I want to learn about FM circuits from simulation applications (like Spice)
so I installed "QUCS" (because I have Ubuntu-Linux at home,
and Qucs is native on linux, and also it's free.. ).
My problem is that I simulated a basic FM transmitter circuit,
and the output looks like AM. (i.e. the carrier only changes in amplitude
and not in frequency).
In the screenshot attached, I'm showing the circuit diagram with
the components values, and the 4 graphs show the input to the transistor
and the output from the oscillator (after removing the DC with the last cap)
in 2 time scales (0 to 0.5 uSec, and 0 to 10 uSec).
The modulating input is a 10 MHz signal and the carrier is 100 MHz.
They both appear in the graphs with correct frequencies,
but the modulation looks like AM, not FM... (not even AM really)
- DoesntLookLikeFM.png (55.07 KiB) Viewed 4798 times
Link to the screenshot: http://www.electronic-circuits-diagrams.com/forum/download/file.php?mode=view&id=1042
Is the FM transmitter circuit wrong (I copied it from some website)
or is it the application that's not simulating it right?
(I tried changing the values of all the elements and even different
BJT models, but it didn't make an FM shaped output).
And also, why is the output so weak?
it's less than 10mV p-p... Is it supposed to be that small?
I'm beggining to feel like I don't really understand
why the transistor is supposed to modulate the frequency
and not the amplitude...
Thanx a lot for any explanations,
fasti
