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):

a busy cat ![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.

At Start

After Few Seconds

Few More Seconds

Then is would be repeating this shape

Any ideas how to edit or fix the above circuit to have this output is similar to what it should be?

  • \$\begingroup\$ Describe the weird results you are getting. What was weird and what did you expect? \$\endgroup\$
    – Andy aka
    Dec 7, 2018 at 9:20
  • \$\begingroup\$ Expected a regular increasong of power for the signal.. while the results giving a static and unchangeable amplutide and behaviours abd acts randomly. Can you check the circuit connections in the program? Do I have any errors? \$\endgroup\$
    – B Happy
    Dec 7, 2018 at 9:29
  • \$\begingroup\$ You need to be more detailed in what you expected and what you got. What you have just said makes no sense to me. FM does not produce a changing power in the carrier. Maybe you are thinking of AM? \$\endgroup\$
    – Andy aka
    Dec 7, 2018 at 9:33
  • \$\begingroup\$ Sorry, I couldn't express the right description . I mean the signal is acting like random. I meant the amplitude shouldn't change while my simulation changes it randomly from 2 to 1.8 in the start of the running. As you also answered a part of my question. Is the circuit above in my Multisim Design is correct or just missing anything? \$\endgroup\$
    – B Happy
    Dec 7, 2018 at 10:23
  • 1
    \$\begingroup\$ It has to be connected period. \$\endgroup\$
    – Andy aka
    Dec 7, 2018 at 11:53

1 Answer 1


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.

  • \$\begingroup\$ Thank you for the answer.. Distance isn't important me as much as the accurate results. This project doesn't have to be built in real life.. Its just an Analog communications simulation project that needs to be discussed in the class. I've tried the 10uF in series with the signal generator .. the result has been corrupted. Noting that I am using 1K Freq. with 10K Sampling rate. Any suggestions to the circuit above? I mean if you can suggest any changes with a draw or something .. cuz I am newbie to this kind of projects. \$\endgroup\$
    – B Happy
    Dec 7, 2018 at 14:07
  • \$\begingroup\$ Is this input & output signal analyzer makes since as an FM Transmitter? i28.servimg.com/u/f28/12/32/61/62/test10.png \$\endgroup\$
    – B Happy
    Dec 7, 2018 at 14:44
  • \$\begingroup\$ The question you perhaps should be asking is "what sort of FM modulator can be cleanly simulated" \$\endgroup\$ Dec 7, 2018 at 14:44
  • \$\begingroup\$ @ChrisStratton Mmm.. I guess not.. Do you think the modulator and the circuit used in my question cannot be cleanly simulated???? \$\endgroup\$
    – B Happy
    Dec 7, 2018 at 15:05
  • 1
    \$\begingroup\$ @BHappy I don't have multisim and it is the wrong tool for high frequency simulation anyway. Seriously, first do a design that does not suck, or model something more meaningful. The lumped element model used to derive the telegraphers equations springs to mind as something that you could simulate and explore usefully, and it would demonstrate interesting things about transmission line behaviour. This is going to be far more interesting and far easier to reason about in a discussion then something that relies on parasitics that your sim may or may not include. \$\endgroup\$
    – Dan Mills
    Dec 7, 2018 at 19:29

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