Simple(?) RF transmitter. Am I foolish? - Page 1
Forum section: electronics / beginners

author: tehtehteh
author_rank: Regular Contributor
timestamp: June 09, 2021, 06:45:03 pm
content: Firstly I would like to mention that I don't really know much 'real' electronics. All the projects I have worked on mainly involve simple, logic level electronics.I have a project involving a key fob where I would like to take the circuitry and integrate it onto another board..... And it still needs to unlock the same thing afterwards. I have reproduced the circuit diagram of the original below (most values currently unknown). The microprocessor side of the circuit doesn't concern me and isn't included. The RF side........ does concern me.I have been trying to read up about this but the information I've found so far is, understandably, aimed at people who know more about the subject than I do. From what I can guess by looking at it, the resonator/transistor/capacitor circuit is producing a 433.92Mhz wave that is modulated by the data signal (which I measured to be 4kbps), what I think is known as 'on-off keying'. This is going through a PCB trace acting as an antenna.I think this is some form of Colpitts oscillator but Google brings up many variations and this isn't one of them. This looks deceptively simple in the circuit diagram but I understand these are carefully tuned taking into account capacitance and inductance of components and pcb traces etc. Looking at the original PCB there is a wiggly trace coming from the resonator which is probably doing something important, and the ground areas have been separated around certain components probably for a good reason too. To think I could directly copy this and have a working result would be naive of me, especially since I only have a 100khz oscilloscope to (not) test with and no appetite to throw money at iterations.So I'm thinking about transferring just the microprocessor side of the circuit over to the new board and making the RF side of it new. Is the idea plausible at least? If I keep traces short, use a full ground plane, consider impedance etc maybe RF circuitry doesn't have to be impossible for people like me?A big constraint is I cannot change the microprossor, so the data pin is all I have to interface with. I have looked at RF ICs but they need to have automatic switch on and shut down, and of those I have not found any that have a switch on time compatible with 4kbps data (They are designed for the microprocessor to send a pulse first).So what I would like to know is how the oscillator works, and if it's possble to implement a similar oscillator sucessfuly as a DIY job, possibly with a chip antenna? Or if anybody has knowledge of a compatible RF IC? I would appreciateanyinput from people who are more knowledgable about the subject than myself.

author: mikerj
author_rank: Super Contributor
timestamp: June 09, 2021, 08:02:49 pm
content: You can buy basic 433MHz AM transmitter modules for very little money, though usually as part of a TX/RX pair. e.g.ebay. If you don't mind the shipping wait they can be bought from Aliexpress etc for about a third of that price.

author: tehtehteh
author_rank: Regular Contributor
timestamp: June 09, 2021, 08:15:46 pm
content: I have looked into these and the simplicity is very appealing, but I should have mentioned I have 16mm by 26mm of board space to fit this all in including the microprocessor, and also 2.8mm clearance above the board. They also suffer from the same switch on issues as the RF ICs, probably because they're using similar ones on board. Another problem is the supply voltage being 3v to 12v puts my 3v battery immediately in danger of not meeting the lower end of the range.

author: Manul
author_rank: Super Contributor
timestamp: June 09, 2021, 08:50:10 pm
content: As I know, these modules which mikerj suggested (TX side, with round SAW resonator) is almost same like in your schematic. They work with OOK. You may also be able to find them in some local electronics part shops, bacause they are really popular. You can use them for testing, proof of concept or reverse engineer them.Making such oscillator as DIY is totally possible, nothing fancy here. Basically it may need some tuning of feedback capacitors, and a reasonable layout. Getting clean RF signal is another topic (harmonics filtering), but might not be important here.

author: bob91343
author_rank: Super Contributor
timestamp: June 09, 2021, 09:43:40 pm
content: What makes it a Colpitts oscillator is the capacitive voltage divider from collector to emitter. The inductor is provided by the loop and resonates with the series combination of those capacitors and the other one in parallel with them. Due to resonant rise of voltage, the collector to emitter signal path causes positive feedback. The frequency is stabilitzed by the crystal, but otherwise it has no function.

author: tehtehteh
author_rank: Regular Contributor
timestamp: June 10, 2021, 08:38:46 am
content: Quote from: bob91343 on June 09, 2021, 09:43:40 pmThank you for telling me this. I can see my understanding was quite far out.