--- In [email protected], "Patricia Wilson" <[EMAIL PROTECTED]> wrote: > > Thank you Carlos, a useful answer. > > I am rather new to this SDR thing and have a really newby question. > Where in the chain between antenna and speaker should the signal be > sampled by an SDR? The Nyquist limit imposes a sample rate of at > least twice the highest frequency component so if you want to sample > at the input RF and that RF is 29 MHz then you need a sample rate of > 58 MHz which is not only pretty fast but will also produce a LOT of > data very quickly. On the other hand if you mix it down to a first > IF you lose the possibility of digitally filtering out unwanted > signals earlier in the signal path before they have a chance to > overload something. > > As with nearly everything in engineering it is a trade-off. But > where do most SDR's do the sampling?
It depends on what you mean by "most". There are probably more "baseband" SDRs (in effect direct conversion) "in the field" as they are reasonably cheap to make and can be very versatile. The Softrock series of boards http://www.wsplc.com/acatalog/SOFTROCK-RXTX.html each of which can be squeezed to cover most of an amateur band using a single crystal, are probably the most populous. There are also a number of home brew designs. I myself have the Elektor (Dutch Magazine) design which is general coverage. Providing you use a pair of mixers with 90 degree out of phase carriers you can easily eliminate unwanted side bands and even demodulate FM digitally. By putting the mixer close to the antenna you don't get the same blocking issues you get with traditional multiple conversion Receivers, and the image problems are much reduced. These are also useful as add-ons to traditional receivers when tuned to the IF frequency and used to replace the filtering function. In this case you may still get issues from the higher level mixer, but you do get a very versatile set of filters. In addition you can use software (I forget its name and google is no help) that will simultaneously decode and display multiple CW signals at once... On the other hand I feel that direct sampling receivers must offer the way forward. Sampling directly at RF will give the best fidelity of signal. Trouble is at present its more expensive :-( Dave G4UGM
