Martin, I'm not sure what you have in mind exactly, but I believe you may be either misinformed, or uninformed. Take this quote "The reason I chose AM is that its frequency band is still digitizable - I don't have to mix it down like I would with FM or whatever." I'm not sure what you mean here by "its frequency band is still digitizable". Unless you mean that since you are dealing with AM, you only have amplitude to be concerned about. And that indeed is digitizable. However, FM is also just as digitizable. And where you say I don't have to mix it down like I would with FM or whatever.". What does that mean? In a typical radio receiver, there are a wide range of frequencies that need to be receivable, and decodable. One problem with amplification for various radio circuits is that at the frequencies involved, there is a substantial dependency on frequency regarding the amplification factor of a given stage. To aleviate this problem to a great extent would be to have a fixed frequency to amplify. This would give the needed amplification, but not have to worry about the frequency dependency. Now, how can that single frequency be obtained, and still convey the original information that was to be conveyed via the radio channel? Through the process of hetrodyning. How this works is that there are two (at least) circuits that are tuned when you change the frequency dial on a radio. One of these circuits is the tuned circuit that selects the frequency that you want to receive. The second is the Local Oscillator. The L.O. is tuned, in the case of the AM broadcast band in the US, 455 Khz higher than the frequency of choice. The L.O signal, and the tuned radio signal are mixed in a non linear component. The output of this circuit contains 4 frequencies. The original signal tuned, the L.O signal, The difference between the L.O and the tuned signal, and the sum of the L.O and the tuned signal. Through filtering, the difference signal is preserved and sent to the I.F amplification stages. The I.F, or intermediate frequency, stages has a single frequency to work with, namely 455 Khz. So this signal is amplified many hundreds to thousands of time in the IF stage, and then sent to the detector, where is is detected (rectified), and sent to the AF (Audio Frequency) amplifier(s). That is the main reason in US AM broadcasting for mixing an RF frequency down. A direct conversion set operates basically the same way except that it doesn't really use the IF stages. In the Direct conversion receiver, the incoming and the L.O. frequency differes by only the audio range. Therefore, When you tune a signal from the air, the L.O is only audio frequencis away from the incoming frequency. When they are mixed as above, they produce 4 frequencies also. But here the audio difference is sent through to the A.F stages. All the other frequencies are eliminated by filtering. And you hear the original performance as it was intended when broadcast. Of course, this "as it was intedned when broadcast is contingent on the bandwidth of the receiver and the original signal. Suffice it to say, it will sound very limited in audio bandwidth because of the receiver design, but mostly because of the limitations of the badwidth limits of the broadcasted signal which is dictated by the FCC. Anyway, this is a long winded way of describing why mixing and downconverting are needed and how they work. If you have any other questions, just ask. Either myself, or one of the other very knowledgable people on this list, will try to answer you. Regards, Jim -----Original Message----- From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of Marcel Birthelmer Sent: Tuesday, November 25, 2008 1:00 PM To: Microcontroller discussion list - Public. Subject: Re: [EE] ADC development module recommendations Martin, my objective is to do as much as I can in software/FPGA. That is, I get the digitized antenna feed (the ADC input stage itself will probably require a LPF, but that's it), tune/detect/filter it using DSP magic, and spit out some sort of signal to an audio DAC. I realize that this is not the way "proper" radios work. The reason I chose AM is that its frequency band is still digitizable - I don't have to mix it down like I would with FM or whatever. But I'm still reading as I go, and you're right, it may be unfeasible the way I have it planned right now. Thanks, - Marcel What's your front-end look like? > You can't just feed an antenna into an ADC and expect to get a good > signal (or any).. I'm not an RF guru but you at least need an input > filter (band-pass) and maybe (maybe, it's a modern idea) - just a > first-stage amplifier if you're going for direct conversion. I'm not > sure if this works the same for AM. I'm not an RF wizard. > > Normally, an RF amplifier has a local oscillator (or two) that gets > mixed with the input signal. A low pass filter then removes the high > frequency parts so you're left with audio-range frequencies. You'd > then only have to sample at something like 48kHz. > > If I were you I'd first work on how you're going to receive the RF in > the first place. Get an ARRL Handbook and also read about the > "SoftRock-40" project. > > - > Martin > -- > http://www.piclist.com PIC/SX FAQ & list archive View/change your > membership options at http://mailman.mit.edu/mailman/listinfo/piclist > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist