Software defined radio-How does it work? - Page 1
Forum section: electronics / beginners

author: djsb
author_rank: Super Contributor
timestamp: September 29, 2012, 09:35:16 am
content: I watched a couple of Jeri Ellsworth's youtube video's on Software defined radio last night and came away non the wiser.Can someone explain to me how SDR works in simple language. For instance what is I and what is Q?David.

author: Bored@Work
author_rank: Super Contributor
timestamp: September 29, 2012, 10:20:30 am
content: It works with math. A lot of math. And so much for the claims you read here again and again that an EE never needs math once out of uni ...The simplified version of SDR for a receiver:The RF signal is sampled as early as possible (i.e. with as little analog preprocessing as one can get away with), and as broad as needed into a digital data stream. In fact, it is typically two streams, the I and Q streams, but we skip that for now (just makes the math more interesting).The sampling theorem applies, so you should sample at least more than twice as much as the bandwidth of the incoming signal. In practice even higher. If your input starts at 0 Hz, then you should sample more than twice as much as the highest frequency you want to receive with your radio.The rest is math. You use digital filtering to extract your desired signal out of the sampled data. This is your modulated signal. You need to demodulate it.Every modulation technique can be described with math. So can be the demodulation process required to extract the message from the modulated signal. You take the mathematical description of the demodulation process, convert it into the digital domain, form an algorithm to perform the decoding, optimize the algorithm, possibly incorporate the required digital filtering, and implement it so it processes the digitized signal. You get your transmitted message. And while you have your message in digital form, you can apply more digital processing. E.g. for voice communication some noise filtering, or for digital communication some data decoding.People get a hard one over SDR, because it "only" requires to change the software to support other modulation techniques. That is the "software defined" part. Which is a rubbish term. The software doesn't define anything. The modulation/demodulation techniques do, and the software in an SDR is just implementations of demodulation algorithms. And of course, you need to have enough processing power to run the necessary decoding algorithms, otherwise your radio remains pretty "undefined".

author: M0BSW
author_rank: Frequent Contributor
timestamp: September 29, 2012, 11:09:59 am
content: As a fully licensed Radio Amateur , I must admit I don't really understand it, in my shack everythings like me old & big enough to work on, like my Kenwood TS820 valve Hybrid,lovely. this new stuff i guess is OK,oh you need a computer and I think a good one at that, boxs are quite small, I call it cheque book amateur radio , ok if you havent a lot of room, not sure if you can build stuff and add it to iit without damaging the box, anyway OK for the rich amateurs , doesn't really interest the dinisaurs like me.

author: ejeffrey
author_rank: Super Contributor
timestamp: September 29, 2012, 11:26:07 am
content: The real interesting thing with SDR is the ability to implement modulation techniques that would be difficult or impossible to do in the analog domain, given enough overkill in your DAC/ADCs. For instance, frequency hopping techniques are much easier to implement with SDR since changing your carrier frequency only requires altering the outgoing bitstream rather than tuning an oscillator which may take time to stabilize. A lot of hobbyists use SDR for the relatively mundane task of implementing AM/FM or similar radios without having to generate a carrier. This is all fine, but it is not the real value of SDR.I and Q are the 'in phase' and 'quadrature' outputs of a demodulator. They are the real and imaginary components of a Fourier transform. I couldn't begin to cover all the details of modulation techniques here, but the idea is that an I/Q (de)modulator can generate/detect any signal within the bandwidth of the the mixer surrounding the local oscillator. The classic AM/FM/SSB/etc modulation techniques are all very simple special cases of I/Q modulation. Beyond that, as B@W says, it is a lot of math.

author: madires
author_rank: Super Contributor
timestamp: September 29, 2012, 01:58:28 pm
content: Quote from: ejeffrey on September 29, 2012, 11:26:07 amI always thought, that I and Q are the outputs of two mixers driven by the same local oscillator, while the signal of the local oscillator is phase-shifted 90 degrees for the Q mixer. So I would be the in-phase baseband and Q would be the baseband phase-shifted, allowing to determine amplitude and phase later on. And since you got amplitude and phase you can demodulate virtually any signal in software. Demodulating AM would be just a simple sqr(I^2 + Q^2). But I might be totally wrong ;-)

author: Bored@Work
author_rank: Super Contributor
timestamp: September 29, 2012, 03:11:42 pm
content: Quote from: madires on September 29, 2012, 01:58:28 pmHa, this is where the math fun starts. Euler couples a cos() and a sin() (aka, a 90 deg. phase shifted cos()) together in a great equation: e^(-j*x) = cos(x) - j * sin(x).Multiplying (mixing) a periodic signal f(x) with e^(-j*n*x) (aka a cos(n*x) - j * sin(n*x)) and integrating it over a period of two pi gives the complex Fourier coefficients for the signal. The complex Fourier coefficients can be split in real and imaginary coefficients (difference and sum of two consecutive complex coefficients), and you can call this your analog I/Q signals. Sample, and you have your digital I/Q data stream.

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: September 29, 2012, 03:16:01 pm
content: I am a ham also and would like to buy either or both of these but cannot afford either at this time .These are basically the top rigs at this time , by -http://www.sherweng.com/table.htmlhttp://www.elecraft.com/KX3/kx3.htm

author: djsb
author_rank: Super Contributor
timestamp: September 29, 2012, 03:25:40 pm
content: My heads hurting already

author: Lukas
author_rank: Frequent Contributor
timestamp: September 29, 2012, 03:48:28 pm
content: For getting started with SDRs , look for rtl-sdr. Some smart people discovered that DVB-T usb receivers based on the rtl2832u (cheap, ~20$) can stream the raw IQ data via usb, so you can receive virtually everything from about 60MHz to 1800MHz. The bandwidth of the signal has to be <2MHz.

author: M0BSW
author_rank: Frequent Contributor
timestamp: September 29, 2012, 04:10:25 pm
content: How would I like to own Elecraft Radio's, I thought my Kenwood's were top dogs until I had the chance to use an Elecraft for a week, i didn't want to give it back

author: M0BSW
author_rank: Frequent Contributor
timestamp: September 29, 2012, 04:17:40 pm
content: I just watched the video, where's is there anywhere with SDR for the constructor who wants to modify or build units to add to it answer No Where,it saddens me really Amateur Radio the man in the street who has an interest in electronics builds equipment uses it understands how it works, this is what kept me interested in amateur Radio all my life, watching the video I lost interest in around 3 seconds, just an opinion.

author: IanB
author_rank: Super Contributor
timestamp: September 29, 2012, 04:27:42 pm
content: Quote from: madires on September 29, 2012, 01:58:28 pmAs with many things in mathematics, it's just a different way of looking at the same thing.When people talk about "real" and "imaginary" components and use complex numbers in signal processing, they are just using a convenient mathematical tool to help with the calculations. The imaginary components are just as real as the real components.If you look at any given (frequency) sine wave you can describe it in terms of amplitude and phase angle (the phase angle is how far it is shifted away from a reference wave of the same frequency). Since a complete period of a sine wave is 2pi radians or 360 degrees you can treat amplitude and phase angle as a pair of polar coordinates on a 2D graph or x-y diagram. Conveniently it happens that complex numbers are also described as a pair of numbers you can plot on a 2D graph (an Argand diagram). So any time you have x-y or polar coordinates you can use complex numbers to represent them, and then you can do calculations with these complex numbers and convert the result back to polar coordinates when you are finished.Now the vertical axis of the x-y graph (or the imaginary direction in the Argand plane) is 90 degrees or a quarter turn away from the horizontal axis. So imagine Q as meaning "Quarter turn" and you can understand where quadrature comes from, and why Q is the "imaginary" component.To pull all the pieces together, Fourier analysis shows how every periodic signal of any shape can be represented as a sum of sine waves of different frequencies and phases, in other words a bunch of the previously mentioned 2D coordinates.In a really simple way then, if you wanted to extract the wanted signal from a high frequency carrier wave, you could do a Fourier analysis to resolve the signal into a sum of sine waves of different frequencies and then simply ignore the frequencies you didn't want. Doing it for real is a bit more complicated, but the essential theory is the same.

author: poptones
author_rank: Frequent Contributor
timestamp: September 29, 2012, 07:20:54 pm
content: You don't necessarily need to sample the rf signal at Nyquist. Because the bandwidth of the signal is limited what you need to sample at is a rate twice the bandwidth (roughly). You can do SDR with nothing more than a front end, a limited IF and the sound card inputs to your PC. It's less flexible than the higher end SDRs because you're limited by a tuned front end, but it still allows much of the same functionality.http://youtu.be/mfdgxZPc2BI

author: Bored@Work
author_rank: Super Contributor
timestamp: September 29, 2012, 08:05:41 pm
content: Quote from: poptones on September 29, 2012, 07:20:54 pmWhat part ofQuote from: Bored@Work on September 29, 2012, 10:20:30 amdidn't you like?

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 29, 2012, 08:20:02 pm
content: Quote from: Bored@Work on September 29, 2012, 08:05:41 pmhe means you dont necessarily sample at Nyquist, but what you need to do is sample at Nyquist. in another way of saying... "i dont disagree, but i agree"

author: poptones
author_rank: Frequent Contributor
timestamp: September 29, 2012, 08:22:53 pm
content: Exactly.But no....

author: gregariz
author_rank: Frequent Contributor
timestamp: September 29, 2012, 09:13:49 pm
content: I = in-phase componentQ = quadrature (90 deg shifted) componentTraditional direct conversion receiver has RF in --> mix with local oscillator down to baseband ---> filter ---> speakerSDR has instead RF in ---> power split 2 ways ----> mix with local osc at both inphase and at 90 deg shifted ----> sample both IQ at twice information speed (depends on modulation for AM ~ 6KHz) ----> demodulate ---> filter ----> speakerThe reason you go to this extra trouble is that by having I and Q you can implement a receiver to demodulate (almost) anything- AM, FM, digital mods..For the simple AM case on the video its dead easy... demodulator is simply AM = sqrt(I^2+Q^2). Job Done... pump it out to your speaker through a DAC.