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Hack Chat Transcript, Part 1

A event log for Learning DSP and SDR Hack Chat

To the frequency domain, and beyond

Dan MaloneyDan Maloney 11/11/2020 at 21:040 Comments

OK all, big crowd today. A warm welcome to all of you who came over via RTL-SDR.com, and to everyone else. I'm Dan, a staff writer here at Hackaday, and I'll be the mod today for Dr. Marc Lichtman as we dive into Learning SDR and DSP!

Welcome, Marc! Can you tell us a little about your background to start us off?

clint.cole joined the room.12:01 PM

Marc Lichtman12:01 PM
sure

Marc Lichtman12:02 PM
My name is Marc Lichtman and I wear three different hats- 1) I'm an adjunct prof at University of Maryland, within the CS dept, where I teach a course on hands-on SDR, 2) I'm one of the leads for the GNU Radio project, a free and open source SDR framework, and 3) I'm a research engineer at Perspecta Labs as my full time gig where I do a lot of SDR, machine learning, and spectrum sensing related work.

Marc Lichtman12:02 PM
I recently put together a free online textbook, www.pysdr.org, to help teach DSP and SDR concepts in a hands-on and less mathy way, compared to traditional textbooks on the topics. It uses Python for all code examples, and there are some PlutoSDR-specific exercises. And it's based off what I learned teaching my course at UMD

jwahar bammi joined the room.12:03 PM

Jeffrey Forbes joined the room.12:03 PM

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Marc Lichtman12:03 PM
I think that coders and CS students shouldn't feel like they can't dive into DSP and SDR just because they didn't go through a EE graduate program =P

Aaron12:04 PM
I went through an EE graduate program.. I WISH they went this far... ;)

Marc Lichtman12:04 PM
a large fraction of the actual work being done in DSP/SDR (and jobs), requires good coders, like your typical CS students

wildblueyonderman joined the room.12:04 PM

Marc Lichtman12:04 PM
hah yeah, you have to be a EE grad student, specializing in wireless/DSP

How do you find the CS students reacting to a hardware topic that at first seems out of the typical CS wheelhouse?

Marc Lichtman12:04 PM
I mean there might be an intro to comms class at the undergrad level, but that's about it

Marc Lichtman12:05 PM
well, the CS students I had didn't have any experience related to the topics, so they needed to be slowly introduced in a hands-on way, that involved a lot of code examples and exercises

Marc Lichtman12:05 PM
it's very foreign to them

Marc Lichtman12:05 PM
luckily, even though an SDR is hardware, what you actually do with it is mostly software

clint.cole12:05 PM
My daughter is studying this for her PhD in Electronics Engineering at Stanford

Mark J Hughes12:06 PM
What's going to be the next great development in SDR? I know I'm seeing it appear in Amateur radio equipment.

Marc Lichtman12:06 PM
ah very nice

David Tarnoff12:06 PM
What math background did they have at the point they came into your class?

clint.cole12:06 PM
Which is partly why I want to learn about it

Aaron12:06 PM
I started reading your book and haven't gotten very far yet, but so far you did an excellent job!

Stony Yakovac joined the room.12:06 PM

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Marc Lichtman12:06 PM
Not sure what the next great development is, depends if you are looking at low cost SDRs, or like space-hardened, or stuff focusing on hams

Barry Duggan12:06 PM
I second that!


https://pysdr.org/index.html

Pysdr

PySDR: A Guide to SDR and DSP using Python

Read this on Pysdr

Marc Lichtman12:07 PM
but usually ham stuff is at lower sample rates, and it's all about the antenna, from my little xp

Marc Lichtman12:07 PM
so you dont need like the latest and greatest SDR that can do 200 MHz instantaneous bw

Dave Sanderman joined the room.12:07 PM

Aaron12:07 PM
What SDR platforms do you use and what can you recommend?

Marc Lichtman12:07 PM
As far as math bg, my course wasn't too mathy so what they had was enough

Mark J Hughes12:07 PM
@Marc Lichtman I have no actual use case. I just like to learn :)

Barry Duggan12:08 PM
HF bands for hams are not covered very well with existing hardware

Marc Lichtman12:08 PM
I personally spend most of my time with the lower cost SDRs, so your PlutoSDRs and USRP B200/B210

curiousmarc12:08 PM
Can you give an overview of what the status of DSP and SDR hardware is? Up to what frequency do they go? When does it make sense to drop a classic hardware implementation for a software defined one?

Marc Lichtman12:08 PM
yeah you need the downconverters like ham-it-up, right?

Mark J Hughes12:08 PM
@Marc Lichtman I believe that is correct.

Aaron12:08 PM
I have a HackRF and love it... However, there are some Chinese radios that give you the IQ output..

Marc Lichtman12:09 PM
depends how much money you spend, but up to 6 GHz center freq is a common one, because of the mass produced RFICs used in the low-cost SDRs

Collin Avidano joined the room.12:09 PM

curiousmarc12:09 PM
wow, that high?

Thomas Propst joined the room.12:09 PM

For the current crop of RTL-SDR type dongles, yes. But IIRC there are some purpose-built SDRs for the ham bands that don't need downconverters.

Marc Lichtman12:09 PM
remember you have the SDR's RF freq range, and then also it's sample rate range (which is the same as it's bandwidth range)

Barry Duggan12:09 PM
that works for receive, but there is no equivalent for transmitting - no ham it down :)

Marc Lichtman12:09 PM
hah yeah someone needs to make that

jtrash1337 joined the room.12:10 PM

Marc Lichtman12:10 PM
but if you look at what makes an SDR hard to build, its the upper RF freq range, not lower, from what Ive seen

Marc Lichtman12:10 PM
and obviously higher sample rates will cost more

Marc Lichtman12:10 PM
your Plutos and USRP B200's do up to 56 MHz, but there are SDRs that do up to 200 MHz or even higher

Marc Lichtman12:10 PM
RTL-SDRs will always be limited by the chip they are based on, to 2 GHz RF, and 2.4 MHz sample rate, I believe

Aaron12:11 PM
Sorry for not reading all of your book before this hack chat.. So hopefully our questions are not going to be redundant...

Marc Lichtman12:11 PM
so like for $10 you get 2.4 MHz sample rate, and for $200+ you get 56 MHz

Marc Lichtman12:11 PM
oh I wouldn't expect anyone to have read the book already

Marc Lichtman12:11 PM
but after the chat feel free to check it out, www.pysdr.org, free and no ads

Collin Avidano12:11 PM
As a CS person who knows nothing do I need to buy a different antenna than the pluto-sdr comes with to transmit at wifi frequencies?

cuzeau12:11 PM
sorry, a beginners question : is the typical SDR equipment capable of dealing with UWB schemes ?

Aaron12:11 PM
Personally, I would love to get a good idea of the hardware and software that you use for your testing.

Barry Duggan12:11 PM
I got to read an early release. great material

Marc Lichtman12:12 PM
the antenna that comes with the pluto will transmit at 2.4 GHz, but read up on the legality https://pysdr.org/content/pluto_advanced.html#transmitting-over-the-air-legally

Marc Lichtman12:12 PM
what do you mean by UWB

Marc Lichtman12:12 PM
depends how WB

cuzeau12:12 PM
Ultra Wide Band

Marc Lichtman12:12 PM
I use a Pluto and USRP B210 for testing/work, with a variety of antennas

Marc Lichtman12:12 PM
how wide band? =P

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Marc Lichtman12:13 PM
they can only do up to 56 MHz, for example, at one time

Marc Lichtman12:13 PM
so thats your limit there

Marc Lichtman12:13 PM
and the RTL-SDR can only do up to 2.4 MHz at one time

Aaron12:13 PM
My internet is spotty... Not sure if you received this question since it said it failed to send: Personally, I would love to get a good idea of the hardware and software that you use for your testing.

cuzeau12:13 PM
and there probably is a sensitivity issue too

curiousmarc12:13 PM
What’s an RTL.

Jeffrey Forbes12:13 PM
I think that with an amateur license one can do experimental radio at the shorter wavelengths,

curiousmarc12:13 PM
?

Marc Lichtman12:14 PM
software wise, I mostly use Python and GNU Radio

Marc Lichtman12:14 PM
like to create apps

curiousmarc12:14 PM
RTL-SDR?

Marc Lichtman12:14 PM
but I still play around with receivers like GQRX and the SATNOGs software stack

Mark J Hughes12:14 PM
What kind of apps?

@curiousmarc - https://www.rtl-sdr.com/about-rtl-sdr/

Aaron12:14 PM
RTL is the chipset on most USB dongles...

And hi!

Marc Lichtman12:15 PM
well, drone stuff, LTE surveying, spectrum sensing

Marc Lichtman12:15 PM
it varies over time

curiousmarc12:15 PM
Thx Dan

Marc Lichtman12:15 PM
some apps are for work, some for fun

Dave Sanderman12:15 PM
Question: I work with a non-profit that does flood monitoring using LoRaWAN-connected sensors; have you (or anyone here) had any experiences using SDRs with LoRaWAN (900MHz)?

Marc Lichtman12:15 PM
sometimes its not actually a standalone app but a piece of software that goes into something else

Marc Lichtman12:16 PM
All I know is that there's a GNU Radio module for doing LORA I believe, https://github.com/rpp0/gr-lora

Marc Lichtman12:16 PM
I don't know how well-maintained it is or how far they got

More info on the RTL2832U chipset: https://www.realtek.com/en/products/communications-network-ics/item/rtl2832u

Marc Lichtman12:16 PM
but something like a PlutoSDR or USRP B210 should be able to tx and rx at 900 Mhz at the sample rates you're talking about just fine

Marc Lichtman12:16 PM
RTL-SDR might be able to rx it? not sure

Marc Lichtman12:17 PM
if you need to tx as well then RTL-SDR won't fly

Aaron12:17 PM
I have found that GNU Radio is a pain to install for Windows and for Linux. Is there any kind of virtual machine or an image that has everything installed?

markatwpi12:17 PM
Do you suggest any type of math background courses (free hopefully) for understanding the DSP?

Jie Zhang12:17 PM
Hi Marc, I'm reading your book. I'm a software engineer entering RF area. It's a very good book to start. Thank you. Is there a forum or mailing list for book readers to discuss problems, solutions to the exercises, etc?

RoGeorge12:17 PM
@Marc Lichtman gave it a browse already and it looks great! Seems like it covers all the key concepts. My hat off to your book, sir!

For now, the only "not really" spotted is the figure of the Gaussian shape, in the figure from this page https://pysdr.org/content/noise.html That doesn't look like normal distribution to me, but I'm nitpicking, I know

:o)

An example of a Gaussian (interactive): https://www.geogebra.org/m/ehkwndma

Aaron12:17 PM
I think MATLAB is a very good course...

Dave Sanderman12:17 PM
I've been able to see signal with RTL-SDR but haven't gone into any greater depth; thanks for the response.

Marc Lichtman12:17 PM
RE math courses, for my textbook the idea was to get by with very little math, but basics of complex numbers will help, and some trig

Marc Lichtman12:18 PM
and if you want to get into more complex stuff like MIMO you'll need a lot of mutlivar xp

Marc Lichtman12:18 PM
like a lot of matrix math

perronemajr joined the room.12:18 PM

Marc Lichtman12:18 PM
it depends how deep you want to dive

Aaron12:18 PM
MATLAB does very well with matrices and is fast..

Marc Lichtman12:18 PM
No mailing list currently, for the textbook, just the github issues/PR pages

Marc Lichtman12:18 PM
so you could use github issue for stuff that isn't actually an issue

Marc Lichtman12:19 PM
yeah I would highly recommend Python over MATLAB if you want to get into SDR

Mark J Hughes12:19 PM
@aaron I just bought one of those 64GB ultra small PCs and made a stand-alone machine. All it does is run GNU Radio off the recommended linux distro.

Marc Lichtman12:19 PM
Python will do whatever you want to do in MATLAB, unless you're talking about comms systems toolbox or something

Marc Lichtman12:19 PM
just as fast

AVR (lordKiCAD)12:19 PM
Is anyone doing SDR with JUlia ? Julia will eventualyl replace MATLAB

curiousmarc12:19 PM
OK I got instant education on RTL-SDR thks to the link. But is there something you can use for the transmit side at 2GHz-ish frequencies?

Mark J Hughes12:19 PM
I also have it running on my windows PC -- but you're right , it's not trivial to get going.

jtrash133712:19 PM
Hi Marc, I am a CS student and I want to thank you for your really clear and well written course, it provides a good starting point for everyone who want to start with RF stuff. I'm used to using the HackRF for that purpose, is your python module fully compatible with that tool (i'd rather user HackRF than Pluto,

Marc Lichtman12:19 PM
Julia is fast but you want to use a lanuage other people use...

Jeffrey Forbes12:19 PM
What about Julia? It is free and faster than Python and in many cases Matlab.

Aaron12:20 PM
So a good start for a receiver that will play well with GNU Radio, what would be your recommendation? On UHF or VHF bands?

perronemajr12:20 PM
What hardware would you recommend for generating and reading squeezed quantum states? Parametric generator and balanced homodyne reciever?

Marc Lichtman12:20 PM
like the speed is not going to make up for the fact that other folks will be using Python and C/C++

Marc Lichtman12:20 PM
I would stick with Python or C/C++ imo

Miguel Escobar12:20 PM
What are some of the applications that someone interested in SDR and DSP could accomplish after reading your book?

Marc Lichtman12:20 PM
MATLAB is OK for simulation only

@curiousmarc - I think HackRF will do that

Marc Lichtman12:20 PM
RE "OK I got instant education on RTL-SDR thks to the link. But is there something you can use for the transmit side at 2GHz-ish frequencies?" look into PlutoSDR and USRP B200

curiousmarc12:21 PM
Thx will look it up right now


https://greatscottgadgets.com/hackrf/

Greatscottgadgets

Great Scott Gadgets - HackRF One

HackRF One from Great Scott Gadgets is a Software Defined Radio peripheral capable of transmission or reception of radio signals from 1 MHz to 6 GHz. Designed to enable test and development of modern and next generation radio technologies, HackRF One is an open source hardware platform that can be used as a USB peripheral or programmed for stand-alone operation.

Read this on Greatscottgadgets

Marc Lichtman12:21 PM
RE "Hi Marc, I am a CS student..." my textbook does NOT include a python module

Thomas Shaddack12:21 PM
LimeSDR Mini is also quite good.

Aaron12:21 PM
HackRF will do up to "6GHz".. But the sample rate might not be as fast as others.

Marc Lichtman12:21 PM
I specifically didn't make a python module to go along with my textbook, like other books have done, e.g. Think DSP

Marc Lichtman12:21 PM
yeah i've had issues with LimeSDR's support...

Marc Lichtman12:21 PM
HackRF is good too, its an older one

Barry Duggan12:22 PM
for those who want to know more about GNU Radio, look at https://wiki.gnuradio.org/index.php/Main_Page

Marc Lichtman12:22 PM
LimeSDR mini use to be the go-to for low budget, but now I think that has gone to PlutoSDR if you are OK with not doing 100% duty cycle, since its over USB 2.0

Aaron12:22 PM
GNU Radio is SO powerful..

Marc Lichtman12:22 PM
yeah GNU Radio is great for making SDR apps that you can share with others

Marc Lichtman12:22 PM
or even just specific blocks

Marc Lichtman12:22 PM
RE "What are some of the applications that someone interested in SDR and DSP could accomplish after reading your book?", good question!!! I like to think they can do the kind of stuff I do, like spectrum sensing

Marc Lichtman12:23 PM
what you won't be able to do, is set up an actual comms link

Marc Lichtman12:23 PM
when you get into RX/TX over the air, there are more details that go into it, that I don't cover in my book (yet)

Jeffrey Forbes12:23 PM
Marc, Do youhkave any knowledge or thoughts of where DARPA funding may take Gnu Radio

Marc Lichtman12:23 PM
like with my book you can simulate a QPSK signal over a wireless channel, but if you actually tried to transmit/receive, you might run into issues

Aaron12:23 PM
As long as it is still free.. Who cares.. =)

Collin Avidano12:23 PM
When about to ask the question on projects I did find this page from rtl sdr

Collin Avidano12:23 PM

https://www.rtl-sdr.com/about-rtl-sdr/

rtl-sdr.com

About RTL-SDR

What is RTL-SDR? RTL-SDR is a very cheap ~$25 USB dongle that can be used as a computer based radio scanner for receiving live radio signals in your area (no internet required). Depending on the particular model it could receive frequencies from 500 kHz up to 1.75 GHz.

Read this on rtl-sdr.com

Marc Lichtman12:24 PM
DARPA funding will indirectly help GNU Radio in many ways

Marc Lichtman12:24 PM
including having GNU Radio support heterogeneous platforms like FPGAs/DSPs

@Marc Lichtman - What kind of problems? Legal issues with license requirements?

Marc Lichtman12:24 PM
but a lot of what DARPA does is research-oriented, and a lot of those projects use GNU Radio =)

Barry Duggan12:24 PM
for hams who might be interested, look at https://wiki.gnuradio.org/index.php/HamRadio

Marc Lichtman12:25 PM
well legal issues, yeah, but I was thinking more along the lines of equalization

Marc Lichtman12:25 PM
like since you will probably have a strong multipath fading channel

Aaron12:25 PM
Do they have any FPGAs that are strickly filtering only? As is, RCL?

Marc Lichtman12:25 PM
my textbook covers sync, but not equalization (yet)

Whitney Knitter joined the room.12:25 PM

Marc Lichtman12:25 PM
they have DSPs that are, not FPGAs per se

Marc Lichtman12:25 PM
but lots of system on chips include both

kiggins.chris joined the room.12:26 PM

Harrison joined the room.12:26 PM

Marc Lichtman12:26 PM
I mean you can implement a digital filter in an FPGA pretty easily, without taking too much of the fabric

Aaron12:26 PM
I think an FPGA strictly for filtering should be deisinged.

Marc Lichtman12:26 PM
I don't think it would be an FPGA if it was strictly for filtering

Marc Lichtman12:26 PM
check out the latest RFSoCs from Xilinx

Marc Lichtman12:26 PM
they probably do what you want =)

Marc Lichtman12:26 PM
but they are expensive

Marc Lichtman12:27 PM

https://www.xilinx.com/products/silicon-devices/soc/rfsoc.html

Xilinx

Zynq UltraScale+ RFSoC

Zynq UltraScale+ RFSoCs integrate multi-giga-sample RF data converters and soft-decision forward error correct (SD-FEC) into a MPSoC architecture.

Read this on Xilinx

Aaron12:27 PM
I am sure they are..

kiggins.chris12:27 PM
Comments on RFNoC?

Marc Lichtman12:27 PM
insanely powerful

Marc Lichtman12:27 PM
I think RFNoC is great, and its slowly coming along

Marc Lichtman12:27 PM
it's an Ettus (USRP) thing, if anyone is curious

Aaron12:27 PM
Once they become cheaper, I am sure we will be seeing a HUGE jump in SDRs..

Marc Lichtman12:27 PM
so not GNU Radio specific, it's more Ettus specific, although GNU Radio supports it

Marc Lichtman12:28 PM
yeah I tend to use SDRs in a way where the samples are delivered to my host machine

AVR (lordKiCAD)12:28 PM
Best open SDR on the market ?

Marc Lichtman12:28 PM
versus doing any DSP on the SDR

kiggins.chris12:28 PM
It sounds like your experience mirrors my own with lime v ettus re:support

perronemajr12:28 PM
What do you think of the NMR spectrometer that guy made with the LimeSDR? https://limemicro.com/community/ultra-low-cost-limesdr-based-nmr-spectrometer/

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