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Re: [softrock40] RXTX v6.3 improved signal to noise

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  • Alan
    ... Subject: Re: [softrock40] RXTX v6.3 improved signal to noise ... John, It does not seem that simple to me. My initial thought was After many years of
    Message 1 of 19 , Oct 15, 2012
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      ----- Original Message -----
      Subject: Re: [softrock40] RXTX v6.3 improved signal to noise


      >Those 10 ohm resistors before the mixer are dropping precious nanovolts of wanted signal. Worsening signal to noise ratio.

      John,

      It does not seem that simple to me.
      My initial thought was "After many years of Softrocks how could this be?"
      6dB is a big difference.
      From a practical point of view with a Softrock connected to a good antenna what difference will it make?
      On a sufficiently sensitive low noise receiver SNR is established at the antenna input. Signal/atmospheric etc. noise. Provided the
      receiver adds no noise the SNR is fixed. Indeed with the lower frequencies an attenuator can be used with no effect on SNR. Tony
      uses attenuators in the Ensemble RX.

      Now, maybe on the higher frequencies a Softrock might need to be a little more sensitive. I understand this is sometimes done by
      using a low noise card and reducing op-amp gain. So why, apart from Tom, has nobody suggested removing the resistors?
      What are they doing? Is there something else that is degraded if they are removed? Are they an integral part of the mixer?

      73 Alan G4ZFQ
    • John
      Alan, If I get a chance I will put the resistors back in and measure the minimum detectable signal and compare. DG8SAQ also mentions that the impedance looking
      Message 2 of 19 , Oct 15, 2012
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        Alan,
         
        If I get a chance I will put the resistors back in and measure the minimum detectable signal and compare.
         
        DG8SAQ also mentions that the impedance looking into the mixer is closer to 50 Ohms without the resistors than with them as measured on his vector network analyzer. 
         
        John
      • Milt Cram
        ... Hi All, Shouldn t impedance matching be done with reactive components (transformers, capacitors and inductors)? If the 10 ohm resistors are being used for
        Message 3 of 19 , Oct 15, 2012
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          On 10/14/2012 12:01 PM, John wrote:
          I was reading one of DG8SAQ's pages http://www.mydarc.de/dg8saq/switchimpedance/index.shtml where he measures the input impedance to the RX mixer.
          On the same page he mentions replacing the two 10 Ohm resistors on the mixer inputs with shorting links.
           
          I have just tried this with the folowing result:
           
          With the two 10 Ohm resistors:
           
          Frequency 1.977 MHz
          With a 50 Ohm resistor connected to the SoftRock antenna input HDSDR noise peaks are about -125dB
          With a signal generator set to -93 dBm connected to the SoftRock antenna input the signal peak is about -110 dB 
           
           
          Without the two 10 Ohm resisors:
           
          Frequency 1.977 MHz
          With a 50 Ohm resistor connected to the SoftRock antenna input HDSDR noise peaks are about -125dB
          With a signal generator set to -93 dBm connected to the SoftRock antenna input the signal peak is about -104 dB
           
          This appears to have improved signal + noise / noise ratio by 6 dB
           
          Has anyone else tried this?
           
          John G3UGY

          Hi All,

          Shouldn't impedance matching be done with reactive components (transformers, capacitors and inductors)?  If the 10 ohm resistors are being used for impedance matching, they will both attenuate the signal, and contribute noise power.  This will reduce the S/N.  Removing them should improve S/N....?  If necessary, adjust the turns ratio of the input transformer to provide the impedance match.

          73, Milt
          W8NUE
        • John
          Agreed... John G3UGY Hi All, Shouldn t impedance matching be done with reactive components (transformers, capacitors and inductors)? If the 10 ohm resistors
          Message 4 of 19 , Oct 15, 2012
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            Agreed... John G3UGY
             
             
            Hi All,

            Shouldn't impedance matching be done with reactive components (transformers, capacitors and inductors)?  If the 10 ohm resistors are being used for impedance matching, they will both attenuate the signal, and contribute noise power.  This will reduce the S/N.  Removing them should improve S/N....?  If necessary, adjust the turns ratio of the input transformer to provide the impedance match.

            73, Milt
            W8NUE
          • Milt Cram
            ... Additional comments regarding the 10 ohm resistors-- I m guessing that the 10 ohm resistors are also used to swamp the variations in the on-resistance of
            Message 5 of 19 , Oct 15, 2012
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              On 10/14/2012 12:01 PM, John wrote:
              I was reading one of DG8SAQ's pages http://www.mydarc.de/dg8saq/switchimpedance/index.shtml where he measures the input impedance to the RX mixer.
              On the same page he mentions replacing the two 10 Ohm resistors on the mixer inputs with shorting links.
               
              I have just tried this with the folowing result:
               
              With the two 10 Ohm resistors:
               
              Frequency 1.977 MHz
              With a 50 Ohm resistor connected to the SoftRock antenna input HDSDR noise peaks are about -125dB
              With a signal generator set to -93 dBm connected to the SoftRock antenna input the signal peak is about -110 dB 
               
               
              Without the two 10 Ohm resisors:
               
              Frequency 1.977 MHz
              With a 50 Ohm resistor connected to the SoftRock antenna input HDSDR noise peaks are about -125dB
              With a signal generator set to -93 dBm connected to the SoftRock antenna input the signal peak is about -104 dB
               
              This appears to have improved signal + noise / noise ratio by 6 dB
               
              Has anyone else tried this?
               
              John G3UGY

              Additional comments regarding the 10 ohm resistors--

              I'm guessing that the 10 ohm resistors are also used to swamp the variations in the "on-resistance" of the switches.  At low HF frequencies, external noise probably dominates the S/N and the resistors "stabilize" the input impedance.  At higher frequencies, it would seem desirable to eliminate, as much as possible, resistors in the input circuitry.

              73, Milt
              W8NUE
            • Kees & Sandy
              I agree. Any input impedance matching to the antenna will help and the VNA is a good tool to find out, but I assumed those resistors were in there also to
              Message 6 of 19 , Oct 15, 2012
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                I agree. Any input impedance matching to the antenna will help and the VNA is a good tool to find out, but I assumed those resistors were in there also to maintain balance between the two inputs. Since you can have some resistance variation between the two FET switches, windings, etc adding 10 ohms will swamp those values. Another approach might be to take a good commercial input RF transformer from someone like Mini-Circuits and use it. You can also parallel FETs to "average" the resistance variation. As Milt suggests, you can adjust the turns ratio and then reduce the 10 ohm resistance so you see 50 ohms with the VNA. Another item that I've found makes a difference is to use bifilar wire for the secondary. Regardless of how much/little you twist the wires, it's not as good as bifilar wire.
                 
                73 Kees K5BCQ


                ---------- Original Message ----------
                From: Milt Cram <w8nue@...>
                To: softrock40@yahoogroups.com
                Subject: Re: [softrock40] RXTX v6.3 improved signal to noise
                Date: Mon, 15 Oct 2012 09:21:55 -0500

                 

                On 10/14/2012 12:01 PM, John wrote:
                I was reading one of DG8SAQ's pages http://www.mydarc.de/dg8saq/switchimpedance/index.shtml where he measures the input impedance to the RX mixer.
                On the same page he mentions replacing the two 10 Ohm resistors on the mixer inputs with shorting links.
                 
                I have just tried this with the folowing result:
                 
                With the two 10 Ohm resistors:
                 
                Frequency 1.977 MHz
                With a 50 Ohm resistor connected to the SoftRock antenna input HDSDR noise peaks are about -125dB
                With a signal generator set to -93 dBm connected to the SoftRock antenna input the signal peak is about -110 dB 
                 
                 
                Without the two 10 Ohm resisors:
                 
                Frequency 1.977 MHz
                With a 50 Ohm resistor connected to the SoftRock antenna input HDSDR noise peaks are about -125dB
                With a signal generator set to -93 dBm connected to the SoftRock antenna input the signal peak is about -104 dB
                 
                This appears to have improved signal + noise / noise ratio by 6 dB
                 
                Has anyone else tried this?
                 
                John G3UGY

                Additional comments regarding the 10 ohm resistors--

                I'm guessing that the 10 ohm resistors are also used to swamp the variations in the "on-resistance" of the switches.  At low HF frequencies, external noise probably dominates the S/N and the resistors "stabilize" the input impedance.  At higher frequencies, it would seem desirable to eliminate, as much as possible, resistors in the input circuitry.

                73, Milt
                W8NUE

                 

                 

              • vbifyz
                ... I agree with Milt. Shorting these resistors will improve SNR, but may degrade image rejection. There is a tradeoff, which is also frequency dependent. 73,
                Message 7 of 19 , Oct 15, 2012
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                  --- In softrock40@yahoogroups.com, Milt Cram <w8nue@...> wrote:
                  >

                  > Additional comments regarding the 10 ohm resistors--
                  >
                  > I'm guessing that the 10 ohm resistors are also used to swamp the
                  > variations in the "on-resistance" of the switches. At low HF
                  > frequencies, external noise probably dominates the S/N and the resistors
                  > "stabilize" the input impedance. At higher frequencies, it would seem
                  > desirable to eliminate, as much as possible, resistors in the input
                  > circuitry.
                  >
                  > 73, Milt
                  > W8NUE
                  >

                  I agree with Milt. Shorting these resistors will improve SNR, but may degrade image rejection. There is a tradeoff, which is also frequency dependent.

                  73, Mike
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