Hi Jan, > My experiences are limited. For AM reception > I can vary the band (e.g. more lower side band, less upper > side band or vv) around the virtual center, that helps a lot in handling > noisy signals nearby the frequency received. Yes, with a conventional AM receiver you have two parameters: Center frequency and bandwidth. That is the same in analog and digital systems:-)
> I think I mean improving S/N ratio, esp for smaller signals, > receiving rtty (b)psk is my favorite. With an SDR you might remove various kinds of non-random noise by use of clever software but otherwise there is in principle no difference between analog and digital receivers. To receive RTTY you need a linear receiver (SSB mode.) The detect process would be in some external software that would work exactly the same regardless of whether the linear receiver is an SDR or an old conventional receiver. > My background in information theory is limited. > In the series of articles in QEX on software radio for the masses > (that inspired me quite a lot) there were some mysterious > comments in the description of the front end signal handling where > he spoke about "improving the signal" without elaborating on it. Improvements in the front end could be better rejection of image frequencies and other spurious responses as well as lower noise floor (higher sensitivity) In principle it is trivial. Just add a preselector with narrow bandwidth and low noise figure. With high enough gain any radio would get ideal performance for those parameters. (As long as dynamic range is not a problem. With high enough selectivity it would nearly always be OK.) There is no particular article that comes to my mind where you could read about those very basic aspects of radio. Just read "here and there" until you find something specific that you do not understand. Then you would be able to formulate a specific question that you might post on this or some other mailing list:-) The theory of an ideal receiver is quite simple. Most of what you can find in the literature is about non-ideal behaviour (dynamic range.) Very often receivers are good enough to be considered ideal. The signals we hear are the real ones and not spurs, images or artefacts caused by poor dynamic range. Then the radio is just a linear process that shifts the frequency and removes signals outside some selected filter bandwidth. The linear radio is always followed by a non-linear detection process which could be inside a human brain (CW, SSB) or in a computer (RTTY, JT65,...) or a combination (AM, FM.) The theory of detection is the exciting and difficult part which is right now under development. 73 Leif / SM5BSZ
