[linrad] RE: USB peripheral Radio

[Leif Asbrink <leif.asbrink@xxxxxxxxxxxxxxxxxxxx]

> Linrad would be the back end.  This is a wide band A/D, with only
> 12 bits so it would need to narrow the bandwidth in the decimation
> filters for both processing gain and so Linrad could stick its
> teaspoon into the Niagara Falls coming at it from the USRP.  The
> code for the decimation, downsampling, etc. runs in an FPGA on
> board.  In addition to this,  the board plugs into USB.  We would
> need to modify Linrad to accept data that gazinta USB without
> it being a sound card.  The USRP is a naked front end.  It would
> need a front end (low noise figure) and  mixer to bring the band(s)
> of interest to the A/D front door.
I have actually designed Linrad for use with this kind of hardware
and I think I have written it explicitly at several places. This 
particular unit has a very limited dynamic range and I do not think
it will fit the needs of amateur operators except in special cases.
One can of course add roofing filters in front of the A/D converter
but that will increase complexity. On 40 meters one would have
to use narrow roofing filters to exclude the strong BC stations
for example.

> Sounds like a great idea.  BTW,  I have a USRP.  I want to do many
> different things with it: some professional and not discussed here,
> and some amateur which I am happy to discuss.  I will give but one
> example.  Sit on (say) 20 meters, scan the entire CW band, detect and
> decode all usable signals (TBD) and display same to the (say) contester.
Yes. With low enough gain this will work fine. I think the noise figure 
will not be comparable to an ordinary radio, the A/D noise floor will 
dominate, not the antenna noise, but showing all except the really weak
CW signals would provide the contester with a lot of information in
real time so he can use his high dynamic range radio more cleverly.


> On weak signal work, the story is still out if we can get enough 
> processing
> gain from the A/D and decimation filtering to make it useful.  Further,
> it needs a front end to give it a noise figure on the VHF+ bands.
On 2 meters the dynamic range requirements are even tougher than on
40 meters except in rural areas far from fellow amateurs, but on 
microwave bands I think it would be an excellent solution because
saturation that occurs during a radar pulse is no problem at all
as long as the pulse is not lengthened by slow recovery of amplifiers
that were saturated. I have been told that strong narrowband signals
do not occur on the microwave bands. Local amateurs are very unlikely
to point the antennas into each other. I think Linrad can process 
0.5 MHz or more on a modern computer without problems but I do not 
know if it would be of any interest to microwave enthusiasts.

I will be happy to add support for the USRP (Is this the name of 
the unit or a short for some general description of it?) when the
hardware is stable and produced in some quantity. There is just
one problem and that is in the Gnu license. Linrad is "free for 
anyone for any purpose" and I do not know if I will be allowed to 
incorporate the Gnu Radio hardware control code and the FPGA code 
into Linrad.

73

Leif / SM5BSZ



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