mstechca said:
It seems that the 1nF coupling capacitor almost makes my circuit useless. I changed it to 15pF and I have to add a capacitor between + and gnd just to get the transmitter working.
The circuit will work fine without the coupling cap. Its function is to prevent shorting the battery if the antenna touches ground.
Your transmitter has two caps in series between + and ground so adding another one won't make any difference unless the two have wiring much too long.
Also, with this setup, I wont be able to provide optimum transmitter results with a capacitor connection between base of the NPN and ground.
Your oscillator is operating in a common base mode so of course it needs a cap from its base to ground.
The coupling cap to the lowpass filters has nothing to do with the oscillator's base, unless your circuit has ground wiring much too long.
When I tested everything, it seemed to stop TV interference, however, I turned on the radio which is about 1/2 meter away from the transmitter and flicked through the entire FM band, and came across the transmitter signal only twice.
The transmitter has only a single fundamental RF frequency. Any more frequencies picked-up by a nearby radio are caused by the radio giving its "image" frequency pick-up due to its poor design or by the radio's front end overloading also due to its poor design.
Try receiving your transmitter with a good quality home stereo tuner or a car radio and the transmitter will be picked-up at only one spot on the dial.
Why is it that when low pass filters are chained on, they go down to 1/4, and 1/16? why those fractions?
The inductive and capacitive reactances of an LC lowpass filter cause its output response to drop at the rate of 12dB per octave. -12dB is exactly 1/4 of the voltage. One octave is double the frequency.
The load impedance for an LC lowpass filter is very important. With a load impedance too high, the filter is a series resonant circuit which is like a dead short to ground. That is why the two filters in the article are different. The 2nd filter is the load for the 1st one, and the antenna is the load for the 2nd one.
when I introduced the coupling capacitor, there were some oscillations being transmitted to the receiver.
What kind of oscillations?
Is there a way I can do this without a coupling capacitor from collector to the filter section?
Certainly, but keep the antenna away from ground.
Calculate the capacitive reactance of a 1nF cap at 100MHz. Its 1.6 ohms is pretty much like a short piece of wire.
If I were to transmit at 150Mhz, and I set some high pass filters to 120Mhz or so, how many of them do I need to prevent a radio about 1/2 a meter away from picking up harmonics of the transmitter?
You don't want highpass filters. They would pass the harmonics. You want lowpass filters to attenuate harmonics.
The required number of filters and their cutoff frequency is determined by how dirty (distorted) is the fundamental frequency and its level.
A simple LC lowpass filter isn't perfect, it attenuates its cutoff frequency too, then has much more attenuation for harmonics. :lol: