Re: [Discuss-gnuradio] frequency tuning word quantization

[David I. Emery]

On Sun, Mar 05, 2006 at 12:03:33PM -0800, Eric Blossom wrote:
> Good question.  I've wanted to revisit it myself too.
> As I recall it was to reduce the spurs in the DDC output.
> If you get a chance, please see if you can find relevant papers.
> I suggest searching for "DDS spurs", or something like that.
> 
> In looking at the DDS's from Analog Devices, etc., they maintain a
> full precision frequency tuning word and phase accumulator, and then
> use only the high part to feed into the sin/cos generator.  We also
> do that in the FPGA.  The question is whether we should be doing the
> coarse quanitization of the frequency tuning word the way we are, or
> some other way, or not at all.

        Obviously the sin/cos generator probably has  only limited
precision and certainly any DAC involved certainly does, so feeding in
only an approximation of the phase angle makes some sense.

        The actual accumulated phase, however, can have arbitrary
precision depending on the number of bits in the phase increment and
phase accumulate registers and adders.   It may  be significantly more
precise than the smallest resolvable unit of phase angle at the sin/cos
level or hardware DAC level.

        And the higher the precision of the accumulated phase (more bits
past the decimal point) the smaller the frequency step that can be set
(which in our USRP case would be a big win) - or seen another way, the
more precise the frequency setting can be, given an arbitrary frequency.

        Certainly there is no reason why the NCO should be limited by
anything but bit width of the register/adder path.

        As for spurs resulting from this - I would have to do a
literature check.   But the mechanism for generating them is not
obvious.


> 
> Anybody else got something to contribute to this discussion?

        The above is my attempt...

-- 
   Dave Emery N1PRE,  address@hidden  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
celebration of what could have been, but wasn't and is not to be now either."