Some questions about simple Transmitter-Receiver RF AM circuits - Page 1
Forum section: electronics / beginners

author: Tricka90
author_rank: Regular Contributor
timestamp: March 15, 2014, 11:41:39 pm
content: Could you please answer some questions about an AM Transmitter and Receiver circuit I found?The first one is the Transmitter, you can go to 0:22 to see the circuit diagram:What does the Oscillator Chip do exactly? I know it should mix the Audio Signal with the Carrier Wave. Is this a complex process or is it very simple? I would like to know what happens inside it or at least have the equivalent circuit diagram.The second one is the Receiver, go to 0:26 for the circuit diagram:My question is: if I substitute the DIY capacitor and inductor with commercial ones, with a fixed value, would the circuit work the same? Of course I wont be able to select different frequencies, but would it work properly?My last question is: with this super-simple Transmitter-Receiver system will I be able to reach a communication distance of some meters, if I use big antennas and good signal amplification on the Receiver?Thanks a lot to anybody will help :-D

author: Hideki
author_rank: Frequent Contributor
timestamp: March 16, 2014, 02:32:51 am
content: Oh my.. that's really nasty.. but it worksThe oscillator is this:http://www.foxonline.com/pdfs/f1100e.pdfIt outputs a TTL square wave and is meant to run on 4.5-5.5 volts. Here he uses more like 6 volts (depends on how used the batteries are), but it appears to handle that just fine.To perform the amplitude modulation, the transformer is connected in series with the supply voltage, so it can add and subtract the amplified audio signal. If we say the output of the transformer is +/- 1 volts peak, then the oscillator will run on a varying 6 +/-1 volts supply (5 to 7) and the amplitude of the square wave will change accordingly.This is abuse of a poor clock chip for a digital system, but it very roughly does what you need for an AM transmitter.I didn't watch the second video yet, but you can use fixed components. Not sure why you would do that however, as you would have very little idea of what it would be tuned to, and thus little chance of picking up your transmitted signal.

author: Tricka90
author_rank: Regular Contributor
timestamp: March 16, 2014, 07:46:06 am
content: Thank you for the replyBut could you please be more precise about how the Chip does the mixing?Let's say I want to do the mix using a very simple 2 PIN oscillator like this one:https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcSPg4qe120QRqJUyxmVCqPOxV4-a3FdZFdP2sPqwMQ_XO4mTpBvwould it be possible or the Chip hides some other processes?

author: Simon123
author_rank: Regular Contributor
timestamp: March 16, 2014, 08:42:55 am
content: The "chip" is oscillator and quartz on your link needs support circuitry, which "chip" already has inside.The transformer does the mixing, not the oscillator.

author: PA0PBZ
author_rank: Super Contributor
timestamp: March 16, 2014, 09:26:49 am
content: There is no real mixing involved, the signal is amplitude (=strength) modulated.What the circuit does is it varies the supply voltage of the oscillator, and by doing that it also varies the output strength of the signal, forget about mixing for now.

author: Unknown
author_rank: Unknown
timestamp: March 16, 2014, 09:36:40 am
content: If you google "simple AM transmitter" you'll see plenty of other sites discussing the same or similar circuits. In far, far too many cases the transformer is described as the "modulator" or "mixer" without further explanation.(e.g.here,here, and the comments on the first video.)That may be the source of the OP's confusion.