[Ground-station] Baseband => decimation - questions

Zach Leffke zleffke at vt.edu
Sun Jan 27 14:25:33 EST 2019


This is cool.  Ron maybe you can school me a bit here.....

1.  Got it with the Quadrature ADC, running at half the required rate, 
but producing I and Q, so Nyquist is happy.....

2.  So you are saying the ADCs are clocked at variable rates? Here is my 
major disconnect......with a B210 and UHD, I can request sample rates 
(complex) from something like 200 kHz to 20 MHz (but have to make sure 
the clock rate of the USRP divided by the sample rate requested is an 
even integer).  I thought the ADC clocks ran at a fixed rate and then 
decimation was performed in the FPGA, and that the 'even integer' 
requirement was derived from the DSP going on in the FPGA (and integer 
math being more computationally efficient).  It sounds like you are 
saying that there is maybe a bank of acceptable pre-scalers that when I 
request a particular rate, the appropriate pre-scaler is applied to the 
master clock, and that is the rate the ADC is clocked at. Am I getting 
that right?

Seems like there may be pros and cons to either technique.  With a fixed 
ADC rate that is massively oversampling the input signal, can't we 
achieve increased dynamic range?  For every factor of 4 oversampling we 
gain 1 bit in ADC resolution, or 6 dB in dynamic range.  I thought this 
was how the Ettus products worked and similarly how the Flex Radio 
systems worked (with the FPGA handling the half-band filtering and 
decimation to the desired output rate requested by the host computer).  
I have no experience with the BladeRF or LimeSDRs.  I know in the UHD 
source block you can tweak the clock rate of the motherboards, but 
generally I thought that's how they worked.  I feel like there might be 
another pro in there in terms of Aliasing and relaxed filtering 
requirements when oversampling (images are farther apart)?  A con to 
this technique I would guess is higher power consumption (ADCs running 
at a higher rate and needing the pre-processing for the decimation).  If 
the clock rate of the ADC can be controlled on the other hand, you would 
lose the oversampling capability, but would decrease power consumption.  
Seems like the hardware would have to be a little more complicated in 
order to handle the multiply or divides to achieve the desired clock 
rate (that I assume is derived from a reference master clock that is 
higher than the ADC clock rate).  That control capability though is 
probably not that much more taxing though in terms of power (just 
picking the right prescaler).

Could it be that some SDRs use one technique (like the one I described, 
massive oversample then decimate, maybe USRPs), and others use a 
different technique (like the one you described, maybe BladeRF, LimeSDR, 
etc.)?


If I'm roughly accurate in the above description, then isn't the 
question Michelle is driving towards is what technique would we want to 
use?  Presumably for a ground station implementation, power isn't a 
concern (compared to the payload) and therefore it would be desirable to 
oversample and have the added benefits of higher dynamic range.  In 
other words, maybe the power consumption trade-off is worth it.  Then 
again, maybe higher dynamic range isn't really necessary (only a single 
downlink) and its not worth the added DSP complexity.

On the payload side, I would guess that having a higher dynamic range on 
the uplink is actually desirable and worth the tradeoff in power 
consumption.  I'm basing this on the assumption that if 1000 hams build 
ground stations, the uplink powers will be wildly different and we may 
need to handle very strong and very weak uplink signals, and thus a 
higher dynamic range would be desirable.  On the other hand.....using 
something like an AD9361 with 12 bits of resolution is 72 dB of dynamic 
range out of the gate (maybe a little less when determining 
ENOB).......maybe that's enough and oversampling isn't necessary.........


This is good stuff, I like learning new things.  Am I roughly getting 
this right or am I out in left field some where?


-Zach, KJ4QLP

Research Associate
Aerospace Systems Lab
Ted & Karyn Hume Center for National Security & Technology
Virginia Polytechnic Institute & State University
Work Phone: 540-231-4174
Cell Phone: 540-808-6305

On 1/27/2019 4:07 AM, Ron Economos via Ground-Station wrote:
>
> SDR's like the Ettus B2x0, bladeRF and LimeSDR are direct conversion. 
> Here's a block diagram of the architecture.
>
> direct conversion
>
> The PLL is tuned to the frequency of interest. The IF is 0 Hz. Each 
> ADC runs at the desired sample rate and provides 1/2 the sample rate 
> of bandwidth. The analog baseband low-pass filters are set to 1/2 the 
> bandwidth. No decimation required.
>
> In fact, the bladeRF has no filtering at all in it's FPGA. If you set 
> the baseband analog filters to wider than the desired bandwidth, you 
> can see the aliasing (the TX is also direct conversion). The sample 
> rate for this OFDM signal was 6.86 MHz.
>
> aliasing
>
> Same signal with the baseband filter set properly to 5.5 MHz (each 
> low-pass filter set to 2.75 MHz).
>
> no alias
>
> Ron W6RZ
>
> On 1/26/19 23:51, Zach Leffke via Ground-Station wrote:
>>
>> I'll attempt to clarify without adding to much noise......I think I'm 
>> seeing both sides of this here......Though I'm no DSP expert, so fair 
>> warning I might misspeak a bit.
>>
>> A lot of what has been discussed so far is about down conversion, not 
>> decimation.........
>>
>> The trick is the 'guts' of the SDR, specifically the RFIC and the ADC 
>> used.  Say we have an IF of 700 MHz coming out of an LNB.  The upper 
>> edge of our signal would be at 705 MHz. According to Nyquist, you 
>> would have to sample that IF signal at 1410 Megasamples per second 
>> (at least).  Any lower than that, and images of the sampled signal 
>> will overlap and corrupt the data (Aliasing).  Say your ADC had 14 
>> bits.......that's a lot of data, probably more than most FPGAs can 
>> handle.
>>
>> The trick in the SDR (pick your vendor, I'm staying generic here) is 
>> that it has a tunable front end that performs another level of down 
>> conversion in the analog domain before sampling (for example, the 
>> AD9361 in Ettus B210s, E310s, etc).  This takes the 700 MHz signal 
>> down to say 20 MHz (made that number up), with an ADC running at a 
>> fixed rate..... say 80 MHz (also made that number up).
>>
>> At this point, my DSP-fu is weak, but where I'm going with this is 
>> that the 80 MHz sampling is real samples, represents 40 MHz of RF 
>> bandwidth (Nyquist).  This can be represented as a stream of complex 
>> samples (IQ) running at half the rate, though each sample is twice 
>> the size, one for I and one for Q .......(Nyquist isn't violated, 
>> because even at half the rate, you have two samples, one In-phase, 
>> and one Quadrature)......
>>
>> So now we have a stream at 40 MSPS (complex), spanning from 0 to +40 
>> MHz, with our signal camped out at +20 MHz.  So then we tune 
>> digitally to center our signal at 0 MHz...aka complex baseband.  Now 
>> we have a stream still at 40 MSPS (complex), but spanning from -20 
>> MHz to +20 MHz.  Negative frequencies are OK in the complex domain.  
>> Our signal of interest is now centered at 0 MHz, but is spanning from 
>> -5 MHz to +5 MHz.
>>
>> I think what Michelle is getting at is that we don't want to have an 
>> FPGA processing a 40 MHz wide stream of data, when we only need to 
>> worry about 10 MHz.  Since our signal is centered at 0 MHz, we can 
>> start throwing away samples........Decimation.  We can toss 3 out of 
>> every 4 samples out (decimate by 4, and filter), which leaves us with 
>> a 10 MSPS stream of complex samples.  This also has some benefit in 
>> that we are also rejecting the noise contributions of those 
>> unnecessary samples that we tossed out (linked to the B in kTB).
>>
>> Everything that was (horribly) described above is handled 'under the 
>> hood' by the UHD drivers in Ettus products for example.  When you 
>> tell a UHD source block the sample rate you want (usually the 
>> 'samp_rate' variable that shows up in the flowgraph) what you are 
>> actually telling it is the 'requested' sample rate on the output of 
>> the above process.  The onboard ADC still runs at the fixed higher 
>> rate (80 Mhz in my example), but based on your input into the UHD 
>> source block, it will automatically select the right parameters to 
>> ensure that after sampling, conversion to complex baseband (including 
>> tuning your requested center frequency to 0 MHz), decimation and 
>> filtering, that the rate you requested is fed out to the host 
>> computer.  This is why you have to be careful about selecting sample 
>> rates with UHD.........some of you have probably seen the debug 
>> output where it warns you that the division of the ADC clock rate by 
>> the requested sample rate is not an even integer and to expect 'CIC 
>> rolloff'........basically it couldn't cleanly do the decimation you 
>> requested (again....something here about the value of half band 
>> filters in the FPGA that are part of the conversion to complex 
>> baseband and decimation.....sorry for my weak DSP-fu).  You know you 
>> got it wrong when you see a 'hump' of spectrum when the spectrum 
>> should be 'flat'.
>>
>> Now we can run that 10 MSPS complex stream of samples through the 
>> demodulation process and extract the 'frames of interest' for any 
>> particular user........that would be demultiplexing (throw out the 
>> frames for everyone else, I only want the frames for me).
>>
>> I would offer that on the ground side, there is no channelizer in the 
>> mix.....you must receive the entire 10 MHz signal to recover the full 
>> downlink data stream, since there is only one time multiplexed, 10 
>> MHz wide signal.
>>
>> On the satellite payload on the other hand....you WILL want a 
>> channelizer.  Lets say the uplink is 10 MHz wide, and supports 1000 
>> channels, so each 10 kHz wide.  In order to demodulate 1000 channels 
>> in parallel, somewhere in there you need to tune 1000 times to center 
>> each uplink signal to complex baseband and decimate to 10 ksps 
>> (complex).  executing 1000 'tunes' in parallel at the full 10 MSPS 
>> rate is very very wasteful........Think of the channelizer as a 
>> really efficient way of performing the tuning and decimation so that 
>> each output channel of the channelizer is only 10 ksps and it is 
>> properly 'centered' on the desired uplink channel.  it will require 
>> more horsepower than a single tune and single decimate 'string' but 
>> is less processing intensive than 1000 of those strings running in 
>> parallel.
>>
>>
>> I didn't mention anything about other sampling tricks (Nyquist zones, 
>> and potential spectral inversion issues), or the benefits of 
>> oversampling (dynamic range), integer vs float 
>> representation.........maybe on another thread one day.
>>
>> So if you are going to roll your own hardware..........first a lot of 
>> downconversion from 10 GHz (maybe an LNB to get to say L-Band or 
>> maybe something to get to a 'ham band IF' at 432 or 144).  Thats not 
>> enough.  You'll have to then downconvert again to something that can 
>> be handled by the selected ADC and whatever clock rate it is running 
>> at (Nyquist rules apply here).  Filtering and careful consideration 
>> of mixing products will matter!  phase noise of the LOs will matter 
>> in all that downconversion.......Also, gain gain gain... will matter 
>> to make sure that you are fully exercising the full range of the ADC 
>> and not just toggling the the lowest couple of bits  (but not too 
>> much gain.....clipping).  Then the complex baseband conversion and 
>> decimation and on to demodulation, demultiplexing, etc. etc.......A 
>> lot of the above is what is handled 'under the hood' by most of the 
>> commercial SDRs out there (i.e. UHD) so that the end user can easily 
>> get up and running with the 'more interesting' stuff downstream.........
>>
>> Hopefully this helped clarify the issue.....sorry if it added more 
>> noise (my DSP-fu is weak).  Not sure if I actually answered any 
>> questions.......
>>
>>
>> -Zach, KJ4QLP
>>
>> Research Associate
>> Aerospace Systems Lab
>> Ted & Karyn Hume Center for National Security & Technology
>> Virginia Polytechnic Institute & State University
>> Work Phone: 540-231-4174
>> Cell Phone: 540-808-6305
>> On 1/25/2019 7:27 PM, Ron Economos via Ground-Station wrote:
>>>
>>> Okay. De-multiplexing is a much better and less confusing 
>>> terminology. As you stated, decimation is a DSP thing and 
>>> channelizing the downlink payload has nothing to do with DSP (all 
>>> the DSP has already been down in order to deliver payload packets).
>>>
>>> Ron W6RZ
>>>
>>> On 1/25/19 16:17, Michelle Thompson wrote:
>>>> 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 
>>>> <mailto: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>
>>>>>     <mailto: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>
>>>>>>         <mailto: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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>>>>>>         Ground-Station at lists.openresearch.institute
>>>>>>         <mailto:Ground-Station at lists.openresearch.institute>
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>>>>>>
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>>>>>>         http://lists.openresearch.institute/mailman/listinfo/ground-station
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