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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>This is my first post to the HPSDR group so before giving my thoughts
it may be useful to give my SDR background. I designed and marketed Radware, an
SDR using I&Q mod/demod and a sound card, under Comer Communications Inc.
thirteen years ago. Radware was a business failure for many reasons most of
which have no relevance today. </span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> </span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>The most serious limitation to the performance of this early SDR were
second order products, IMD2. I played with the SDR 1000 to see how much
better it did on this issue. At first I was unable to find any IMD2 products.
After some thought I realized that the dithering introduced by the spur
reduction process was spreading the IMD2. When the spur reduction is turned
off, IMD2 can be measured. Although the mixer selections have improved
over the years IMD2 is still an issue, typically worse than IMD3, with low IF
receivers. Dithering produces a very significant improvement in this
respect, although it is increasing the noise floor when present.</span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> </span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> Now to my real comment. </span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> </span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>Reading the </span></font>PHOENIX article synthesizer choices, I liked
the “Microwave PLL divided down” choice, similar to Radware, but
then I got to thinking. If this is used dithering will be much harder and
I still think IMD2 is a bigger problem than the spurs produced by the AD9951.</p>
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12.0pt'> </span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>As a second consideration, dividing by 4 using a single AD9951 puts an
upper limit of less than 50 MHz on the receiver. Using 2 AD9951’s puts 6
and 2 meters within range.</span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'> </span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>73 Brian KF6C G3ZVC</span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'> </span></font></p>
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