[PVRCNC] SKIMMER/K3

[Brian Alsop]

Guys,

Been playing with of couple of the $18 SDR kits.  One for 20M and one 
for the K3 IF.

Looking at just a spectrum display is kind of boring.  Add call signs 
and it becomes useful.  So I decided to try SKIMMER.  Having spots for 
stations actually heard at your QTH is quite useful.

The below observations have probably been recorded elsewhere. Every 
generation seems to have to relearn what the OT's already knew. I'd like 
feedback.  The below may show my ignorance. I may have gone down faulty 
paths and there are work-around's.

-1) You need a decent stereo sound card. Here, one PC's motherboard 
worked only at 48K sampling rate.  Crummy dynamic range too. Another 
PC's motherboard sound card was mono!  You need stereo for SDR's I and Q 
outputs.  Ended up with a $100 PCI sound card which was suppose to be 
"low cpu use".  The E-MU 202 USB sound card apparently is a buss hog. 
They recommend using it in these applications only an multicore CPU 
machines.
0) When hooked up to the 20M SDR, SKIMMER blew my socks off.  Decoding 
the entire 20M CW band at once and putting it on a band map.  Even Don 
Miller couldn't do that.  I suspect these could be exported via TELNET.
The sensitivity and selectivity of the combo were quite astounding.
The decode worked well even with signals that were faint on the 
waterfall.  It also used 80% of the CPU cycles.
1) SKIMMER and N1MM do not work together on a 2.8 GHz, 2GB memory, 1 CPU 
machine running WIN-XP.  Both load OK but N1MM then looses the radio and 
then become unresponsive
2) SKIMMER is pretty useless until you get it to export spots to N1MM. 
Telnet is the suggested way.  Some additional program has to merge 
ordinary spots and SKIMMER spots.  WRITELOG apparently allows two 
sources of TELNET spots and an external spot merge program isn't needed 
for WRITELOG.
3) When hooked up to the K3 IF (via a Clifton labs z-10000 
preamp/isolation buffer) SKIMMER will only decode +/- 12 KHz from the 
present K3 transmit frequency.  This is a SKIMMER, not K3, limitation.
You have to tune to get more of the band.  So if you're running them, 
you have to find a way to get more of the band decoded. See (4). If you 
use LP-BRIDGE, you can share K3 frequency info and click on the SKIMMER 
band map to pounce on a spot.  Likewise these can be exported to TELNET 
and imported to N1MM.
4) Given observation (1) above. Either get a macho PC or do as Mark N2QT 
just finished doing.  Run SKIMMER on a standalone PC and link to the K3 
via LP-BRIDGE over a null modem RS232 cable between the contest and 
SKIMMER PC.  Mark has hooked the K3's subrx IF to his SKIMMER to get 
around the +/-12 KHz SKIMMER "IF radio" limitation. He is going to tune 
around with the subrx anyhow.
5) One Caribean DX station is or has planned using stand alone SDR's for 
each band.  These are somehow connected to separate antennas and not 
blown up by the RF when transmitting.  This gets around the +/-12KHz 
limitation.  If these SDR's are muted, it is hard to figure out how they 
get anything to decode -- since these guys "run" nearly 100% of the time.

Practically speaking, having to use SKIMMER via two PC's IMHO is too 
much of a pain.  In fact, the whole thing seems pretty much like a house 
of cards- but promising. Too bad SKIMMER & soundcard were not in one 
standalone box thus avoiding both CPU hits.  Input IF, output spots via 
USB.  SDR-IQ/Spectravue comes closer to this.  Spectravue's 
documentation is quite overwhelming, BTW.

73
de Brian/K3KO