I am new to the telecom simulations system specially multisim, which I am having a project that I want to simulate.
My project in (Analog Communications) is consisting to have FM Transmitter (Modulator) that accepts a generated signal (Sin, Cos) and get me the modulated output as a Graph.
I've found this FM transmitter circuit (3 km range):
![two muppets][1]
and I am building that circuit on Multisim, and I failed. Because I faced many problems, first of, the variable capacitor symbol is weird here in the multisim, and the regular capacitor doesn't have variable options.
Secondly, when I Use signal generator as (Sin, Cos), the output gets weird after 5 second of the simulation.
I want to show the input signal (before modulation) and the output signal (transmitted or modulated).
I guess my output result where not the as expected, as the signal (Sin with phase 90°) is giving weird result of few seconds of simulating.
Below are screenshots of the simulation results within time.
Any ideas how to edit or fix the above circuit to have this output is similar to what it should be?
Best Answer
That dogs dinner of a circuit will generate a horrific mix of FM and AM as it relies on the changing parasitics with bias point to do what is essentially AM to PM conversion.
No way are you getting more then a few tens of meters of range out of a single transistor common base oscillator like that in any case.
You need about a 10uF of so capacitor in series with your signal generator to avoid messing up the things bias, and you may wish to consider that the analyser sample rate MUST be greater then twice the highest frequency present at the output.
You will in any case find that simulation is not usually all that useful with these kinds of circuits as they are far too sensitive to parasitics. I would suggest just dead bugging the thing on a bit of copper clad (use the copper clad board as the ground plane), but be prepared to be very disappointed about range, I am thinking a few meters at best.