Is this a working transmitter? - Page 1
Forum section: electronics / beginners

author: SolderOcelot
author_rank: Contributor
timestamp: November 25, 2018, 01:36:10 pm
content: Reading about (crystal) oscilators gives me the confidence they're really simple and I could build a simple RF transmitter. However after looking at simple example circuits I see they jam them up with resistors, capacitors, transistors, paralel lines etc. It's driving me nuts.I drew this simple schematic. Shouldn't this be a simple working RF transmitter?

author: Benta
author_rank: Super Contributor
timestamp: November 25, 2018, 03:16:50 pm
content: I see a crystal but no oscillator. You might exchange the crystal for a resistor and ask: "Is this a working amplifier?"

author: HB9EVI
author_rank: Frequent Contributor
timestamp: November 25, 2018, 03:22:12 pm
content: just greatthat remembers me some voodoo devices you patch on the rear of your cellphone to change bad radiation to good radiation

author: SolderOcelot
author_rank: Contributor
timestamp: November 25, 2018, 03:28:51 pm
content: Quote from: Benta on November 25, 2018, 03:16:50 pmI thought the "crystal" is an oscilator. It takes in DC and outputs AC as I understood. Correct me if I'm wrong, I'm here to learn.Quote from: HB9EVI on November 25, 2018, 03:22:12 pmIf you help me learn how to create a simple oscilation that goes through the antenna I promise to build you a voodoo device one day.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: November 25, 2018, 03:36:57 pm
content: That's not so simple like you imagine.Basically an oscillator is an amplifier with a positive feedback, and yes, an amplifier needs DC to operate, but it can ampify DC and also AC signalsif you already studied schemes of oscillators you see, that there are resistors, capacitors and transistors - they are needed. there is no amplifier without at least one transistor, but no transistor can work properly without resistors.Capacitors are needed to generate the necessary phase shift to turn the amplifier into an oscillator.check once for 'pierce oscillator'

author: ebastler
author_rank: Super Contributor
timestamp: November 25, 2018, 03:48:15 pm
content: Quote from: Faloude on November 25, 2018, 01:36:10 pmHmm, so where did you "read about crystal osciallators"? Apparently that source did not give you the full picture.If you want to think about them in terms of a mechanical analogy, a crystal oscillator behaves roughly similar to a tuning fork: If you "excite" it by a signal pulse, it will oscillate at a resonance frequency. (Yes, that's a bit oversimplified...) But that will only give you a short, decaying burst of oscillation. In order to obtain a sustained oscillation, you need a way to keep the oscillator going: Sense where it currently is in its oscillation period, and give it another nudge regularly, at the right moment (i.e. in phase with its current movement).Some active amplification element -- typically a transistor -- is needed to do the "sensing and giving the oscillator a nudge" bit. So you end up with a more complex schematic. This page has a few basic ones, for example:https://circuitdigest.com/tutorial/quartz-crystal-oscillator

author: janoc
author_rank: Super Contributor
timestamp: November 25, 2018, 03:48:31 pm
content: Quote from: Faloude on November 25, 2018, 03:28:51 pmI think you are confusing crystal oscillators with crystals.Crystal oscillators often come in these metal cans but usually have 3 or 4 pins - power, ground and the signal output (the 4th pin is unused). Those will produce some sort of AC signal when you power them up.Crystals have only two pins and there is nothing else inside the can but a piece of quartz in a special holder. This will not do anything by itself, they only act as a very narrow band (and stable) filter/resonator around the frequency printed on the can. You can build an oscillator out of it but it needs some sort of active device to keep the crystal vibrating (think of hitting a gong - unless you keep hitting it it will stop sounding after a while) - typically a transistor oscillator and a something like a buffer stage. That is what you would typically find inside those crystal oscillator cans above.You also don't want to build a transmitter directly out of a crystal oscillator - those things are designed to drive digital logic, they often produce squarish wave signals. If you feed that to any sort of antenna you will produce a wideband interference/EMI that will cause problems. You could even get a hefty fine for it. The circuit needs a low pass filter (and probably a matching driving stage) at the output in order to eliminate the harmonics inherent in the distorted/square wave signal.And finally - a 32kHz oscillator would make a terrible transmitter. That's an almost audio frequency, in order to get any kind of reasonable energy transfer the antenna would have to be enormous - the wavelength is around 9km, so a dipole would be 4.5km long. Or you would need huge power levels to compensate for the inevitable losses.Why do you even want to build a 32kHz transmitter in the first place?

author: ArthurDent
author_rank: Super Contributor
timestamp: November 25, 2018, 03:52:58 pm
content: A quartz crystal is just a resonator, like a tuning fork or a guitar string, that will vibrate at a frequency determined by the physical dimensions of the crystal element. Any vibrating element, like those used in music or the crystal, has to be ‘excited’, as in the crystal, electrically pinged to start it vibrating and if it is only excited by one ping the oscillations will fade fairly quickly. This is the way a piezoelectric ignitor for a gas grill works, it outputs one spark every time it is struck. The reason quartz is used as electrical resonators is the piezoelectric properties of quartz to produce an output voltage at the resonate vibrating frequency. A resonator like a quartz crystal needs a continuous A.C. voltage near the resonate frequency to keep it vibrating and the quartz crystal stabilizes the oscillator frequency. This low-level voltage output by the crystal is amplified and the amplifier output is fed back to the input of the crystal and this higher level voltage keeps exciting the crystal to keep it vibrating at the resonate frequency.So the crystal isn’t an oscillator but just a resonator to stabilize an oscillator circuit. The exciting voltage is an A.C. signal near the crystal frequency to keep it vibrating. The circuit you drew isn’t a circuit at all because in the first place, there is no return to the battery negative plus it is D.C. so it can’t continually excite the crystal as is required. Even the output power of a properly designed oscillator would be far too low to act as a transmitter and R.F. transmitters usually have several stages to amplify and filter to make sure that the output is spectrally pure.Here is one of countless tutorials you can find that describe crystal oscillators and has representative circuits.http://www.philipstorr.id.au/radio/technical/Crystals%20Techo.pdf