jmiles at pop.net
Mon Jun 20 22:51:59 UTC 2011
I'm not familiar with Altera's DDS options, but I will say that Xilinx's DDS
compiler is superb. It can be configured to provide SFDR up to 150 dB (and
I'd notice it if I were getting much less than that in practice.)
As Javier hinted, the reason you can't use the MSB directly is that its
transition point is not necessarily stationary between cycles of the
frequency you're trying to synthesize. It will flop around all over the
place. You need at least a few more bits in most applications -- remember
that in an n-bit word, the magnitude represented by the n-1 LSBs is almost
as much as the bit-n MSB.
When DDS technology was first becoming popular in the 1980s, Qualcomm was
one of the main vendors, and they required external DACs. High-speed DACs
were pricy and used a lot of power, so I imagine that a great many people
tried feeding the MSB directly to the filter, as I did. It could be
feasible at some selected frequencies or at very high clock/output ratios,
but in the general case the output signal is just comically awful.
You would need a truly massive filter to provide the needed flywheel effect
to make up for those missing bits. And it would need to be a BPF, not just
an LPF, because not all of the artifacts associated with output quantization
are above the desired carrier frequency. Sometimes the MSB's toggle period
is going to be shorter than it should be, and sometimes it's going to be
-- john, KE5FX
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-
> bounces at febo.com] On Behalf Of Luis Cupido
> Sent: Monday, June 20, 2011 9:46 AM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] DDS'ery
> Gracias, Javier.
> As you read in my previous email I'm basically
> worried about close-in spurs (those that
> will pass through the PLL loop filter).
> will digest that 4th section... tks.
> Since I'm inside an FPGA... I'm eager to get
> spurs down without leaving the digital world...
> Anyone knows any literature covering that ?
> Luis cupido.
> On 6/20/2011 4:52 PM, Javier Herrero wrote:
> > To reduce the spurii due to quantization distortion. Here is an
> > explanation, in Section 4
> > http://www.analog.com/static/imported-
> > Regards,
> > Javier
> > El 20/06/2011 17:39, Luis Cupido escribió:
> >> Well, if we really need to filter it out
> >> we better filter the MSB and square it
> >> again...
> >> Why having a DAC for ???
> >> Right ?
> >> Luis Cupido.
> >> ct1dmk.
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