[Skimmertalk] Using Skimmer with RTL-Dongle SDR?

[Jan OK2ZAW]

Thanks to great info, 
Thinking abt VHF/UHF skimmer for OL7M too.
Btw if anyone is interested I do have TCXO 28.8 MHz for cheap dongles, price is 2,50 Usd + shipping.
GSM 900 Kal is around 600Hz and good temp stability.
But for 1296MHz the PLL is great idea. Doug, is there anywhere any info abt your construction?

Thank you!

Vy 73 Jan OK2ZAW / OL9A / OL7M

S pozdravem Jan Šustr
Regards Jan

http://www.ok2zaw.com
http://www.qro.cz


-----Original Message-----
From: Skimmertalk [mailto:skimmertalk-bounces at contesting.com] On Behalf Of John Ackermann N8UR
Sent: Friday, January 12, 2018 10:03 PM
To: Douglas Theriault; skimmertalk at contesting.com
Subject: Re: [Skimmertalk] Using Skimmer with RTL-Dongle SDR?

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
----

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.
> 
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