[Ground-station] Baseband => decimation - questions

[Michelle Thompson]

To me, decimation is what we do in order to channelize in the payload.

I don't think that's exactly what I'm being asked about in the ground
station receiver, though.

-Michelle W5NYV




On Fri, Jan 25, 2019 at 4:14 PM Ron Economos <w6rz at comcast.net> wrote:

> I'm not sure we are talking about the same thing yet. So what exactly do
> you expect to decimate and why?
>
> Ron W6RZ
> On 1/25/19 16:07, Michelle Thompson 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>
> <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>
>> <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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