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Newbe question LO clock jitter.

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  • g0opb
    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
    Message 1 of 3 , Jan 1, 2008
      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
    • 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 2 of 3 , Jan 1, 2008
        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 3 of 3 , Jan 1, 2008
          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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