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Re: [softrock40] Newbe question LO clock jitter.

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  • k5nwa
    ... The 74HC74 is rated to work from 60MHz to 75MHz depending on the vendor so 32MHz is not stressing the logic at all. As far as clock skew due to propagation
    Message 1 of 3 , Jan 1, 2008
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      At 07:35 AM 1/1/2008, you wrote:

      >Hi all,
      >I have trawlled through all the files and posts (before getting
      >too bored) and can find no referance to LO clock jitter. I note some
      >ccts use D type flip flops with hysteresis in an attempt to make the
      >edges faster! but this makes the problem even worse as the threshold
      >voltages can move around significantly.
      >
      >Why am I concerned...about to order a softrock IF kit to play with and
      >I dont think 74HC/T logic is fast enough when operating with a 32.5MHz
      >clock (4X my RX 8.125Mhz IF).
      >
      >There is also a need to maintain a consistant Mark/Space ratio for any
      >software I/Q gain-phase correction system to work correctly.
      >
      >Am I missing something, or is this a second order effect compared to
      >other system limitations, like jitter may be completly swamped by my RX
      >PLL 1st LO noise performance!
      >
      >HNY
      >Tony G0OPB
      >

      The 74HC74 is rated to work from 60MHz to 75MHz depending on the
      vendor so 32MHz is not stressing the logic at all. As far as clock
      skew due to propagation issues, that is not a worry since any decent
      software can compensate for it so it's a non-issue also.

      Don't worry, be happy. The SoftRocks work quite well and people are
      amazed how well such a simple hardware setup works, it outperforms
      many commercial rigs.

      Could they be done better? Yes there is room for improvement but it
      will drive up the price and it might not be as noticeable as you may
      think, the key is that the software compensates for imbalances in the
      hardware, on the last SoftRock I built I was getting over 70dB of
      image rejection after a few minutes of calibration.

      The one problem present due to a lack of a pre-amp is LO leakage into
      the antenna terminals, that can be easily fixed when you hook it up
      to your radio, by placing a small buffering amplifier, between your
      radio and the coax leading back to the SoftRock, I used a FET
      follower to isolate the cable from the radio.





      Cecil
      K5NWA
      www.softrockradio.org www.qrpradio.com

      "Blessed are the cracked, for they shall let in the light."
    • brainerd@wildblue.net
      ... You can get 74AC parts that are pin for pin compatible and even available in DIP packages. You can also get 74LVC parts in SOIC that are even faster.
      Message 2 of 3 , Jan 1, 2008
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        On 1 Jan 2008 at 13:35, g0opb wrote:

        > Hi all,
        > I have trawlled through all the files and posts (before getting
        > too bored) and can find no referance to LO clock jitter. I note some
        > ccts use D type flip flops with hysteresis in an attempt to make the
        > edges faster! but this makes the problem even worse as the threshold
        > voltages can move around significantly.
        >
        > Why am I concerned...about to order a softrock IF kit to play with and
        > I dont think 74HC/T logic is fast enough when operating with a 32.5MHz
        > clock (4X my RX 8.125Mhz IF).
        >
        > There is also a need to maintain a consistant Mark/Space ratio for any
        > software I/Q gain-phase correction system to work correctly.
        >
        > Am I missing something, or is this a second order effect compared to
        > other system limitations, like jitter may be completly swamped by my RX
        > PLL 1st LO noise performance!
        >
        > HNY
        > Tony G0OPB
        >
        You can get 74AC parts that are pin for pin compatible and even available
        in DIP packages. You can also get 74LVC parts in SOIC that are even
        faster. Both are available from Mouser and DigiKey. A 74AC74 typically
        clocks to 125 MHz and a 74LVC74 can go to 150 MHz(but operates at 3.3V)

        Dave - WB6DHW
        <http://wb6dhw.com>
        !
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