[hpsdr] Phoenix synthesizer

[Brian D. Comer]

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.  

 

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.

 

 Now to my real comment. 

 

Reading the 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.

 

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.

 

73 Brian KF6C   G3ZVC

 

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