Before, I start into the software definable radio and the standards surrounding the SDR project, I’d like to clean up some loose ends.

Legacy radios, meaning radios that do not operate in the digital domain modulate the radio frequency carrier using A.M. and F.M. there are some r.f. modulation schemes that involve phase changes of the r.f carrier; however, the majority rely on modulating the r.f carrier’s amplitude or modulating its frequency.

When we enter the digital radio era, we convert the analog input (i.e. the human voice) to a digital signal using something like PCM previously discussed. We can then manipulate the digital stream applying algorithms to compress the data stream. We then modulate the carrier frequency with a digital stream representing voice, data or video information. Because we are dealing with a stream of pulses we have to concern ourselves with ways to modulate the carrier frequency so that pulse width, pulse amplitude and pulse repetition rate can be successfully retrieved at the receiving end. For that reason virtually all digital radios use combinations of frequency modulation and phase modulation schemes or amplitude modulation and phase modulation. Phase shifting, involves shifting the r.f carrier along the time domain which is represented on the x-axis of a Cartesian coordinate. We don’t alter the frequency we change its phase.

In my last three posts dealing with JTRS I presented relatively complex technical details about radio communications using some simplistic language. However, if you understood the concepts, you now have a better understanding that the average citizen.

All future posts will be strictly JTRS relevant.