SparkFun Forums

[bachslap]

I'm just beginning with radio frequencies. And I can'tseem to get this ccircuit to work. I can't find the exact proper parts so, I'dlike to improvise, but I don't understand the circuit. I'm semi - new to electronics so can someone please help me?

Thanks in advance!

[lyndon]

So how are you determining that it isn't working? I'll infer from this that you don't have a lot of test equipment? If so, the best suggestion I can make is to get the right parts and build it (which ones are you having trouble finding?) Experimenting is fine, but if you are starting off not knowing what the impact of changes will be, then it's a wild goose chase.

Anyway, C & L set the center frequency of oscillation and the resistors set the bias point of the transistor. All of those components (and the supply voltage) will affect the ability of the circuit to oscillate.

If you just want a cheap transmitter, you may be better off with one of the 418MHz ones that SparkFun sells.

[bachslap]

I can't find the correct capacitors with the proper capacitance. It's not working because a radio isn't picking it up. I have test equipment, I just have a lack of knowledge on how Rf's work.

Also How does L and C change the frequency? I don't really understand inductance and I know I'm asking a lot, but could you please explain it to me?

[waltr]

You need to do lots of google web searches and lot and lots of reading. If you are very interested in RF corcuits then I highly recommend you buy this book. It doesn't matter which year as not much changes from year to year. The basic theory and practice doesn't change.
The ARRL Handbook
http://www.arrl.org/arrl-handbook-2014
used copies can be found on Amazon and many other places.

I'll give you one hint on your last answer: f = 1/(2*pi*SQRT(L*C))

[lyndon]

All of those parts should be available at Digikey. Maybe except the inductor, but that you can wind yourself. The center frequency of the tank circuit is 1/(2*pi * sqrt(LC)) but that gives a minimum frequency of 80MHz, which doesn't seem right, considering that the 2N3904 tops out at 100MHz, so there is something else going on there.

What frequency is your radio tuned to? You are building this on copper clad, right? It doesn't have a snowball's chance of working on a plug in breadboard.

I second, third, millionth waltr's suggestion of the Handbook. It is the bible of ham radio and you will learn tons about RF circuits from it. Also, pretty much every library in the US has a copy so it won't cost you anything.

[bachslap]

I'll look into that book. And I am using a breadboard. Why won't it work with one?

[lyndon]

Breadboards are fine for low-precision DC and low frequency AC circuits. Once you get into RF ranges (or precision DC), the poor contacts, parasitic capacitance from the many parallel paths, and inductance of the wire jumpers cause all manner of problems. Best way to prototype stuff like this is soldered directly on copper clad PCB with the shortest wire and lead lengths needed to connect things together.

Have fun.

[Mee_n_Mac]

And I am using a breadboard. Why won't it work with one?

The grid of wires in the BB will add small amounts of capacitance and/or inductance to your circuit. In this case the amounts added are about the same value as the circuit components. Thus the actual circuit built is different, in some unpredictable way, from the circuit design.

As for understanding the circuit, you need to understand complex impedance, how circuits filter different frequencies and the phenomenon called resonance.
http://www.electronics-tutorials.ws/

[jremington]

That transmitter is for the FM broadcast band. It would be unstable, hard to tune and the component values are rather critical. You would have a much easier time with an AM broadcast band transmitter, which would operate at a much lower frequency and can be built on a breadboard. Google "simple AM transmitter schematic" for lots of ideas, or try this great collection of simple plans: https://home.comcast.net/~filebay1/simple/simple.html which includes instructions for winding the coil. Most of the parts, including the tuning coil and capacitor, can be scrounged from a old AM radio receiver.

Here is another: http://www.eeweb.com/blog/extreme_circu ... ransmitter or this:

[Valen]

In reference to breadboard capacitance:

EEVblog:

https://www.youtube.com/watch?v=6GIscUsnlM0

prototype PCB design (overview) by w2aew:
https://www.youtube.com/watch?v=kH110yjYZ2g

Build a crystal oscillator from schematic thru prototype construction and testing - DIY by w2aew:
https://www.youtube.com/watch?v=blalAktxFoI