pike said: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??
I don't know what the legal situation in Australia is?, but in the UK aircraft model radio systems are covered by very strict legislation, they have their own specific band, channels and modulations.
Bear in mind your helicopter is an EXTREMELY dangerous weapon, and could very easily kill someone. Assuming that happened?, you would quite rightly be guilty of murder!, and presumably would be sentenced accordingly.
I would STRONGLY suggest that you look in to the legal implications of this! - also the licence free radio modules would NOT be legal for this purpose!.
I would also strongly suggest you buy a commercial aircraft radio system, if you want to experiment with radio, DON'T do it with an aircraft.
So that's that sorted out. How far will that 300mw limit get me??The Radio-controlled Models Class Licence provides for the operation of transmitters that have a maximum equivalent isotropically radiated power greater than 300 milliwatts and not exceeding 1 watt, as such transmitters are typically associated with the control of the more sophisticated types of models. Specifically, the class licence provides for the control of model aircraft, landcraft and watercraft in the frequency band 29.72 to 30 MHz, and model aircraft and watercraft in the frequency band 36 to 36.6 MHz.
In the 36 MHz band, model aircraft and watercraft operations may operate on a carrier frequency obtained using the formula:
35.990 + n(0.020) MHz where n = 1-30,
whereas only model aircraft may operate on a carrier frequency obtained using the formula:
36.000 + n(0.020) MHz where n = 1-29.
Transmitters intended to operate under the Radio-controlled Models Class Licence must not exceed the maximum output power that is specified in the licence. To do so is a breach of the class licence conditions.
pike said:So that's that sorted out. How far will that 300mw limit get me??
Nothing was said specifically about aircraft models and transmitter limitations, apart from the fact that the engine/motor cant have more than 4kw of power behind it. model cars and boats are allowed 8kw.
And according to that rule, my remote control Tank based on 433mhz licence free modules is illegal.
Do I have to use the standard 10.7 and 455khz frequencies or can i select my own IF to work with??
pike said:one last question, The mixing requires 2 inputs, A local oscillator input and an RF input. Do both signals have to be of equal power (as in voltage amplitude). Do they have to be the same. Does this mean I will require an automatic gain control circuit prior to mixing??
pike said:So the detector will work with a square wave??
At 455khz, will that saturation problem matter much?? How about mosfets instead??
Nigel Goodwin said:Why don't you just use an FM IF IC?, there are a number of them (usually Motorola), designed for this exact purpose - dual conversion NBFM.
pike said:The oscillator should work. I pulled it out of the circuit audioguru designed here, the mod4 version that is: https://www.electro-tech-online.com/threads/low-moisture-content-tester-help-wanted.14899/. If it works at vhf I cant see why it wont work at 25.3mhz.
The first diagram I drew looks iffy, probably because the resistor on the base was supposed to be a capacitor.
To save space, I decided to go with a simple prewound choke and capacitor in a series resonant filter setup. The reactance of the capacitor at 36mhz is about 550 ohms where as for the inductor it is about 490. Will this 60 ohms difference matter??
pike said:Yeh harry is a good man, kind enough to share his ideas. His site was the main basis of this project. Most of the stuff i pulled together has come from his site.
From the reactance formula, i can figure that at 25.3mhz that 10pf capacitor has about 600 ohms. Does that sound high for a feedback capacitor??
*back to the drawing board. :x
EDIT: seeing that if I used the transformer on the aerial, do I just connect the base to the secondary output or do i connect it onto the primary. Connecting it onto the secondary sounds like it will be more attenuative, and have lower gain.
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