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Re: Fw: [softrock40] DDS spurs, was Got my 5.0....Now what do I need?

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  • Joe Rocci
    Frank, You asked: Why does the SR5 have spur problems while SR4 does not? The SR4 has to run at X4 Fo so you would think the spurs would be worse. Since my
    Message 1 of 43 , Jan 2, 2006
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      Frank,
       
      You asked:
       
      "Why does the SR5 have spur problems while SR4 does not? The SR4 has to run at X4 Fo so you would think the spurs would be worse. Since my thought was wrong."
       
      I'm sorry Frank, but I haven't been following this group long enough to be knowledgeable about the peculiarities of any specific hardware or software implementation. Perhaps you could direct me to information about these versions you referred to so I can offer an opinion (for whatever that's worth)?.
       
       
      Joe
      W3JDR
       
       
      ----- Original Message -----
      Sent: Monday, January 02, 2006 10:47 AM
      Subject: Re: Fw: [softrock40] DDS spurs, was Got my 5.0....Now what do I need?

      Joe,
      Why does the SR5 have spur problems while SR4 does not? The SR4 has to run at X4 Fo so you would think the spurs would be worse. Since my thought was wrong. frank  

      Joe Rocci <joe@...> wrote:
      "A dds into dividers like flip flops or a comparator eliminates the effect or spurs over most ot the clock cycle. It only can change the time of the switch slightly. The rest of the cycle is not efected"
       
      Francis,
       
      How do you explain the above statement? I believe that most of the DDS spurs are phase related, which means that they are manifested as jitter in the zero-crossings of the sine-wave output. Squaring-up the signal in a digital circuit might remove amplitude-related spurious signal components, but I don't see how it can do anything for phase related spurious signal components. Furthermore, even if it were possible to improve the amplitude-related spurs by running the LO through a digital circuit, then the mixer (especially digital mixers such as the Tayloe detector) would accomplish the same thing.
       
      Joe
      W3JDR


    • FRANCIS CARCIA
      Double balanced mixers love square wave LO and produce less distortion due to the speed through the switch point. A square wave is easier to filter into a sine
      Message 43 of 43 , Jan 24, 2006
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        Double balanced mixers love square wave LO and produce less distortion due to the speed through the switch point. A square wave is easier to filter into a sine than going the other way.

        KY1K <ky1k@...> wrote:
        At 09:25 PM 1/22/2006, you wrote:

        >Very true! I'm hoping some sharp fpga programmer figures out how to
        >make a variable frequency square wave because we don't need no
        >stinkin sine wave anyway.

        I'm sure this is highly debatable. If your frequency is low enough
        for a softrock-40 type application, then a square wave is fine.

        But, most of us need sinewaves............

        >The DDS has a way to go before it can compete with a good synthesizer design.
        >It is easy it is quick but it is not clean enough to be called premium grade.


        No argument here. DDS offers much though. A clean DDS would be a
        giant leap forward in the softrock type receiver/transceiver
        development effort.

        Does anyone know what frequency ECL logic can be clocked at?

        Art



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