Interpreting this little RF receiver diagram - Page 1
Forum section: electronics / beginners

author: ivan747
author_rank: Super Contributor
timestamp: January 20, 2016, 02:16:11 am
content: I'm trying to understand this diagram I attached.It's supposed to be an ASK detector. I suspect it is a type of heterodyne detector, but I get lost separating the circuit into blocks I could find online or in textbooks. I expected there to be a crystal tuned at the resonance frequency (which should be 315MHz in this diagram, if it isn't, then it's 433.92MHz), but there isn't one.I'm not very experienced with discrete oscillator and radio circuits.I also suspect the LM386 is simply a comparator that takes the demodulated AM data and digitalizes it. There seems to be some automatic gain control in my experience with the device, because after a while it gets a lot of noise.I also attached a picture of the receiver.Thanks,Ivan

author: Paul Price
author_rank: Super Contributor
timestamp: January 20, 2016, 03:23:21 am
content: This is clearly a super-regenerative circuit. The LM358( a dual op-amp) doesn't seem to be the LM386 (audio power amp) you're talking about. Nothing digital about this circuit at all. If it is supposed to decode ASK, it would probably do a sloppy job it if it worked at all.The freq. is set by coils and caps, and the first transistor on the left seems to be a rather very low-freq. RF amp approach attempting to function as an UHF 300MHz+ freq. amplifier.BTW, the output is giving a chance for a DATE not DATA, and I guess there would be likely something really romantic about this idea, if I were a robot.Are you sure you posted the right schematic?R6 seems to giving the wrong value..I would guess it should be more like 470-ohms, not 47K.R3 seems to be also too high in value, maybe 1800 ohms would be correct instead of 180K.R8 also seems much to high in value, maybe 5.6K instead of 56K.

author: ivan747
author_rank: Super Contributor
timestamp: January 20, 2016, 04:55:02 am
content: Quote from: Paul Price on January 20, 2016, 03:23:21 amWhoops! Mixed those up in my head. You're right.I am 95% certain that the schematic belongs to the transmitter I have in hand. It shares all the prominent components, like the variable inductor, exactly 2 transistors, bot labeled R25 on the actual package and it's got the LM358.Your performance predictions are also right. If you transmit a 10kHz square wave with 50% duty cycle in on-off keying at 10mW, you get something like a 25% (or less) duty cycle wave that has some edges that look like ramps rather than edges. And the included transmitter still manages to occupy 600kHz worth of bandwidth (well I didn't filter my pulses in baseband either but still... it's supposed to be digital in, digital out).I'll try to find other versions of this schematic. They're floating around the web. I can also try to reverse engineer a physical receiver I have in hand.About the DATE thing, a Chinese friend of mine told me that the second chinese character in the "Date" port means "exit".A cross-reference website tells me the transistor is a 2SC3356 microwave low noise amplifier transistor. While technically true, it's an optimist name, isn't it?http://www.s-manuals.com/pdf/datasheet/2/s/2sc3356_nec.pdfTwo questions:Is the super-regen centered on Q2? I suppose it is as Q1 is working as en LNA of sorts, as you said.

author: ivan747
author_rank: Super Contributor
timestamp: January 20, 2016, 05:01:17 am
content: Here's the transmitter that completes the set, plus a bonus spectrum analyzer screenshot. That's 600kHz bandwidth for a 2400 baud/s digital signal.I concluded it's OOK, and that it basically works by literally switching an entire transmitter circuit on and off from the low side. I would have expected something more elegant involving Q1, but instead, it just switches the entire oscillator block on and off like it's 1895. Are my expectations good expectations?

author: Paul Price
author_rank: Super Contributor
timestamp: January 20, 2016, 07:53:40 am
content: I am certain 100% the schematic with the LM358 is not the transmitter, but you posted what could be the transmitter.Modulating by switching the Xtal oscillator on and off is a very bad idea since it takes a finite time for a Xtal to begin to sing, so the transmission rate must be quite slow and the baud rate of transmission must be very low for it to work at all..but the same is true for the receiver, since the LM358 is quite limited by in slew rate so that only very slow transmitted bits (<300 baud) could be recovered.As the circuit is now, if it works at all, the only good thing is that the unexpected high values of three resistors I noticed would result in the receiver having a very long battery life.This long battery life idea and low cost would work fine for making a receiver that could replace a more expensive and maybe harder to obtain cellphone to hack, so among more mundane uses, it could be used to receive a trigger sequence code for an IED.Interesting enough, the only website posting the only info found when googling about this device's posted partnum was in Turkey, one of many countries not without some serious ongoing problems with religious and political factions.