[Lf] DDS and spurs

[Andre' Kesteloot]

Graeme Zimmer wrote:

> Johan Bodin wrote
>
> > At first sight, the DDS may look as an ideal frequency source
> > for LF. This may be true for transmitters but not for receivers!
>
> [snip]
>
> > In-band spurious signals are generated due to the limited DAC resolution.
>
> Having built a number of DDS's using discrete ICs, I feel I can add to
> Johan's comments..
>
> The worst spurs are caused by the fact that the DDS is unable to change it's
> output except at the clock edges.
> At frequencies where the output is an exact sub-multiple of the clock, a
> very clean signal is obtained.
> But at all of the intermediate frequency steps, the DDS is actually
> jittering between the two nearest sub-harmonics.
> It is this FM modulation which causes the worst spurs and this sadly has no
> direct cure.
>
> The spurs caused by the DAC can be reduced by using more bits in the system,
> but the FM spurs can only be reduced by using a (much) higher clock
> frequency (there are various other schemes of course).
>
> > Another option is to make the receiver almost totally digital.
>
> The trouble here is that any digital RX will still need a digital VCO (NCO
> or DDS) which will always have the above inherent FM spur problem. If the
> DDS is in software, it will of course have a much lower clock frequency
> (instruction step) than the hardware DDS chips.
>
> I would love to be proven wrong in this rather pesimistic analysis...
>
> coments anyone ?
>
> ................... Zim .................. VK3GJZ