[Skimmertalk] Using Skimmer with RTL-Dongle SDR?

[John Ackermann N8UR]

Thanks for the added info, Doug!  I will check your repo and may be 
picking your brain.  Though it looks like I have the basic system 
working OK now.

The latest crop of dongles are a lot better than the original ones. 
Last fall I saw a set of six built into a beautiful frame with a common 
master oscillator for use in a radiolocation experiment.

If this were an HF system, I'd use a Red Pitaya.  But since we want to 
monitor 50/144/222/432/1296 the dongles avoid a lot of extra 
complication (though we'll probably stick preamps and filters on at 
least some of them).

Thanks and 73,
John
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On 01/12/2018 03:58 PM, Douglas Theriault wrote:
> As Pete N4ZR mentioned Rick N1GP and I built up a couple skimmers using
> 7 RTL SDR Dongles several years ago.  We wanted to build up a small node
> for CW/RTTY skimming across 7 bands.  Rick blazed the path getting the
> skimmers to work with the sticks and his initial write up n the rtl-sdr
> blog.
> 
> We selected an oDroid C1 to run the rtlhpsdr software after
> experimenting with several variants of  embedded boards.  There is a
> picture of a board set I built for HF skimming using 7 sticks on my QRZ
> page near the bottom.  We wanted to find an embedded board that had USB
> 2.0 speeds and 1Gb Ethernet.  Finding a board that would support 7
> sticks was a challenge but the oDroid C1 in the end worked well for us.
> 
> At the time we could not find RTL sticks with 1ppm xtals or tcxo
> options.  The sticks we had seemed very sensitive to temperature
> swings.  When spotting I was drifting > 200Hz and struggled with this;
> we even tried to compensate dynamically in software but this didn't work.
> When skimming the sticks seems to run hot; Rick had chipset unsolder on
> him in one instance. So keeping the sticks cool but stable was fun.
> 
> For my setup I ended up building a PLL fed by a 10Mhz tcxo to generate
> the 28.8mhz clocks for each stick.  I modified each stick removing the
> xtal and feeding a clock from the PLL.  The PLL also generated a stable
> 125mhz clock for an up converter mixer to move HF spectrum to the 120mhz
> to 155mhz frequency the RTL sticks could receive at.  The PLL effort
> stabilized my HF spots.  With the higher quality sticks now available
> folks may have better luck without going down the route I took.
> 
> Next issue I struggled with were aliasing errors, especially on 10m
> band.  I experimented with various FIR tables to see if I could filter
> them out but in the end did not have good luck.  If the tables were too
> long I could run only 2-4 sticks before dropping packets.
> 
> These alias errors seemed to be worse when there were strong signals.
> This may have been due to my HF up converter mixer design having too
> much gain causing overload on the sticks.  I struggled with my design to
> keep it flat across the HF band.
> 
> As I remember, there is a IF stage in the RTL sticks and am not sure if
> the errors I experienced were due to something in the design of the
> chipset or not.  I was/am new to SDRs not having detailed documentation
> on the chipset led to more questions than answers.  Still was a very fun
> project and I learned a heck of a lot about SDR's.
> 
> At the time the HL v1.2 project caught our interest and became the focus
> of our skimming pursuits.  For HF the HL boards were fantastic option
> for skimming.
> 
> I often thought it would be nice to try the RTL sticks for VHF/UHF
> skimming especially since you do not need an up-converter mixer board.
> Weak signal beacon monitoring would be great.  Unfortunately at my QTH I
> am limited on what I can throw up for antenna's and have not pursued at
> this time.
> 
> I have a repo on github as well for the project Rick and I collaborated
> on.
> 
>     https://github.com/dtheriault/hydra.git
> 
> As John found out there is a config file for the software, rtl_hpsdr.conf
> which defines the IP address for the skimmer instance and several other
> configuration options.
> 
> The copy on my github repo has comments which describe the config
> fields.  In addition there is a wiki that kept track of notes/pictures
> we exchanged during the bring up of the 7 stick HF skimmer.
> 
> Feel free to contact me directly if there's any questions on the
> software.  Its been a while but I'm sure Rick or myself can help answer
> questions you may have.
> 
> Apologize in advance if this email started a new thread, I just joined
> the list this morning in order to  post my comment.
>