this question is the consequence of my previous questions, since I am learning how RF transmitters works. In past I have also built some oscillators (using little transistors and a bunch of components), but now I wish to use a VCO module.
Please also consider that obviously I follow and respect the jurisdiction about the power of these transmitters, firstly because I don't wanna infringe the laws (where I live, in Italy, we can however quietly use low-power transmitters) and last but not least, because I don't need to transmit high power. I also bought a spectrum analyzer and I have fun to learn and see that the circuits that I build are able to give me a signal that I can practically see with this new instrument.
Well, I am interested to build an RF transmitter using a VCO module (this one): for now, for me will be enough and satisfying just to measure and check the output using the spectrum analyzer: I don't want modulate any kind of input signal (eg audio or voice). Looking at the datasheet I can see the pinout: 3,4,5,6,7 to ground, 1 is for VCC, 8 for V-TUNE and 2 is the output.
My plans, for now, are to buy this VCO module and solder it on a common matrix board: then, using some wires, make the connections (grounds, VCC, V-TUNE and another piece of wire as antenna): in this simple way I can expect to see something on my spectrum analyzer? Or I will need some external components? (eg a capacitor to filter the power supply – However I plan to use a battery). And what about the power/distance? The POS-150 claims a power output of about 9 – 9.5dBm. I will also need a particular antenna or a simple piece of wire will be enough? Eg I know that the length of the antenna is related to the frequency that I aim to transmit, and if I go higher with the frequency, there will be the need for short connections on the board, right? And I will also see some harmonics?
Consider that I am a novice and I wish to learn; so would be fine give me a detailed answer for every point (If will be enough make simple connections with wire on the matrix board, using or not extra components, which kind/length of the antenna, which distance of transmitting I can expect and if I will see some harmonics) and obviously any explanation/consideration/advice/tricks.
Many thanks.
Best Answer
An antenna needs to be some fraction of the wave length of the signal being transmitted or received. Quarter length, half length or single length work the best. A wire should work just fine at the beginning.
To calculate the wave length the formula is Speed = Length X Frequency. Radio waves are regarded as moving at the speed of light 2.997 meters/second.
I'm not an RF guy but all of the RF equipped boards I've dealt with recently have a series capacitor in the path between the transmitter and antenna.
The oscillator itself is going to need some bulk/decoupling capacitance at the power input across VCC and GND. A mixture of aluminum and ceramic capacitors is best, at least one of each and as close the the oscillator as possible.
Finally don't let the V-Tune pin float. At the very least use a potentiometer between VCC and GND and connect the output of the pot to the V-Tune pin.
Good luck.