[hpsdr] Horton LO
Phil Harman
pvharman at arach.net.au
Sat Jun 17 20:52:36 PDT 2006
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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