[Ground-station] Baseband => decimation - questions

KENT BRITAIN wa5vjb at flash.net
Fri Jan 25 19:06:28 EST 2019


 Quite a few lads in the UK are using the Octagons with the new EsHal bird.It was worked well with IF's down to 432 MHz.   Some of the more enterprisinglads are removing the 27 MHz xtals and sending a GPS stabilized signal back upthe coax externially locking the down converter section.   OK on a geo bird,but keeping the LO to a few Hz is kind of a waste of time when we will be seeingclose to 500 kHz of Doppler shift during a full overhead pass.
Kent WA5VJB



    On Friday, January 25, 2019, 5:52:03 PM CST, 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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