[time-nuts] Short-Term Stability

Magnus Danielson cfmd at bredband.net
Thu Mar 16 18:50:38 EST 2006

From: "Poul-Henning Kamp" <phk at phk.freebsd.dk>
Subject: Re: [time-nuts] Short-Term Stability
Date: Thu, 16 Mar 2006 18:32:32 +0100
Message-ID: <81211.1142530352 at critter.freebsd.dk>

> In message <006901c6491d$73a48980$0201a8c0 at n4fs>, "Mike Feher" writes:
> >OK, I glanced through the paper. Similar concept to what we used to do with
> >adding analog noise prior to quantization to improve overall dynamic range.
> >I do not understand however what this has to do with reducing spurious in a
> >DDS. Are you saying that you need an extra bit in the wave shaping ROM that
> Let me try to explain the principle:
> When you truncate or round your "theoretical" bits in order to send
> them to the DAC, you essentially add a +/- .5 bit random signal
> which is the the rounding error.

To clarify a bit if I may... the truncation of phase-state results in a +/-
1/2 DDS clock cycle time error, this phase-quantization error becomes an
additional signal to that of the carrier frequency...

> This noise signal, like all signals, has a frequency spectrum,
> which amongst other things, is where most of your spurious lines
> live.
> So instead of just using an "arbitrary algorithm" such as "round to
> nearest" or "truncation", you can choose to use your own bitstream.
> You arrive at that bitstream by calculate the rounding error, and
> filter it to optimize the desired frequency behaviour.
> This filtered version will typically only differ from the "rounded"
> or "truncated" version in few bits, but boy what a difference that
> makes (ie: 20dB better stopband on 69 pole FIR).
> For DDS, all you need is an extra bit to go along and say "round
> the opposite way this time", and precompute that for the desired
> DDS frequency.

Interesting to hear.


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