[hpsdr] Horton LO

[Arthur J. Lekstutis]

That's what I'm trying to say: I think you need to have some analog 
circuitry after the fractional N divider, otherwise you will have spurs. 
Be it a PLL, a D/A (see next paragraph), or some sort of magical low 
pass filters, it's fundamental to the nature of the beast. If the LO is 
going to be driven directly by the fractional N, there is no way 
digitally to remove those spurs. The clocked output of a fractional N 
can only switch on transitions of the reference, not at exactly the time 
required to generate the desired frequency. Thus the output will have 
jitter whose peak value will be plus or minus half the reference clock.

One option that I have considered, but not even tried to model, is to 
use a low resolution D/A at the output of a home made DDS (FPGA). I'd 
arrange the D/A to behave as if there were a high gain amplifier after a 
conventional DDS, driving the output into saturation except during the 
zero transition. Thus the D/A output would be a solid logical one or 
zero output except during the transition, during which the output 
voltage would be proportional to the remainder in the accumulator (a 
clipped sine wave). In other words: the D/A would use the remainder 
portion of the DDS accumulator only, and be saturated otherwise. The D/A 
output would then be low pass filtered, then sent to a comparator. The 
idea being to increase the speed of the zero transition, and to improve 
its resolution, since in a digital system the transition timing is all 
that's important. A QSD doesn't need a sine wave, but a good square 
wave. Hopefully that would reduce some of the spurs and other noise.

That's just an idea, but I have decided not to pursue it and don't know 
if it has any merit. I've realized that for what I'm trying to 
accomplish, I'd be far better of just using a couple crystals.

Later,
Artie Lekstutis
KC2MFS

> 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 
> <mailto: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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