Fw: [softrock40] Re: DDS spurs
- Like Allen says, "In practice, pcb layout, with adequate grounding and active component decoupling is extremely important, so keep this in mind if you wish to do some experimenting".What I've learned so far on making these prototype boards:1) Use short wiring paths everywhere (that's why it pays to just make a littleprototype imbedded board for these designs vs leads all over the place on abreadboard at 100Mhz+2) Look at the GROUND paths (these are all 2 sided boards) and make sure theyare not blocked by clearances and wiring. This is VERY IMPORTANT, not only forminimizing ground loops. Minimize wiring on the ground plane(locating someactive components there is good, also ALL the decoupler caps)3) Locate all the regulators on the bottom for closer proximity to the heat sinkingground plane.4) Always use "fill area" on the ground plane. Not necessarily on the top. Removeresulting "slivers" ....they will just come off and short something.5) Put a decoupler cap at EACH voltage pin, on the bottom, AT THE PIN ....no "wiring"from the pad to the via.6) Punch two vias (one on each side of the lead pad) to the ground plane at eachground pin ....again no "wiring" between the pad and the via.7) Look at the wiring up close and then from far away (can't see the forrest for thetrees situations)....caught a few wiring simplifications like that. Check and re-check, then wait a day and check again ....then get some boards.8) Print out a 1:1 copy and try placing all the components. This will show you ifspacing is too small or if you selected the wrong 28 pin SSOP footprint (#!%%##!!caught me on Pass #2).73 Kees K5BCQ----- Forwarded Message -----From: "allen_fulmer" <afulmer@...>Date: Sun, 22 Jan 2006 16:36:45 -0000Subject: [softrock40] Re: DDS spursMessage-ID: <email@example.com>
divider on spurs, namely 20 Log multiplied by the ratio of the divider.
I did not anticipate anyone having trouble interpreting or using this
to predict the effect on spurs but obviously a couple of members have.
For example if a simple divide by two is used, Log 2.0 = 0.30103, and
20 log 2.0 = 6.0206, so one would expect a spur reduction of
approximately 6 db. (I assume the familiar factor of 20 arises
from the consideration that we are concerned with the power contained
in the spur.)
For each following divide by two the the equation predicts the spur is
reduced by an additional 6 db., so that a divide by four results in a
12db reduction, a divide by eight an 18db reduction, and so on. I
decade or divide by 10 counter results in a 20 db reduction. (log
10.0 = 1, 20 x 1 = 20)
There are other important considerations, both in theory and in
practice regarding spur reduction and noise reduction in general, but
it is my understanding that this is probably not the proper forum in
which to address these. In practice, pcb layout, with adequate
grounding and active component decoupling is extremely important, so
keep this in mind if you wish to do some experimenting.
Hope I have not insulted anyone.
> At 11:22 PM 1/21/2006, you wrote:
> >This topic is veering off topic, but I think enough SoftRock users
> >adapt a DDS to their SR, to make this at least educational.8-)
> >its effect on spurs?
> >Can someone explain the counter/divide process and
> >there normally is one that describes a
> >As I read the specs for DDSes,
> >"Spurious Free Dynamic Range"with values in excess of 40 dBc and
> >significantly morethan that.
> >desired signal is
> >Based on pictures/graphs presented, the
> >bogus signals created as abyproduct of the D to A process within
> >These bogussignals, aka "spurs" are stable in frequency and
> >are repeatable. By repeatable I mean that if you encounter aspur
> >the DDS to a different frequency and return to theoriginal
> >spur likewise returns to its originalfrequency and amplitude.
> >math and formulas should be able to
> >Someone that understands the
> >values, but to theuntrained eye, the magnitude of the spur is
> >ascompared to the desired signal.
> >desired output of a DDS and any spurs are
> >I'm assuming both the
> >or behavelike sine waves. Is this wrong?
> >output has gone thru several tuned circuits and that
> >Also the DDS
> >likethe second and higher harmonics are way, way down in amplitude
> >the signal looks like a sine wave with minimal orno
> >this a bad assumption?the
> >My assumption is that for a greater majority of the cycle
> >is so much stronger than the spur, that thespur has no
impact/effect on the
> >use sensitive enough to
> >Are the counters and/or dividers we are apt to
> >transition/detect signals 40 dB down?60 dB down?
> >desired have any effect
> >Does a signal 40 dB down mixed with the
> >the desired passes thruzero?
> >crossing, then might a Schmitt
> >If the spur only has an effect near zero
> >trigger be a possiblesolution?
> >Do all spurs generated as a byproduct of data truncation within
> >Joe, K9HDE
> >behave the same way, that is, are all spurs frequency andamplitude
> >-----Original Message-----
> > >
> > > From:firstname.lastname@example.org
> > > [mailto:email@example.com]On Behalf Of jabauzit
> > > Sent: Saturday, January 21, 2006 8:27PM
> > > To: firstname.lastname@example.orgRe: Fw: [softrock40] DDS spurs, was Got my
> > > Subject:
> > > 5.0....Now what doI need?
> > >email@example.com, "allen_fulmer"
> > >
> > > --- In
> > ><afulmer@t...> wrote:
> > > >following is an excerpt from "Replacing or Integrating
> > > > The
> > > PLL'swith
> > > > DDS solutions" by Rick Cushing, ApplicationsEngineer, Analog
> > > Devices,is to place the
> > > > Inc.
> > > >
> > > > "Another strategy to control spurs,.....,
> > > > source ahead of adivide-by-N stage to reduce spurs by 20 Log
> > >(Fout/Fin)."
> > > >means that the DDS runs at N times the frequency of
> > >
> > > Which
> > > interest.It
> > > also means that the spurs are worse.Would the result be better than running the DDS at the fundamental?
> > >
> > >Bayona
> > > Jean-Claude PJ2BVU
> > >
> >Yahoo! Groups Links
> KD5NWAwhy doing the same thing over and over and getting the
> "I fail to see
> same resultsevery time is insanity: I've almost proved it isn't;
> only a few moretests now and I'm sure results will differ this time
>radio Yaesu ham radio Shortwave receivers
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