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

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  • 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 1 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 2 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 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

          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 4 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 5 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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