i have got a fm waveform but its amplitude is changing and its not exact fm waveform.
i am sending you the circuit and the waveform that i am geting from this circuit. I am also sending you the waveform that i need to bring at my output.
Hey guys, I didn't read the former thread, so maybe someone already did this. But I used Eric's simulation, and increased the frequency of the modulating frequency. You can clearly see the modulation in this simulation.
The distortion is caused by the output RF amplifier biased wrong and it is clipping. I sim'd it with a tuned circuit for its collector load and it has much less distortion.
The distortion isn't really all that bad compared to some modulators I've seen. That's why lots of filtering is used in the downstream stages. Many modulators source distortion of the carrier signal.
The last one looks like you're on the right track. Using 1Mhz in not practical in a real tramsmitter, but for evaluation your modulator, it will be best for simulation. I think you need to change the time step to 1n in the last simulation. I was getting the same thing, and I think that's what I did to fix it.
The main problem with trying to examine frequency modulated signals in the time domain (ie using an oscilloscope) is that the deviation (the change in frequency due to the modulation) is very small compared with the carrier frequency, in practical circuits.
The result is that usually the best you can see with an oscilloscope is a broadening of the trace at the right hand side of the display.
What you will not see is a trace like that in the first post of this thread.
You should not blast an extremely high modulating signal (4V p-p) into the oscillator. Try 20mV peak (40mV p-p) because the input is from a microphone.
You won't see much modulation at 20mV. My sim uses a 2V signal, but it doesn't show the over modulation. I think you're problem is a simulation setup one, and not a circuit error.
Where do you see 75MHZ deviation? Not in any of my simulations you don't. The O/P asked about a demonstration of FM modulation, he didn't say anything about listening. In fact, that's irrelevant, since you can't hear a 1MHZ signal anyway. The levels and frequencies are chosen to demonstrate the process of frequency modulation, as per the O/P's request. I haven't seen any voice FM transmitter that connects a microphone directly to the modulator's input.
A transistor Colpitts oscillator used as an FM broadcast band transmitter uses a very low modulating signal, not anywhere near 4V p-p.
Most simple FM transmitters use a preamp transistor so that they can be used as a "bug" and detect voices anywhere in a room.
The first link in Google (there are probably many more) shows this FM transmitter that connects the low level output of an electret mic directly to the modulation input of the oscillator transistor: