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Well the Delta 44 is now on its way here, but I have a long way to go for
my change over to Linrad. I am still planning and rethinking my aproach to
Linrad. I would like to take a path that minimizes wasted purchases.
The Time machine would be such a waste.
I have been on a number of web pages, mostly Leif's, and am getting
to the point that the information is jumbled and hard to find again what I had
read already. I think I understand the need for a pure LO and good mixer (AM and FM noise
rejection), so I will probably be getting an RX2500. The question will be going from
144 Mhz to 2.5 Mhz. I do not fully understand why so many conversion steps
are recommended. I can see the difficultly for spurs if frequencies are not carefully
chosen. Certainly it is useful to be able to QSY using the crystals that are included
in Leifs design. Since so many conversions will be expensive and difficult (for me)
to build using easy to find components, I am very tempted to use fewer conversions
from 144 to 2.5 Mhz. The obvious simple solution is to use single conversion from
144 to 2.5 Mhz. I think I can provide very good RF selectivity, so image rejection
is not a problem. If this proves to be a poor choice due to intermod, then perhaps
an intermediate conversion into the 25 - 30 Mhz range would help. Intuition says that
the fewer frequencies involved, the less chance of spurs. I have not run any intermod
calculations yet, but I am not limited to the 10 meter band for an HF IF.
If I use the 25 - 30 MHz range as an intermediate IF, I would probably try to use my
CB PLL that has a very wide operating range. This could easily provide IFs in the
27.5 to 30 Mhz range. A crystal oscillator would make the first conversion. A lucky
find in my crystal box along with the wide band CB PLL and a lucky result from a spur study,
and I am off to the races. If I go with single conversion, I can probably synthesis the
needed LO by mixing a crystal oscillator and the CB PLL. Again, another spur study needed.
My guess is that by converting from 144 to 70 Mhz, 70 Mhz to 10.7 Mhz, then finally 10.7 to
2.5 Mhz, this produces a set of numbers that make spurs nearly impossible to detect. Is this
the only reason for it? As I say, I can control RF selectivity at 2m. I use good 5 pole helical
filters for RF. They are available for the LO as well. At $12 a pop for the good mixers,
I would like to minimize them. If this has all been well answered on a web page, please
paste the web page in a reply to me and I'll review it for myself.
I may have missed this too on the web sites, but how much gain ahead of the RS2500 is
needed? Is an LNA at 2.5 Mhz helpful here to keep the mixers after my front end out of
trouble? I suppose that selectivity between mixers is helpful too...?
Per Conrad, it looks like my Pentium 475 Mhz computer will not be fast enough
for proper operation at 96 KHZ bandwidth. I will continue my search for a capable Linrad PC.
73, Jim Shaffer, WB9UWA.