[Skimmertalk] red pitaya, for real?

[kazeringue at aol.com]

OK, so far so good.  RP...for real?  Maybe.  Probably, at the very least 
it initially seems better than my softrock skimmer system.

FWIW....

RP arrived 9/6.  Hacked the only SMA jumper cable I had onto a UHF 
jumper.  Up and running in about 90 minutes, most of that time trying to 
remember linux related stuff to customize RP.

Existing softrock skimmer uses a K9AY for 160/80/40.  K9Ay comes into 
shack through an ICE limiter, into W7IUV style preamp, and into a pair 
of 'magic-t' splitters.  The 80m and 40m softrocks used the outputs of 
the second in series splitter(6db down), 160m the other half of the 
first(3db down).

So I pulled the 80m feed and ran RP in parallel with the softrocks.
No other antenna matching is currently in line.  [more testing needed]
Note:  Ran into a link to a russian site where it was suggested that a 
transformer with 14:1 winding ratio might improve sensitivity by up to 
10 dbm.  More tinkering to follow.  Probably not a help here with the 
high local QRN levels.

RP is running with the pin2-pin5 jumper setting, straight off the feed 
that previously fed the 80m softrock. [Aside: the other jumper 
"jumped"[boingggg!], and is gone for all of eternity.  Spare jumpers, 
not on hand, but ordered... :( ]

So.  Initial tests....
On 160m, RP SNR levels are approximately the same levels as on the 160m 
softrock cw skimmer.

On 40m the RP SNR levels generally run 4 to 8 db higher than softrock cw 
skimmer SNR levels.  Each is finding spots not reported by the other, 
with skim server being more prolific than cw skimmer.

RP was able to pull spots on 80m,30m and 20m.  Don't have comps for those.

The caveat I found was while the RP seemed to produce spots more 
frequently on 40m, it is also producing occasional image spots that are 
either 1.5kc or 2.5kc off of the actual frequency.  I shut down the 
softrock system, and these image spots are much less frequent, but not 
eliminated.  There was definitely interference from the softrock 
oscillators, but there is maybe also some other  sort of interference in 
the shack.

Need to put RP into a shielded enclosure with a good power supply, and 
do more testing.  Besides computers and power supplies in shach, could 
also be problems from 50kw of 680AM a mile away.  Plans for notch filter 
or BCB filter for that.

Running PowerSDR, the interference spikes were obvious, but no image 
signals were obvious.

The skim server ran in parallel to three softrocks over the weekend, 
with each drawing about 10% of the CPU of a core 2 quad Q9400 at 2.66 
Ghz.  Running alone during sprints saturday evening the RP hovered just 
below 16% cpu.

Linux notes:
Issue 1: Root, root, and not wanting a wide open linux OS inside my home 
network.
Having brief flings with debian and Ubuntu linux in the past, it was 
fairly painless.  The one issue I ran into is that the pre-packaged 
SDR's by Pavel Demin would not allow the root password to be changed 
easily(i.e. "PASSWD" command).

Loading the current OS image from the Red Pitaya site, then loading 
Pavel's SDR from the application marketplace solved that issue, as the 
"PASSWD" command leaves a persistent paswword change on the RP with 
their current OS.

Not currently having any linux systems running, on Windows having a 
Virtualbox VM with an Ubuntu client os was helpful in setting up RP.

Not planning on crushing any CPU's with RTTY skimming, although that 
does sound like fun. ;)
73 de w4kaz