- 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 -0000**Subject:**[softrock40] Re: DDS spurs**Message-ID:**<dr0cat+cp2d@egroups.com>`In an earlier post I used an equation for predicting the effect of a`

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.

Allen, K4KDG>

plan to

>

> 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

at times> >significantly more

than that.> >

desired signal is

> >Based on pictures/graphs presented, the

accompanied by> >bogus signals created as a

byproduct of the D to A process within

the DDS.> >These bogus

signals, aka "spurs" are stable in frequency and

amplitude, and> >are repeatable. By repeatable I mean that if you encounter a

spur

and tune> >the DDS to a different frequency and return to the

original

setting, the> >spur likewise returns to its original

frequency and amplitude.> >

math and formulas should be able to

> >Someone that understands the

give exact> >values, but to the

untrained eye, the magnitude of the spur is

quite small> >as

compared to the desired signal.> >

desired output of a DDS and any spurs are

> >I'm assuming both the

sine waves> >or behave

like sine waves. Is this wrong?> >

output has gone thru several tuned circuits and that

> >Also the DDS

things> >like

the second and higher harmonics are way, way down in amplitude

and that> >the signal looks like a sine wave with minimal or

no

discontinuities. Is> >this a bad assumption?

the

> >

> >My assumption is that for a greater majority of the cycle

desired signal> >is so much stronger than the spur, that the

spur has no

impact/effect on the> >counting/division

process.> >

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

except when> >the desired passes thru

zero?> >

crossing, then might a Schmitt

> >If the spur only has an effect near zero> >trigger be a possible

solution?> >

Do all spurs generated as a byproduct of data truncation within

> >Joe, K9HDE

> >

> >PS

a DDS> >behave the same way, that is, are all spurs frequency and

amplitude

stable?> >

-----Original Message-----

> >

> >

> > >> > > From:

softrock40@yahoogroups.com> > > [mailto:softrock40@yahoogroups.com]

On Behalf Of jabauzit> > > Sent: Saturday, January 21, 2006 8:27

PM> > > To: softrock40@yahoogroups.com

Re: Fw: [softrock40] DDS spurs, was Got my

> > > Subject:> > > 5.0....Now what do

I need?> > >

softrock40@yahoogroups.com, "allen_fulmer"

> > >

> > > --- In> > >

<afulmer@t...> wrote:> > > >

following is an excerpt from "Replacing or Integrating

> > > > The> > > PLL's

with> > > > DDS solutions" by Rick Cushing, Applications

Engineer, Analog> > > Devices,

is to place the

> > > > Inc.

> > > >

> > > > "Another strategy to control spurs,.....,

known spur> > > > source ahead of a

divide-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

> >

> >

> >

> >

>

>

> Cecil> KD5NWA

why doing the same thing over and over and getting the

> www.qrpradio.com

>

> "I fail to see> same results

every time is insanity: I've almost proved it isn't;> only a few more

tests now and I'm sure results will differ this time

... ">

radio Yaesu ham radio Shortwave receivers

>

>

> SPONSORED LINKS

> Icom ham

Ham radio>

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