--- In [email protected], k5nwa <[EMAIL PROTECTED]> wrote: > > For example you can have an amplifier and band pass filter followed by > one very high speed A/D converter, the rest of the functions of the > radio are implemented in software, this is a Software Defined Radio, > but there is no I and Q signals going into the computer, you can even > do it without having I and Q signals in the software, but you won't > have image rejection but it will still work as a radio.
Cecil, that is _almost_ correct, but not completely. When you use a high speed ADC to digitize directly the RF (as in SDR-IQ, SDR-14, Perseus), then you have, in the firmware of the radio, to downconvert and downsample the digitized data. To do this, a NCO (Numerically Controlled Oscillator) is used, with a half-complex mixer, all implemented in firmware, which is akin to software. The NCO generates two signals at 90 degrees each other, and the output of the half-complex mixer (which, implementation-wise, is just a couple of multiplications) is an I/Q pair, which, after downsampling, is sent to the PC, usually via the USB port. In this kind of implementation the I/Q pair is generated _after_ the conversion from analog to digital, so there are no problems of unbalancing and poor image rejection. Being all done numerically, everything is balanced (apart from truncation errors, but those are orders of magnitude lower). At the end of the story, you *always* need an I/Q pair, as what you have is a portion of the spectrum centered at zero Hz, and you have to select either the positive part of it, or the negative part when you perform the demodulation. And that is possible only if your signal is analytic, i.e. is composed of two quadrature components, I and Q. 73 Alberto I2PHD
