[Ground-station] Baseband => decimation - questions

David Vieira dpv at ieee.org
Fri Jan 25 19:25:03 EST 2019


 M - I'm hearing pieces of the puzzle.

Ron is stating the standard is 950-2150 for DVB-S2; yet clearly others are putting a lower IF into an SDR (with integrated FPGA).
I also understood some folks are working on a discrete FPGA (or Nvidia??) chip, with TBD front end.  I believe my decimation questions started with that paradigm, but not necessarily limited to that scope.  So can we get the DVB "standard" to work with lower IF's?  (ie, 144, 618, 700ish MHz)  

It makes sense an Off-The-Shelf wideband SDR can just absorb the data directly.  (My impression is we needed more horse power for the DSP backend.)
As for LNB's, I didn't realize some were stable enough for 10G links.I believe the North Amer. Ph4B design target design is Geostationary, so a few Hz may matter.   (I'm still trying to guess what a EsHal bird is?)  If a dish is too narrow to target, a some of us have 10G Flat Panels with ~7-8 degree beam width.  It sounds like there are several sub-projects and many variations in hardware implementations; constraints in one may not apply to another.
Keep the comments coming...
regards,KI6CLA

    On Friday, January 25, 2019, 4:09:21 PM PST, Michelle Thompson via Ground-Station <ground-station at lists.openresearch.institute> wrote:  
 
 The beginning of wisdom being the definition of terms and all, it would be good to make sure we're all talking about the same thing. 

So far, I've used LNBs and USRPs for receive, with the LNB doing an IF at 618MHz (LNB-on-a-Stick) and giving reasonable performance. 

Decimation to me is a DSP thing, or used to reduce power consumption when you don't need to sample as high as you can. 

-Michelle W5NYV




On Fri, Jan 25, 2019 at 3:52 PM Ron Economos via Ground-Station <ground-station at lists.openresearch.institute> wrote:

  
The standard IF for DVB-S2 receivers is 950 to 2150 MHz.
 
DB6NT was selling a down-converter from 10489-10500 MHz to 1129-1140 MHz for P4A.
 
https://shop.kuhne-electronic.com/kuhne/en/shop/new/MKU+LNC+10+OSCAR+P4A/?card=1832
 
I'm not sure what decimation has to do with receiving DVB-S2. The entire 10 MHz signal needs to be demodulated. Individual baseband frames will be selected for processing, but I call that de-multiplexing.
 
 
Ron W6RZ
 
 On 1/25/19 15:32, David Vieira via Ground-Station wrote:
  
  Michelle - Thanks for posting.  I'll frame some of the questions.
  
  Typical 10 GHz terrestrial contesting rigs are Heterodyne; that is a Mixer works with a Local Oscillator (LO) to take the RF down to an IF (Intermediate Frequency). For an SDR, that IF can be digitized by an Analog-Digital Converter. 
  The most popular IF for contesting/SSB rigs is 144 MHz.   For a data BW of 10 MHz that may or may not be a fast enough IF carrier.  If we can digitize and recover the data, it would allow a lot of re-use of existing equipment. 
  I've heard suggestions/proposals up to the 1.2 GHz Ham band. In some sense, the IF carrier could be 144/220/440/915/1200 MHz, or even any Non-Ham frequency in between. 
  There are a lot of proof of existence designs for a 10 GHz Mixed down to an IF; and lots of off the shelf ADC dev-boards.  (catch me off thread for details). 
  Some questions I have are:   ---from an FPGA side of the SDR, what data rate(s) can the FPGA absorb in to a decimator?   
  Must we decide upfront on a single frequency; or  preferably allow flexibility in the RF front end design (ie, Mixer, PLL and Local Oscl hardware choices) by allowing a wide and programmable variety of ADC and decimation rates? 
  {This is where RF and Digital folks must communicate across walls.}  ;-) 
  Comments welcome. 
  regards, David KI6CLA 
  
      On Friday, January 25, 2019, 2:41:54 PM PST, Michelle Thompson via Ground-Station <ground-station at lists.openresearch.institute> wrote:  
  
     While we are striving to enable all sorts of wonderful designs by putting prototypes into GNU Radio, a central goal is to design our own hardware.
 
 We've had a lot of progress on the protocol and algorithm front (GSE, LDPC, some of the polyphase). 
 
 Some fundamental decisions about our own hardware need to be made.
 
 When we receive, we expect to have to decimate. This is because we are receiving at a relatively high frequency (10GHz).
 
 Our bandwidth is (up to) 10MHz. For DVB-S2/X, we fix our sampling rate, depending on what bandwidth we want to support. We have a lot of freedom here.
 
 Picking the right frequencies for the receive chain is therefore important.
 
 What are our options? 
 
 What options make the best sense?
 
 I'd like to build and test as soon as possible, so let's get some discussion going.
 
        -Michelle W5NYV
 
 
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