal opinion at the moment is to go for an external low noise amplifier at the antenna. Followed by a bandpass filter. Feeding the FCB optimised for best strong signal handling rather than absolute sensitivity.

        Regards,

        Martin - G8JNJ

        www.g8jnj.webs.com


        --- In funcube@..., Iz Oos <iz2oos@...> wrote:
        >
        > Hi Martin,
        >  
        > I use a vertical for 144-432-1296Mhz on the roof or a logper 130-1300Mhz with about 14db of preamp gain to overcome the losses of 30meters RG8XX. I had so much intermod from out of band signals that I could not receive anything at all whatever the gain settings of the Funcube. After using helical filters for 144-432-1296Mhz I hear everything, I mean everything everything!
        > The optimal gain setting using the filters are (for me):
        > 144Mhz: LNA Gain between +10 and +20dB, Mixer Gain 4dB
        > 432Mhz: LNA Gain +20dB, Mixer Gain 4dB
        > 1296Mhz: LNA Gain +25dB, Mixer Gain 4dB
        >  
        > the only issue I can now see is intermod from in band signals where front-end filters cannot do much. When I see intermods I set 10db of gain less than I would otherwise. 
        > As for 1296Mhz, today I experimented a high pass filter: LNA Gain could easily set +25dB or more, Mixer Gain 4dB. No signs of intermod detected.
        > Today also I started to look for 70Mhz band as well, using a SAW filter and a Notch filter against FM broadcasters. Best LNA gain was 10-15db.
        >
        > 73
        > Franco iz2oos
        >
        >
        > ________________________________
        > Da: g8jnj <martin_ehrenfried@...>
        > A: funcube@...
        > Inviato: Sabato 17 Marzo 2012 19:14
        > Oggetto: [FUNcube] FCD - Optimum gain settings ?
        >
        >
        >
        >  
        >
        > Hi All,
        >
        > Just got my FCD up and running with SDRRADIO.
        >
        > Using a dual band 145/433MHz antenna on the roof. No pre-amps or external filters (at the moment).
        >
        > Not much luck with the default gain settings, too much intermod from out of band signals. But after some experimentation I'm now getting good results on most VHF & UHF frequencies. With only very occasional bursts of interference.
        >
        > LNA Gain +5dB
        > RF Filter 268MHz
        > Mixer Gain 12dB
        > Mixer Filter 1.9MHz
        > IF Gain 1 +6dB
        > IF RC Filter 1.0MHz
        > IF Gain 2 +3dB
        > IF Gain 3 +3dB
        > IF Gain 4 0dB
        > IF Filter 2.15MHz
        > IF Gain 5 +3dB
        > IF Gain 6 +3dB
        > LNA Enhance Off
        > Bias Current 11 V/U Band
        > IF Gain Mode Linearity
        >
        > Sound card attenuation set to 30dB.
        >
        > I played with various combinations of LNA, mixer and IF Gains, but these settings gave the best S/N ratios for UHF sat downlinks in the 250 and 437MHZ bands.
        >
        > The downside is that the DC offset is rather high with respect to eanted signals. Making tuning at the centre of the screen unusable.
        >
        > I'd be interested to hear of other users experiences, and prefered settings.
        >
        > Regards,
        >
        > Martin - G8JNJ
        >
        > www.g8jnj.webs.com
        >
      • n0jy
        I agree with Martin in keeping the LNA gain low, although last week while traveling on vacation and using two Radio Shack alligator clip jumper wires as the
        Message 3 of 8 , Mar 18, 2012
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          I agree with Martin in keeping the LNA gain low, although last week
          while traveling on vacation and using two Radio Shack alligator clip
          jumper wires as the antenna I did run +20db on the LNA and it did add
          enough oomph to be able to give me decent coverage on railroad
          frequencies (161.520).

          73,
          Jerry
          N0JY
        • iz2oos
          Martin, I thorougly agree with you. The external low noise amplifier will help a lot in terms of sensititivity. In my case I preferred an external amplifier
          Message 4 of 8 , Mar 18, 2012
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            Martin,

            I thorougly agree with you. The external low noise amplifier will help a lot in terms of sensititivity. In my case I preferred an external amplifier that could cope well strong signals. So I selected a single broadband MMIC with just 14db of gain with about 30IP3, which is a respetable value. Using an amplifier with higher gain I would have had lots of intermodulations within the amplifier even if this had a higher IP3. Check the IP3 of the low noise amplifier. I see often low noise amplifiers that have lots of gain and poor IP3, and are also very expensive. They are not my favorite ones as they may likely generate lots of intermods if they don't have band pass filters themselves.

            Using the band pass filter in front of the funcube, I think you will likely prefer to increase the gain setting of the LNA and keep lower the gain of the mixer. I think the mixer gain is somewhat worse than the LNA.

            73,

            Franco iz2oos



            --- In funcube@..., "g8jnj" <martin_ehrenfried@...> wrote:
            >
            > Hi Franco,
            >
            > Thanks for the reply - interesting results.
            >
            > I found that it was better to keep the mixer gain high at +12dB and the LNA gain low at +5dB (+10dB at the most).
            >
            > I suspect that the LNA is not quite as low noise as it should be. So adding additional LNA gain doesn't do much to improve the overall S/N. But it does increase the risk of introducing more intermodulation products.
            >
            > It would be interesting to perform some RX noise measurements with different gain settings.
            >
            > My personal opinion at the moment is to go for an external low noise amplifier at the antenna. Followed by a bandpass filter. Feeding the FCB optimised for best strong signal handling rather than absolute sensitivity.
            >
            > Regards,
            >
            > Martin - G8JNJ
            >
            > www.g8jnj.webs.com
            >
            >
            > --- In funcube@..., Iz Oos <iz2oos@> wrote:
            > >
            > > Hi Martin,
            > >  
            > > I use a vertical for 144-432-1296Mhz on the roof or a logper 130-1300Mhz with about 14db of preamp gain to overcome the losses of 30meters RG8XX. I had so much intermod from out of band signals that I could not receive anything at all whatever the gain settings of the Funcube. After using helical filters for 144-432-1296Mhz I hear everything, I mean everything everything!
            > > The optimal gain setting using the filters are (for me):
            > > 144Mhz: LNA Gain between +10 and +20dB, Mixer Gain 4dB
            > > 432Mhz: LNA Gain +20dB, Mixer Gain 4dB
            > > 1296Mhz: LNA Gain +25dB, Mixer Gain 4dB
            > >  
            > > the only issue I can now see is intermod from in band signals where front-end filters cannot do much. When I see intermods I set 10db of gain less than I would otherwise. 
            > > As for 1296Mhz, today I experimented a high pass filter: LNA Gain could easily set +25dB or more, Mixer Gain 4dB. No signs of intermod detected.
            > > Today also I started to look for 70Mhz band as well, using a SAW filter and a Notch filter against FM broadcasters. Best LNA gain was 10-15db.
            > >
            > > 73
            > > Franco iz2oos
            > >
            > >
            > > ________________________________
            > > Da: g8jnj <martin_ehrenfried@>
            > > A: funcube@...
            > > Inviato: Sabato 17 Marzo 2012 19:14
            > > Oggetto: [FUNcube] FCD - Optimum gain settings ?
            > >
            > >
            > >
            > >  
            > >
            > > Hi All,
            > >
            > > Just got my FCD up and running with SDRRADIO.
            > >
            > > Using a dual band 145/433MHz antenna on the roof. No pre-amps or external filters (at the moment).
            > >
            > > Not much luck with the default gain settings, too much intermod from out of band signals. But after some experimentation I'm now getting good results on most VHF & UHF frequencies. With only very occasional bursts of interference.
            > >
            > > LNA Gain +5dB
            > > RF Filter 268MHz
            > > Mixer Gain 12dB
            > > Mixer Filter 1.9MHz
            > > IF Gain 1 +6dB
            > > IF RC Filter 1.0MHz
            > > IF Gain 2 +3dB
            > > IF Gain 3 +3dB
            > > IF Gain 4 0dB
            > > IF Filter 2.15MHz
            > > IF Gain 5 +3dB
            > > IF Gain 6 +3dB
            > > LNA Enhance Off
            > > Bias Current 11 V/U Band
            > > IF Gain Mode Linearity
            > >
            > > Sound card attenuation set to 30dB.
            > >
            > > I played with various combinations of LNA, mixer and IF Gains, but these settings gave the best S/N ratios for UHF sat downlinks in the 250 and 437MHZ bands.
            > >
            > > The downside is that the DC offset is rather high with respect to eanted signals. Making tuning at the centre of the screen unusable.
            > >
            > > I'd be interested to hear of other users experiences, and prefered settings.
            > >
            > > Regards,
            > >
            > > Martin - G8JNJ
            > >
            > > www.g8jnj.webs.com
            > >
            >
          • rein0zn@ix.netcom.com
            Hello All, I have mainly tested and played with the Funcube at 1296 Mhz with a weak beacon and have been pleasantly surprised by its performance. Is there any
            Message 5 of 8 , Mar 18, 2012
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              Hello All,

              I have mainly tested and played with the Funcube at 1296 Mhz with a weak
              beacon and have been pleasantly surprised by its performance.

              Is there any write-up or info available on all those settings other
              the input LNA gain?

              73 Rein W6SZ


              -----Original Message-----
              >From: n0jy <n0jy@...>
              >Sent: Mar 18, 2012 4:05 PM
              >To: funcube@...
              >Subject: Re: [FUNcube] Re: FCD - Optimum gain settings ?
              >
              >I agree with Martin in keeping the LNA gain low, although last week
              >while traveling on vacation and using two Radio Shack alligator clip
              >jumper wires as the antenna I did run +20db on the LNA and it did add
              >enough oomph to be able to give me decent coverage on railroad
              >frequencies (161.520).
              >
              >73,
              >Jerry
              >N0JY
              >
            • Addy de Jongh
              Hello All, I have the same experience with a weak and pulsing VHF beacon, but at 148 Mhz. The Funcube is surprisingly good with this. I also would like to see
              Message 6 of 8 , Mar 18, 2012
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                Hello All,

                 

                I have the same experience with a weak and pulsing VHF beacon, but at 148 Mhz. The Funcube is surprisingly good with this.

                 

                I also would like to see such write-up about all the settings. Please who has such?

                 

                Best wishes,

                3dotter

                 

                Van: funcube@... [mailto:funcube@...] Namens rein0zn@...
                Verzonden: 18 March 2012 18:03
                Aan: funcube@...
                Onderwerp: Re: [FUNcube] Re: FCD - Optimum gain settings ?

                 

                 

                Hello All,

                I have mainly tested and played with the Funcube at 1296 Mhz with a weak
                beacon and have been pleasantly surprised by its performance.

                Is there any write-up or info available on all those settings other
                the input LNA gain?

                73 Rein W6SZ

                -----Original Message-----

                >From: n0jy <n0jy@...>
                >Sent: Mar 18, 2012 4:05 PM
                >To: funcube@...
                >Subject: Re: [FUNcube] Re: FCD - Optimum gain settings ?
                >
                >I agree with Martin in keeping the LNA gain low, although last week
                >while traveling on vacation and using two Radio Shack alligator clip
                >jumper wires as the antenna I did run +20db on the LNA and it did add
                >enough oomph to be able to give me decent coverage on railroad
                >frequencies (161.520).
                >
                >73,
                >Jerry
                >N0JY
                >

              • Peter Blakey
                I don t think that there any gain settings that are globally optimum - it all depends on the local e/m environment regarding unwanted signals, the
                Message 7 of 8 , Mar 19, 2012
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                  I don't think that there any gain settings that are globally optimum - it all depends on the local e/m environment regarding unwanted signals, the characteristics (including the frequency) of the signal you are trying to receive, the antenna you are using, and whatever front-end filtering you are able to supply.

                  The tuner chip used by the FCD (an Elonics E4000) has very variable performance as a function of frequency. The best noise performance occurs at UHF and is around 3.8dB. At other frequencies it is worse, and at some frequencies it is much worse. This is due to the vagaries of the design, including the influence of the clock frequency. I have heard that the noise figure spikes to over 14dB at some VHF frequencies! Presumably this is why the design includes a Si MMIC front end that is spec'd at 20 dB of gain with a 2.3dB noise figure, to bring down the overall noise figure - but as a result there is a ton of gain (up to 50dB before the mixer!), which with little or no filtering leads to severe intermod and desense problems. If you use a High Sierra or other HF converter you can have up to 70 dB of gain prior to the mixers of the E4000!

                  For what I am interested in, I have pretty much gone back to using VHF and UHF down-converters that feed a regular HF SDR. The FCD is a great concept, and the implementers have done many things heroically well, but the design compromises associated with the limitations of the tuner IC undermine the performance to a level that I find disappointing. A person can usually make the FCD work reasonably well if they know what they are doing - but for any particular application there will normally be other ways to get better performance. 

                  Please feel free to dispute anything I have said, I'm just trying to help people to understand the issues ... 

                  73,

                  Peter Blakey (formerly G8DND)   
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