Another reason for using IF higher than 0 Hz is the high inherent noise level of typical transistors and opamps at low frequencies (so called 1/f-noise). This and the leakage of the VFO signals made me to move away from the zero-IF in my early switching (and Tayloe) mixer experiments. Fortunately, before spending too much time for re-inventing the wheel came Gerald's famous first article in QSX - and here I am!
Now is the time to modify the wheel! 73, Ahti OH2RZ On 21/05/06, Frank Brickle <[EMAIL PROTECTED]> wrote: > The DSP software is already capable of 0 Hz IF, and has been since its > earliest version. IIRC the 11025 Hz IF is primarily a consequence of the > frequency response of typical soundcards, which start rolling off > somewhere in the vicinity of 200-300 Hz. > > 73 > Frank > AB2KT > > Peter Martinez wrote: > >>From G3PLX: > > > > The software that comes with the SDR1000 uses an 11.025kHz intermediate > > frequency. I understand the reasons for doing it this way, but even before > > the SDR1000 appeared I was doing software radio with an I.F. of zero. By > > this I mean that the sine and cosine RF oscillators were set right in the > > middle of the wanted signal, not offset by 11kHz. This may sound impossible > > to those who were brought up with analogue RF, but that's because it could > > never be done with analogue circuitry. With DSP it's actually easier to have > > the 'IF' frequency down in the audio band than to push it up where you can't > > hear it. > > > > The big advantage of zero IF is that the 22kHz image response problem > > vanishes. Any slight amplitude or phasing unbalance in the Tayloe sampler > > just results in an equally-slight amount of in-band distortion. The > > strongest image-frequency signal you ever need to reject is the wanted > > signal itself. You don't need to worry about a much stronger unwanted signal > > 22kHz up the band. > > > > When I got the SDR1000 kit (I got a very early one), I used it with this > > technique, and the results were excellent, except for one thing. It took me > > a while to trace the problem, but I found it in the end and the cause was a > > surprise. The problem showed as noise around the centre-frequency of all > > received signals, but it varied across the bands, and was absent when I > > unplugged the antenna. It was so bad that it made the receiver unusable on > > some bands with some antennas. But if I used the SDR1000 to tap-off and > > demodulate the intermediate-frequency of another receiver, it worked > > perfectly. > > > > The cause was oscillator radiation. The DDS oscillator (right in the middle > > of the wanted signal) was radiating, intermodulating with all kinds of > > low-frequency noise sources external to the receiver, and the resulting > > unwanted products (either side of the oscillator frequency) were re-radiated > > into the antenna. The effect is well-known to anyone who has ever > > experimented with home-brew direct-conversion receivers, where it usually > > shows as a raw power-line buzz in the speaker. It's possible that this > > effect may well have shown in the early work on SDR and it may have been one > > reason for offsetting the passband by 11kHz in the present software. > > > > The fix is to stop the local oscillator radiation. Screening helps a lot but > > another way is to add an RF stage, or configure the receiver as a superhet > > with the Tayloe sampler at the I.F. frequency. My early SDR1000 kit didn't > > have a pre-amp and I understand the current kits do. The local oscillator > > radiation is probably considerably lower on the present kits, so the zero-IF > > technique would probably work a lot better than it does on mine. > > > > Has anyone here who is writing his own SDR software tried this on the latest > > hardware? I can provide more details of the zero-IF technique if required. > > All the well-known modes can be implemented this way, both for receive and > > transmit. Maybe the present SDR software could be patched to implement > > zero-IF, or my own zero-IF software could be run in parallel on another > > soundcard. Would anyone like to have a go? > > > > 73 > > Peter > > > > > > _______________________________________________ > > FlexRadio mailing list > > FlexRadio@flex-radio.biz > > http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz > > Archive Link: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/ > > FlexRadio Homepage: http://www.flex-radio.com > > > > > > > _______________________________________________ > FlexRadio mailing list > FlexRadio@flex-radio.biz > http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz > Archive Link: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/ > FlexRadio Homepage: http://www.flex-radio.com > _______________________________________________ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://mail.flex-radio.biz/pipermail/flexradio_flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com
