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Ensemble RXTX Output Linearity ?

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  • Jim Whitaker
    Anyone know what the maximum output power still within linearity is? I ve seen as high as 3 watts output (too much drive audio) and wondering what the overall
    Message 1 of 6 , May 28, 2012
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      Anyone know what the maximum output power still within linearity is? I've seen as high as 3 watts output (too much drive audio) and wondering what the overall safe zone is... I run digital at basically 100% duty cycle for 47 seconds on and 73 seconds off...


      de n8xyn
    • Alan
      ... From: Jim Subject: [softrock40] Ensemble RXTX Output Linearity ? ... I find a true 3W difficult to believe. All my RXTXs start to show compression below 1
      Message 2 of 6 , May 28, 2012
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        ----- Original Message -----
        From: "Jim
        Subject: [softrock40] Ensemble RXTX Output Linearity ?


        > Anyone know what the maximum output power still within linearity is? I've
        > seen as high as 3 watts output (too much drive audio)

        I find a true 3W difficult to believe.
        All my RXTXs start to show compression below 1 watt. I do not think I have
        seen 1.5 watts from any using full drive from a D44. But I never use more
        than 1 watt.
        0.5 watts is probably the safest, especially if driving an amplifier.

        73 Alan G4ZFQ
      • Jim Whitaker
        ... Might or might not have been true, I killed it fast for obvious reasons. ... The only oscillascope I have is a handheld 5 Mhz unit. Would using a second
        Message 3 of 6 , May 28, 2012
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          > I find a true 3W difficult to believe.

          Might or might not have been true, I killed it fast for obvious reasons.

          > All my RXTXs start to show compression below 1 watt. I do not think I have
          > seen 1.5 watts from any using full drive from a D44. But I never use more
          > than 1 watt.
          > 0.5 watts is probably the safest, especially if driving an amplifier.
          >
          > 73 Alan G4ZFQ
          >


          The only oscillascope I have is a handheld 5 Mhz unit. Would using a second receiver be of any use to see where the sine wave starts to distort?

          de n8xyn
        • KF4BQ
          ... Hi Jim, An oscilloscope will be of limited use for measuring distortion at low levels. Ideally you can use either a SDR or other audio program to generate
          Message 4 of 6 , May 28, 2012
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            --- In softrock40@yahoogroups.com, "Jim Whitaker" <jimawhitaker@...> wrote:
            >
            > The only oscillascope I have is a handheld 5 Mhz unit. Would using a second receiver be of any use to see where the sine wave starts to distort?
            >
            > de n8xyn
            >

            Hi Jim,

            An oscilloscope will be of limited use for measuring distortion at low levels. Ideally you can use either a SDR or other audio program to generate a two tone test. Measure the output level with a single tone or both(average or peak important) Example: with a single tone at .25W; putting out two tones will be .5W average and also 1W peak(tone voltages sum, making 4X power peak).

            Using a external receiver (as a spectrum analyzer) you can measure the levels of each tone (thru proper power attenuators/dummy loads..) and compare to the 3rd order products that are spaced away from the desired two tones (by the tone frequency difference). 3rd order products can be specified relative to one of the two desired tones or .. as ARRL does to the PEP level which gives 6dB "better" results. A 3rd order product of maybe -20dB or -26dBPEP would be average/poor linearity. -30dB is real good.

            73, Mike Collins KF4BQ
          • Alan
            ... From: Jim Subject: [softrock40] Re: Ensemble RXTX Output Linearity ? ... I think looking at the trace on an oscilloscope is not going to show distortion
            Message 5 of 6 , May 28, 2012
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              ----- Original Message -----
              From: "Jim
              Subject: [softrock40] Re: Ensemble RXTX Output Linearity ?



              >
              > The only oscillascope I have is a handheld 5 Mhz unit. Would using a
              > second receiver be of any use to see where the sine wave starts to
              > distort?
              >

              I think looking at the trace on an oscilloscope is not going to show
              distortion before it gets severe. But I do not often use one.

              The method I use is to feed the Softrock from something that gives a
              calibrated output.
              Some soundcard mixers give dB steps, WSPR does and IQout. I checked my D44
              and the others with an attenuator and these seemed reasonably accurate.
              Using a diode probe across a dummy load enables power to be measured. Well,
              calculated from the voltage readings. A 10M ohm meter will show peak
              voltage.
              You can then see when a dB increase in input does not produce a dB extra in
              output. Then the 1dB point can be estimated.

              However, there are better methods, I see Mike has contributed one. I have a
              homemade "poor man's spectrum analyser" that seems to work well with a USB
              scope.

              73 Alan G4ZFQ
            • Jim Whitaker
              ... Thanks for both of the responses, now I have to go find a few tools and get my ambition up. I ve been running at 1 watt PEP for the last month and didnt
              Message 6 of 6 , May 28, 2012
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                --- In softrock40@yahoogroups.com, "KF4BQ" <mikecol@...> wrote:
                >
                > --- In softrock40@yahoogroups.com, "Jim Whitaker" <jimawhitaker@> wrote:
                > >
                > > The only oscillascope I have is a handheld 5 Mhz unit. Would using a second receiver be of any use to see where the sine wave starts to distort?
                > >
                > > de n8xyn
                > >
                >
                > Hi Jim,
                >
                > An oscilloscope will be of limited use for measuring distortion at low levels. Ideally you can use either a SDR or other audio program to generate a two tone test. Measure the output level with a single tone or both(average or peak important) Example: with a single tone at .25W; putting out two tones will be .5W average and also 1W peak(tone voltages sum, making 4X power peak).
                >
                > Using a external receiver (as a spectrum analyzer) you can measure the levels of each tone (thru proper power attenuators/dummy loads..) and compare to the 3rd order products that are spaced away from the desired two tones (by the tone frequency difference). 3rd order products can be specified relative to one of the two desired tones or .. as ARRL does to the PEP level which gives 6dB "better" results. A 3rd order product of maybe -20dB or -26dBPEP would be average/poor linearity. -30dB is real good.
                >

                Thanks for both of the responses, now I have to go find a few tools and get my ambition up. I've been running at 1 watt PEP for the last month and didnt even think about looking at purity until yesterday when I saw a station drifting and producing harminics of his signal no more then a few cycles apart. Made me wonder since without voice no one ever reports on quality just quanity...

                de n8xyn


                > 73, Mike Collins KF4BQ
                >
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