#286 How does Software Defined Radio (SDR) work under the Hood? SDR Tutorial

The whole world goes wireless. And digital. One technology is right in the center of these two megatrends: Software Defined Radio or short: SDR. Do you know how it works? And how you can take advantage of it? If you do not want to take the time to watch Michael Ossmann's (Great Scott Gadgets) series, then this SDR tutorial is for you
Software-defined radio is a relatively new technology and was extremely expensive in the past. Then came this small USB dongle for 10$ and changed all. And other, more capable devices like the HackRF One for 300$ pushed this technology into the mainstream. Today, most modern radios used for example in Smartphones, in Amateur radio equipment, or Satellites are SDR radios. This is the reason for the tutorial.
I am a proud Patreon of GreatScott!, Electroboom, Electronoobs, and others.
#SDR #Tutorial #Electronics

Links:

SD Card Image with SDR Tools (Buster):

[zvpunry]

Just some information that may be helpful for beginners:

0.) Programs like GNU Radio allow the use of sound-cards to learn the basics of software defined radio. Build simple digital transmitters and receivers that use the microphone and speakers instead of a SDR.

1.) There is not much difference between the blue/plastic and the silver/metal rtl-sdr. The bandwidth and the software is the same, both are useful up to 2.2 MSps but can be pushed up to 3 MSps (no difference here). The difference is, that the metal one has a _much_ better connector, SMA instead of MCX. The blue plastic one also has a really cheap crystal oscillator, it wanders around whenever the temperature changes. The silver/metal version has a temperature compensated oscillator, it measures the temperature and uses this to correct its drift, so it is much more stable. The drift normally is a few parts per million (it it is 10ppm, this would be 10 Hz at 1 MHz, but already 1000 Hz if you receive something on 100 MHz or 10 kHz if you are interested in 1000 MHz signals).The TCXO has only 0.5 ppm drift and the cheap crystal up to 20ppm.

2.) The most important things are antennas and short good coaxial cables. They decide about the signal quality. The cheapest RTL-SDR with a good antenna (and band-filters, see #3) is better than a 10000€ SDR with a crappy antenna. Beginners should go for the cheapest SDR and use the saved money for the antenna. With RTL-SDRs it is cheaper to use a long USB cable and get the SDR close to the antenna and away from the PC (which may create interfering emissions) than a good coax.


3.) Almost all SDR have wide open inputs. All signals received together (in case of the HackRF everything between 1 and 6000 MHz) must not exceed the maximum allowable signal strength. It's a common source of problems for beginners. They get interested in weak signals from far away and enable the AGC (automatic gain control) to get a nice strong signal but the signal strength (including the noise floor) jumps up and down because the AGC constantly increases and decreases the gain. Then they switch the AGC off and manually set the gain to some value, but then some mystery signals appear, signals that aren't there. That is what happens if the sum of the received signals exceed the maximum allowable signal strength, which turns amplifiers and ADCs into mixers that mix different received signals and produce these mystery signals.


Do it yourself band-stop filters however can be extremely cheap. The cheapest is the quarter wave stub. If you have only one interfering FM station at 100 MHz, which is a wavelength of 3 meters, add a 75cm (1/4 of 3 meters) piece of coax to the antenna cable. The waves will travel to the open end, get reflected and come back 180° out of phase. They will destructively interfere with the offending signal and thereby stopping it from reaching the SDR. The waves travel only 2/3 the speed of light in coax, so the 75cm will be way too long (the final length will be around 50cm). To tune such a stub, you can connect a noise source instead of the antenna and look at the dip in the noise. If the frequency of the dip will be too low, cut a bit of coax off and see how that dip rises in frequency. If it gets too high you have cut off too much (just push some copper wire into the open end of the coax).

4.) Almost all SDR have wide open outputs! The produced signal may contain spurious emissions and harmonics. If a signal is produced, say at 145 MHz, you need at least a low-pass filter above your frequency but below twice that frequency (i.e. 200 MHz) to get rid of all harmonics. Then look at the signal with a spectrum analyzer (i.e. another SDR)! If you have tuned your SDR to 145 MHz (center frequency), modulated some NFM (narrow frequency modulation) signal at +500 kHz and send it to the SDR and expect it to only transmit at 145.5 MHz, you might also see a copy of it at 144.5 MHz. This happens due to I/Q imbalance and this error can be corrected. Then you might produce a good signal and send through an amplifier, which again might add harmonics that must be filtered with another low-pass filter.

[MattBrunton]

I'm only 6 minutes in, and I've never seen the concept of mixer and IF explained so clearly. Thanks Andreas!

[jeroenjeroen]

Hats off for this great video on such a complex topic.
I have been staring at my sdr dab dongle for months, but after watching this episode I'm ready to experiment. Thanks!

[briancrane]

A delightful summary! I was a radar systems engineer for 30 years...we were using I/Q demod in the '80's...I am retired 10 years now...this video was like going home for a nice meal and a chat! Many thanks!

[avejst]

Impressive overview as always
You are doing a great job, of opening new doors for the community
Thanks for sharing👍😀

[murrij]

One of the best (possibly THE best) SDR intro videos there is.

[TchFlicks]

You are the teacher I never had. Keep doing what you do! :)

[hardickbora]

The best video on the subject i have seen so far. Its a one-stop shop for anyone looking to understand,
1. What is a radio?
2. What it does? (In simple terms)
3. How does SDR improve upon the prevalent solutions?

And the narration and the narrator is fantastic. Thank you for the video.

[Vikas_Singh_Kushwaha]

Don't know whenever I search some tricky rf concept, I always find one of your videos on it ! Thanks for the great content and efforts.

[MoTown]

Andreas, I chuckled when you mentioned 'we' began listening to FM radio at a young age. FM came into my life as a teenager around 1962. AM broadcast stations were king of the airwaves and it took almost a decade for FM to overtake the throne. I enjoyed this video. Thanks for taking time to present it.

[wmh]

Digital Radios with these capabilities have been around since the 1980’s. A Software Defined Radio is a Digital Radio that has been processed by a Marketing Department. We can start calling them Digital Radios again with no loss of generality—as we say in math class.
Thank you for the summary charts.

[darthvader]

I did my radio trade apprenticeship 40 years ago. I had heard of SDR but never lifted its skirt. You have blown my mind with the ingenuity of the people who developed it, and with the possibilities.

[MerlinFox]

A great explanation, thank you! SDR revived my interest in a radio, and I’m now an M0 in the UK 👍

[Panakotta]

I'm now in for about a year in the SDR world (RTL-SDR v3, now upgraded to an SDRplay RSP1B). And I usually tried to understand what is happening, but only tinkering really helped. This video explained it really well how such SDRs are built. Thanks!

[waltsteinchen]

Thanks Andreas again for such a great video! .. It must be incredible time consuming to deal with such a complex area and summarize it in such a great way for us!

[OldDogNewTrick]

My mind is blown. Here it is November 2019, and I am a 79 year old tech junkie but I never knew about SDR until now. I built crystal radios when I was a boy in the 1950s, worked for a while as a radio-TV technician in the early 60s, and with mainframe computers from mid 60s until I retired. I am amazed at how far technology has advanced in my lifetime. Thanks for the great info.

[Fagrache]

Great job Mr Andreas, you have explained that as simple as possible. Respect from Morocco.

[williambudd]

A friend suggested that I learn the 60 to 80 abbreviations used in SDR menus as a way to get started with using SDR’s. I thanked him for the warning and pointed out that I have better things to do with my time and if a SDR is not intuitive and easy to use, I don’t want one. Such a suggestion is appropriate because during my time working as an electronic engineer, I learned that programmers could turn an intutive concept into somethng no one can understand in no time at all !!!

[alexanderl.]

Great video! Small but important correction at 7:55. In order to digitize a 10.7 MHz signal, the ADC (Analog Digital Converter) sampling frequency must be at least 2*10.7 MHz (21.4 MHz) due to the Nyquist criterion. Even if you'r interested in only the 150 kHz FM signal the Nyquist criterion must be satisfied up to the highest possible frequency within your signal, otherwise you will get aliasing effects.

[borisj]

Wonderful to find someone who really knows what they are talking about.