On Sat, 5 Jul 1997 23:02:38 EDT Harold M Hallikainen writes: [...] > Thanks for the responses... Guess I didn't make the >requirements >clear. What we're trying to do is make a very small transmitter and a >very small receiver. The receiver needs to let us know if the >transmitter is "out of range" (about 10 feet away). Distance itself >is pretty noncritical. since this is a very short distance for RF, I >was thinking I could just do a damped sine wave generator by stepping >a PIC output from 0 to 5 volts or back. This output would probably >drive a series LC tuned to some RF frequency (as allowed by part 15 of >the FCC rules for unlicensed devices). The idea is that at the >resonant frequency there would be a high circulating current in the >LC, causing radiation from the L as the current decays due to losses. >We're kinda pulling an RF harmonic off the step waveform. A single damped sine wave is going to be impossible to differentiate from impulsive noise, which is common in the 49 MHz band. At the very least, send a series of several pulses with known timing. Also at 49 MHz the typical PCB mounted coil's physical size will be too small to act as much of an antenna. Some sort of wire antenna will likely be needed. You're also going to have to fight with a lot of strong FM signals from cordless phones, baby monitors, fire trucks and police cars, etc. A higher band would have less interference. At 312 MHz, about all that could interfere are garage door openers and the Air Force. >That's kinda what I have in mind for the transmitter. For the >receiver, another LC driving something. Perhaps a current input type >RF amplifier. The L would again serve as the receive antenna. The RF >amp would drive some sort of detector which would drive the receiving >PIC where some coding would identify the unique transmitter. this is >a little like keyless entry stuff. Could also use "transponder" >technology, but it seems that a separate transmitter and a separate >receiver is more cost effective here since they would always be sold >as pairs. I think transponders are typically used where there a bunch >of real cheap transponders and a few relatively expensive units for >polling them (like theft prevention tag systems). So... anyone Theft prevention tags are just nonlinear antennas that resonate with the transmitter's strong signal and generate harmonics which are detected by the receiver. The tags themselves are completely passive. Some of them have a coil and a diode and others have a piece of mu-metal biased with a magnet. Transponder tags which send a code such as for toll payment or implementing the New World Order use an antenna connected to with a switch driven from open to short by the code at about a Kbaud. The polling system has a transmitter and receiver tuned to the same frequency. The transmitter sends a continuous signal and the receiver looks for AM modulation in the returned signal resulting from the variation in the reflection from the tag. Both of these systems make the polling unit expensive because (a) transmitter power on the order of watts is required and (b) precise RF components are needed to keep the transmitted signal from overwhelming the receiver. >have some ideas in this area? Cost is very >critical, but if we are only transmitting a few feet, it doesn't seem >that we should have to go to triple conversion superhet receiver >design! Go with AM in the 312 MHz garage-door opener band. The transmitter is just a single transistor oscillator with a printed coil operating as both the inductor and the antenna. The receiver is a regenerative design with a single transistor doing all the RF, typically followed by an op-amp for baseband gain. Since the receiver is an oscillator, it is conceivable that the same circuit could be used for both transmit and recieve. However, because the receiver is an oscillator, it will likely interfere with other nearby receivers (garage door opener receivers often use an RF amp to improve sensitivity a bit and also reduce the regen oscillator leakage back out the antenna.