[hpsdr] Horton LO

[Phil Harman]

HI Guido,

This is a really good suggestion. I also understand that the noise due to fraction N is removed by the LPF response of the PLL by moving the noise to the high end of the spectrum. If  there was a way to remove the noise without the need for a PLL this would be a real breakthrough.

73's Phil...VK6APH 


  ----- Original Message ----- 
  From: Guido 
  To: hpsdr at hpsdr.org 
  Sent: Sunday, June 18, 2006 4:49 AM
  Subject: Re: [hpsdr] Horton LO


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  Bob, this is a very interesting thought. Still trying to get the complete idea.

  Artie, fractional spurs may be compensated by noise shaping in case of a phase-lock loop. The higher the order delta sigma modulators, the more the fractional spur energy (quantization noise) is moved from the phase-lock loop band towards a higher frequency. 

  Assume that there is no phase-lock loop in Bob´s Horton LO idea: There is just a stable crystal clock which is divided by a fractional-N, where the output is fed to a QSD for mixing. Would it still be possible to noise shape in order to compensate for the fractional spurs ? 

  Guido.


  On 6/17/06, Arthur J. Lekstutis <Artie at lekstutis.com> wrote:
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    Hi,

    I can't keep up with you guys as I'm too busy with other projects at the
    moment, so I mostly lurk on this list. Perhaps I misunderstand or missed 
    an email some place.

    Wouldn't a purely digital "fractional N" divider generate significant
    spurs? The duration of any digital signal from a synchronous device
    must, by definition, be multiples of the clock cycle. Trying to get 
    around this with a fractional N divider will result in uncertainty of
    the desired output of approximately the clock period. The jitter would
    equal the clock period, the magnitude would depend on the current
    fraction. I would expect spurs from that. Or am I missing something? 

    Adding a random number generator to a fractional N divider to choice
    when to switch from one frequency to another (which is what I think a
    fractional N divider basically does) might spread out the spurs out, but 
    they'll still be there.

    I think you need to be happy with an integer divider, or introduce an
    analog PLL, if you want to minimize noise.

    Back to lurking....

    Later,
    Artie Lekstutis
    KC2MFS


    >***** High Performance Software Defined Radio Discussion List *****
    >
    >Guido:
    >
    >Doing all of this fractional stuff digitally and never having to convert
    >back to analog is a big win, a huge win.   The facts are the fractional 
    >N, without having to go back to analog, but retaining an all digital
    >character is a huge win since we remove tons of nastiness due to
    >attempting to produce analog waveforms.  Our ULTIMATE resolution should 
    >by a 2^16 word size and in this case that is 2.5 kHz.  It is going to be
    >a big win.
    >
    >Bob
    >
    >
    >


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