i2phd wrote: > > > --- In soft_radio@yahoogroups.com <mailto:soft_radio%40yahoogroups.com>, > 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. > I stated so in fewer word, no image rejection unless you have I and Q says it all in fewer words. It's not the best way of doing it, it's actually foolish since the cost is so little, but it would be functional. I was trying to illustrate the I and Q is not the sole definition of a SDR you can have one without I and Q but it will have image issues, we have seen that before with radios in the past, it's nothing new.
-- Cecil K5NWA www.qrpradio.com www.softrockradio.org "Blessed are the cracked, for they shall let in the light."
