Tag changed to EE as it should have been in the first place. Jose Da Silva wrote: > An AGC is needed unless you do FM Not necessarily. If it's simple binary AM modulation, AGC is only used for the purpose of deciding where the threshold between on and off should be. There is no need to actually normalize the full carrier on level to some preset value, only to determine what it is. This is commonly called a "data slicer" circuit. Of course this can also be done in software with very little electronics. One of my current projects is a receiver for 434MHz ISM band low power instruments. The modulation scheme is simple binary carrier on/off at 10Kbit/second with machester encoding. The previous design used a bunch of opamps and filters and comparator to convert the output of the RF receiver into a digital signal. My new design does this all in software on a dsPIC. The dsPIC samples the RF signal strength at 100KHz, and the rest is software. In the process, it also measures the full on RF signal strength used further downstream in other parts of the system. > Radio passes through walls. > Your 100mW may still be overshadowed by a transmitter a mile away > pumping out 5W. Very unlikely. *Think* about it instead of just blurting something out. 5W / 100mW = 50 power ratio. SQRT(50) = 7.1, which is the distance ratio required between a 5W and 100mW transmitter to have the same received signal strength. 1 mile / SQRT(50) = 747 feet = 228 meters. I thought the OP said his transmitter would be "nearby" in the same room. Now of course in practise received signal strength doesn't go down smoothly with the square of the distance, but that's the general trend. Obsorption of the signal by material between the transmitter and receiver will make this fall off even more quickly. One study showed that in a hospital instead of falling off with the distance to the power of 2 it's more like 2.8. Multipath distortions can cause hot spots and cold spots at particular locations, but these are essentially noise on top of the overall attenuation with distance trend. > ... unless your frequency of choice is dedicated to > low-power stuff, for example garage-door openers or particular bands > like that. If you use those bands, then, yes, you're likely okay. This is called the ISM band, and is a good idea for low power short distance stuff like this. Note however there are some restrictions in this band too, particularly on tranmission duration and repitition rate. If you're doing this for private purposes on your own property, just keep the power low and don't worry about it. Technically it's illegal, but as long as the signal is well attenuated by the time it gets to the edge of your property, nobody is going to notice or care. The FCC has better things to do unless you become a nuisance to your neighbors by jamming all their garage door openers and outside thermometers. > I'm not sure why you need freq stability. In a product you definitely do. There are a lot of demands on the RF spectrum, so keeping everyone to their frequency is important. In practice, transmitters need to be crystal based. Good frequency stability and accuracy of the transmitter also allows the receiver to discriminate better between the signal and noise. > AM is what I thought you meant from your original posting. > FM FSK PSK is much much better, so I'm glad you chose it. That's a pretty irresponsible statement without more qualification. FM is absolutely not "better" than AM in all circumstances. As with everything, it's a tradeoff and each has their strong and weak points. FM circuits are more complex and therefore more expensive. It will also take more power for the same number of bits sent. On the other hand, FM has more immunity to interference from weaker sources. I don't remember what the OP wants to do, but both could be viable choices. >> The simplified requirements are: I need two black boxes that would >> let me connect two terminals 10 meters apart, and allow communication >> at 1200 baud. I know you guys tend to assume there's a pic on each >> end that I can squeeze some modem code into, but that's not the case >> here. :-) Ah. You must always think of a radio link as an error-prone channel. At only 10 meters and 1200 bits/second, simple circuits will work fine most of the time. The only way to guarantee reliability is to have the bi-directional communication. The higher level protocol uses ACKs and NACKs and retries until the data gets thru. If you can tolerate some gaps in the data stream, then you can just use a simple transmitter and receiver with packets and checksums. The receiver tosses out any packet that isn't right. What is the application? ****************************************************************** Embed Inc, Littleton Massachusetts, (978) 742-9014. #1 PIC consultant in 2004 program year. http://www.embedinc.com/products -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist