<div dir="ltr"><div>I actually have never built a direct conversion receiver but since reading Dan's paper of his "Tayloe detector", which doesn't even need a Sine/Cosine and Analog Mixer pair, I got intrigued by his concept and wanted to know more about it.</div><div>So these are my findings about direct conversion, never tried but it makes sense to me:</div><div><br></div>Do we have to worry about other signals in the vicinity of the frequency of interest?<div>If no, then the roll-off of the analog filters should be enough and I would also go with Ron's suggestion of direct conversion.</div><div>Then besides equalization, only one thing remains, that can be taken care of in the digital domain:</div><div>Dealing with the unmatched input amplitudes and phase shifts, caused by the diverging signal paths and component tolerances.</div><div><br></div><div>So one has to decide either to use a higher (at least twice the) sampling rate for the IF to feed the FFT (or DEMOD) with one (real) signal to avoid phase ambiguity or do the Zero IF direct conversion to get the analytic signal pair with the problems described above.</div><div><br></div><div>And as always: Please tell me if I got something wrong :)</div><div><br></div><div>Ahmet</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Sun, Jan 27, 2019 at 10:08 AM Ron Economos via Ground-Station <ground-station@lists.openresearch.institute> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div bgcolor="#FFFFFF">
<p>SDR's like the Ettus B2x0, bladeRF and LimeSDR are direct
conversion. Here's a block diagram of the architecture.</p>
<p><img src="cid:168909b759f5fd5757a1" alt="direct
conversion" width="768" height="483"></p>
<p>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.</p>
<p> 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.<br>
</p>
<img src="cid:168909b759fc1f1a8d12" alt="aliasing" width="800" height="480">
<p>Same signal with the baseband filter set properly to 5.5 MHz
(each low-pass filter set to 2.75 MHz).</p>
<p><img src="cid:168909b759fc6e5ec2b3" alt="no alias" width="800" height="480"><br>
</p>
<p>Ron W6RZ<br>
</p>
<div class="gmail-m_-4901761508123658200moz-cite-prefix">On 1/26/19 23:51, Zach Leffke via
Ground-Station wrote:<br>
</div>
<blockquote type="cite">
<p>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.</p>
<p>A lot of what has been discussed so far is about down
conversion, not decimation.........<br>
</p>
<p>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.</p>
<p>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). <br>
</p>
<p>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)......</p>
<p>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.<br>
</p>
<p>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).<br>
</p>
<p>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'.<br>
</p>
<p>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).</p>
<p>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.</p>
<p>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. <br>
</p>
<p><br>
</p>
<p>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.<br>
</p>
<p>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.........<br>
</p>
<p> </p>
<p>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.......<br>
</p>
<p><br>
</p>
<p>-Zach, KJ4QLP<br>
</p>
<pre class="gmail-m_-4901761508123658200moz-signature" cols="72">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</pre>
<div class="gmail-m_-4901761508123658200moz-cite-prefix">On 1/25/2019 7:27 PM, Ron Economos
via Ground-Station wrote:<br>
</div>
<blockquote type="cite">
<p>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).<br>
</p>
<p>Ron W6RZ<br>
</p>
<div class="gmail-m_-4901761508123658200moz-cite-prefix">On 1/25/19 16:17, Michelle Thompson
wrote:<br>
</div>
<blockquote type="cite">
<div dir="ltr">To me, decimation is what we do in order to
channelize in the payload. <br>
<br>
I don't think that's exactly what I'm being asked about in
the ground station receiver, though. <br>
<br clear="all">
<div>
<div dir="ltr" class="gmail-m_-4901761508123658200gmail_signature">
<div dir="ltr">
<div>
<div dir="ltr">
<div>
<div dir="ltr">
<div dir="ltr">-Michelle W5NYV<br>
<br>
<div dir="ltr"><br>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
<br>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail-m_-4901761508123658200gmail_attr">On Fri, Jan 25, 2019 at
4:14 PM Ron Economos <<a href="mailto:w6rz@comcast.net" target="_blank">w6rz@comcast.net</a>> wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div bgcolor="#FFFFFF">
<p>I'm not sure we are talking about the same thing yet.
So what exactly do you expect to decimate and why?<br>
</p>
<p>Ron W6RZ<br>
</p>
<div class="gmail-m_-4901761508123658200gmail-m_8520444453280004698moz-cite-prefix">On
1/25/19 16:07, Michelle Thompson wrote:<br>
</div>
<blockquote type="cite">
<div dir="ltr">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. <br>
<br>
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. <br>
<br>
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. <br>
<br clear="all">
<div>
<div dir="ltr" class="gmail-m_-4901761508123658200gmail-m_8520444453280004698gmail_signature">
<div dir="ltr">
<div>
<div dir="ltr">
<div>
<div dir="ltr">
<div dir="ltr">-Michelle W5NYV<br>
<br>
<div dir="ltr"><br>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
<br>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail-m_-4901761508123658200gmail-m_8520444453280004698gmail_attr">On
Fri, Jan 25, 2019 at 3:52 PM Ron Economos via
Ground-Station <a class="gmail-m_-4901761508123658200gmail-m_8520444453280004698moz-txt-link-rfc2396E" href="mailto:ground-station@lists.openresearch.institute" target="_blank"><ground-station@lists.openresearch.institute></a>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div bgcolor="#FFFFFF">
<p>The standard IF for DVB-S2 receivers is 950
to 2150 MHz.</p>
<p>DB6NT was selling a down-converter from
10489-10500 MHz to 1129-1140 MHz for P4A.</p>
<p><a class="gmail-m_-4901761508123658200gmail-m_8520444453280004698gmail-m_-6643074664132559776moz-txt-link-freetext" href="https://shop.kuhne-electronic.com/kuhne/en/shop/new/MKU+LNC+10+OSCAR+P4A/?card=1832" target="_blank">https://shop.kuhne-electronic.com/kuhne/en/shop/new/MKU+LNC+10+OSCAR+P4A/?card=1832</a></p>
<p>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.<br>
</p>
<p>Ron W6RZ<br>
</p>
<div class="gmail-m_-4901761508123658200gmail-m_8520444453280004698gmail-m_-6643074664132559776moz-cite-prefix">On
1/25/19 15:32, David Vieira via Ground-Station
wrote:<br>
</div>
<blockquote type="cite">
<div class="gmail-m_-4901761508123658200gmail-m_8520444453280004698gmail-m_-6643074664132559776ydp1f38d99eyahoo-style-wrap">
<div>Michelle - Thanks for posting. I'll
frame some of the questions.<br>
</div>
<div><br>
</div>
<div>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).</div>
<div>For an SDR, that IF can be digitized by
an Analog-Digital Converter.</div>
<div><br>
</div>
<div>The most popular IF for contesting/SSB
rigs is 144 MHz. </div>
<div>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.</div>
<div><br>
</div>
<div>I've heard suggestions/proposals up to
the 1.2 GHz Ham band.</div>
<div>In some sense, the IF carrier could be
144/220/440/915/1200 MHz, or even any
Non-Ham frequency in between.</div>
<div><br>
</div>
<div>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).</div>
<div><br>
</div>
<div>Some questions I have are: </div>
<div>---from an FPGA side of the SDR, what
data rate(s) can the FPGA absorb in to a
decimator? </div>
<div><br>
</div>
<div>Must we decide upfront on a single
frequency; or </div>
<div>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?</div>
<div><br>
</div>
<div>{This is where RF and Digital folks
must communicate across walls.} ;-)</div>
<div><br>
</div>
<div>Comments welcome.</div>
<div><br>
</div>
<div>regards,</div>
<div>David</div>
<div>KI6CLA</div>
<div><br>
</div>
<div><br>
</div>
</div>
<div id="gmail-m_-4901761508123658200gmail-m_8520444453280004698gmail-m_-6643074664132559776ydp40e62e6byahoo_quoted_8708381549" class="gmail-m_-4901761508123658200gmail-m_8520444453280004698gmail-m_-6643074664132559776ydp40e62e6byahoo_quoted">
<div>
<div> On Friday, January 25, 2019, 2:41:54
PM PST, Michelle Thompson via
Ground-Station <a class="gmail-m_-4901761508123658200gmail-m_8520444453280004698gmail-m_-6643074664132559776moz-txt-link-rfc2396E" href="mailto:ground-station@lists.openresearch.institute" target="_blank"><ground-station@lists.openresearch.institute></a>
wrote: </div>
<div><br>
</div>
<div><br>
</div>
<div>
<div id="gmail-m_-4901761508123658200gmail-m_8520444453280004698gmail-m_-6643074664132559776ydp40e62e6byiv6388574106">
<div dir="ltr">
<div dir="ltr">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.<br>
<br>
We've had a lot of progress on the
protocol and algorithm front (GSE,
LDPC, some of the polyphase). <br>
<br>
Some fundamental decisions about
our own hardware need to be made.<br>
<br>
When we receive, we expect to have
to decimate. This is because we
are receiving at a relatively high
frequency (10GHz).<br>
<br>
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.<br>
<br>
Picking the right frequencies for
the receive chain is therefore
important.<br>
<br>
What are our options? <br>
<br>
What options make the best sense?<br>
<br>
I'd like to build and test as soon
as possible, so let's get some
discussion going.<br>
<br>
<div>
<div dir="ltr" class="gmail-m_-4901761508123658200gmail-m_8520444453280004698gmail-m_-6643074664132559776ydp40e62e6byiv6388574106gmail_signature">
<div dir="ltr">
<div>
<div dir="ltr">
<div>
<div dir="ltr">
<div dir="ltr">-Michelle
W5NYV<br>
<br>
<div dir="ltr"><br>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
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</blockquote></div><br clear="all"><div><br></div>-- <br><div dir="ltr" class="gmail_signature"><br>Ahmet Inan<br><br>Co-founder and CEO of aicodix GmbH<br><a href="https://www.aicodix.de/" target="_blank">https://www.aicodix.de/</a><br></div>