pike
Member
HI guys i want to build an FM superhet receiver for my RC helicopter. Range will have to be about 200m. The transmitter is no problem thats already built. One problem though is the amount of infromation that has to be relayed to the reciever about 20,000 bits/second. I'm not asking for a circuit diagram or anything but i do have some questions:
Seeing that a superhet design revolves around the idea of mixing signals down to a much more manageable signal frequency, How exactly do the signals get mixed ?? Do i just connect the outputs from the RF tuner and the LO together and then feed them into an amplifier?? Someone told me that this was adding, not mixing. :?
Say the modulation bandwidth is -+5khz at 20mhz, after the mixing reduces this signal down to 455khz will the modulation bandwidth still be 5khz?? or this be reduced down as well. (ie, will it stay 450khz to 460khz)
Why do some recievers use 'dual-conversion' as in the RF signal heterodynes to form 10.7mhz, which is then heterodyned again to form a 455khz signal. cant you go straight from RF to 455khz, and just skip 10.7mhz??
I know there are license free modules available that could easily cover the requirements, but the prime reason in doing this is to learn about RF. Getting the model helicopter in the air is just a bonus.
edit: And why do AM receivers sometimes have 3 IF filters all based on 455khz, couldn't you use just one filter?? Does this have something to do about the Q of the IF tuners and the selectivity??
Seeing that a superhet design revolves around the idea of mixing signals down to a much more manageable signal frequency, How exactly do the signals get mixed ?? Do i just connect the outputs from the RF tuner and the LO together and then feed them into an amplifier?? Someone told me that this was adding, not mixing. :?
Say the modulation bandwidth is -+5khz at 20mhz, after the mixing reduces this signal down to 455khz will the modulation bandwidth still be 5khz?? or this be reduced down as well. (ie, will it stay 450khz to 460khz)
Why do some recievers use 'dual-conversion' as in the RF signal heterodynes to form 10.7mhz, which is then heterodyned again to form a 455khz signal. cant you go straight from RF to 455khz, and just skip 10.7mhz??
I know there are license free modules available that could easily cover the requirements, but the prime reason in doing this is to learn about RF. Getting the model helicopter in the air is just a bonus.
edit: And why do AM receivers sometimes have 3 IF filters all based on 455khz, couldn't you use just one filter?? Does this have something to do about the Q of the IF tuners and the selectivity??