[hpsdr] Horton LO

[Guido]

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:
>
> ***** High Performance Software Defined Radio Discussion List *****
>
> 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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