Electrical – Where does SDR IQ data come from

radioRFsdrsoftware defined radio

I have never really done much RF design before, and am looking into getting an SDR and messing around with it. I have been doing some research into how they work, and havr found that IQ modulation and demodulation is very important. One thing that I do not quite understand is where the IQ data comes from. How does the digital logic in the SDR capture both the real and imaginary components of the signals being fed into it?

Best Answer

Short answer: The transmitter encoder/modulator e.g. QAM enter image description here

IQ modulation is used everywhere including SDR.

Understand some roots in phone data modems since above 1200 baud then up to 56kbps/64kbps then cable and DSK modems , Analog and digital phones , Analog TV since colour was broadcast and now digital TV and fast forward later to SDR's.

IQ modulation in phase and amplitude is simply a way of compressing signal bandwidth into a smaller spectrum at the expense of minimum SNR needed.

Since bandwidth is limited and expensive IQ modulation offered the original HDTV in the same bandwidth as Analog, 64K phone modems in the same bandwidth as 300 baud modems, colour TV and hundreds of other radio modulation schemes that use IQ modulation.

The purpose of IQ modulation is significant and pervasive to all communication channels where bandwidth compression is not only desirable, but essential. Although I eluded to the fact it is used in ANalog and Digital communication, I'll touch on the digital radio side for you.

The main figure of merit and purpose of IQ modulated data is bandwidth compression at a tradeoff for excess SNR.

It has a long history that includes telephone modems that use 1200 and 2400 Hz in a 4KHz bandwidth. The computer uses a UART's to send serial asynchronous data such as; 7 or 8 bits data + 1 start + 1 stop and 1 parity bit (opt.), to the Modem. THe Modulator then strips out the start/stop bits & encodes it into synchronous waveform in IQ modulation. The demodulator does the reverse. This is how one can get 64kbps in a 4KHz BW. For EMI concerns the FCC limited 64K modems to 56kbps.

I and Q of course represent In-phase 0deg and Quadrature 90deg signals.

Let's look at the QAM-16 IQ channel. The Modulator for QAM-16 creates 16 states of amplitude and phase in each symbol period which is a 4x4 matrix of 16 states. Each state is used according to 16 different 4-bit sequential bit patterns.

See below. http://en.wikipedia.org/wiki/Constellation_diagram enter image description here The eye diagram shows the 4 sequential binary patterns assigned for each dot where X-Y patterns represent I-Q phase and amplitudes.

There are hundreds of modulation methods which use IQ modulation but they each follow the same basic method of I&Q channels but with many levels not just 2 (binary).

http://www.scribd.com/doc/43027465/Modulation-Coding-Scheme-MCS-Table

Above are some basic ones defined the Modulation & Coding Schemes up to 64 levels.

There are many more at 256 levels and above, and each method has a tradeoff for quality, spectral efficiency, cost in terms of higher Signal to Noise ratio required.

This is defined by the Shannon–Hartley theorem which defines the solution for the minimum channel bandwidth vs SNR vs Bit Error Rate (BER), which are significant communication tradeoffs.

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