--- In [email protected], "[EMAIL PROTECTED]" <[EMAIL PROTECTED]> wrote: > > Alberto > Hello, I am thinking about how SDR seems to work from a non > software programmers perspective. Which means in laymans terms. > And I could have the wrong idea here but my experience with the > softwares so far leads me to believe that the software is a kind of > DSP ware that acts like a i.f. mixer by injecting a beat frequency > into the base band signal in the sound card. --- snip ---
Dan, you have it basically right. The software receives a range of frequencies, let's say from -48 to +48 kHz. The frequency can be negative, being the signal described by quadrature components, the famous I and Q. Inside this range, the task of the software is to isolate those 3 or 4 kHz of interest to the user, and bring them to baseband, i.e. to a zero IF. To do this, nothing more, nothing less than a mixer is used, implemented in software instead of hardware. And a mixer of course needs a LO. So inside the program there an NCO (Numerically Controlled Oscillator) which produces the needed samples of a sinusoidal signal, together with its counterpart shifted by 90 degrees. The shifted signal is needed as also in this final conversion we want to eliminate any image response. Being now in software, implementing a mixer is quite straightforward. It is just a multiplication between two complex numbers, that's all. And this is a _perfect_ mixer... differently from its hardware counterparts, where some higher-order terms are inevitable, leading to spurious products and hence to intermodulation, this mixer does not suffer from such problems. So what is done when the user selects a different frequency to listen at, is just to change the value of signal generated by the NCO. Then the now converted-to-baseband signal is routed to the demodulators for further processing. Conceptually it is more or less identical to how it could have been implemented it in hardware. Only you don't need to breath the solder smoke when building it...:-) And making changes is somewhat faster and less expensive... 73 Alberto I2PHD
