This question already has an answer here:

I've asked my question before, and I am writing this new question with some edits based on my previous problem.

I am building a small FM Modulator ( Transmitting ) simulation on (Multisim). I've used a circuit that I had from Google, and welling to use it for my simulation. Note that this simulation is only for (Educational and Learning Purposes), Therefore, I've replaced the mic and the antenna with some signal generator and oscilloscope or signal analyzer.

The circuit that I used for my transmitter as follows:

 FM Modulator Circuit

When I built that circuit on Multisim, I've made many variable capacitors to notice any changes on the result and how the signal would be effected when the simulation is running.

I've placed function generator with ( Sinusoidal Function ) with ( 1 KHz ) Frequency on the input. On the other hand, The output has both of an oscilloscope and a Signal Analyzer. the output was weird on oscilloscope. But signal analyzer has almost a Sin function signal same as output. So the system has worked as Integrator, Not a modulator.

Example of the built circuit including the input and output stuff:


After having many changes on the circuit and too many tries, I've tried to add a capacitor attached to resistor in series with a signal generator to check if it removes the DC or ripples. and tried to connect the grounding to the circuit directly.. I had some other weird results as Follows:

Weied Results

Also, I've used another circuit (just a test), Used a function generator on the input, and added a carrier function generator on the emitter of the transistor, and put some oscilloscope to analyze the output as follows:

Another circuit

The output was also weird. Therefore, I guess the problem is how I am using the circuit and how I apply the input and output devices.

As for my information, when I am applying a (Sin) function signal to the input. the modulated output should be as follows:

FM Modulated signal

Any suggestions whats wrong in my circuit and what changes do I have to make to have the expected results?


marked as duplicate by Chris Stratton, Dave Tweed Dec 14 '18 at 21:27

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • \$\begingroup\$ 1- I have deleted my previous question. 2- They have told to change the circuit, they didn’t answer or suggest any changes on that simple circuit. 3- Asking for changes means that I know it wouldn’t give a pretty good results and I need help not such a notes similar to yours. \$\endgroup\$ – B Happy Dec 14 '18 at 21:23
  • \$\begingroup\$ Deleting and reposting is against the rules, especially when you've already received a good answer explaining your error. \$\endgroup\$ – Chris Stratton Dec 14 '18 at 21:25
  • \$\begingroup\$ My question has been edited and made new changes to my post. Both questions had different images and descriptions !!!!!! If you had a problem with my question then just ifnore it :) \$\endgroup\$ – B Happy Dec 14 '18 at 21:26
  • \$\begingroup\$ Edit your original question, don't repost. It's that simple. \$\endgroup\$ – Chris Stratton Dec 14 '18 at 21:27
  • \$\begingroup\$ Next time then... \$\endgroup\$ – B Happy Dec 14 '18 at 21:28

There may be nothing wrong.

The circuit relies on the fact that the audio signal applied to the base of the transistor has an effect on its internal base-collector capacitance, which changes the resonant frequency of the external tank circuit by a small amount.

It's possible that your simulation is not specifically modelling this capacitance change — it requires a very detailed model of the transistor in order to do this, and the default model is usually much simpler than this for performance reasons.

Also, even if it's happening, the modulation (Δf) is very tiny relative to the carrier frequency. You might not be able to see it without some very specific analysis techniques. Your crude detector is almost certainly not going to do the job.

Note also that a 0.1 µH + 100 pF tank circuit is going to resonate at around 50 MHz. Your analysis clock is going to have to be much faster than that in order to make any sense of the results.

  • \$\begingroup\$ Very useful answer.. depending on what you said, I should edit the values of the resonant circuit of the Capacitor and the Inductor to make sure it has a good frequency difference that makes it able to change the input signal depending on that difference. the frequency is measured as f=1/2pi sqrt lc . I have tried to make decrease the (Δf) in the resonant circuit by increasing the capacitance to 100uF, I had the following results: raed.net/img?id=120 ... any suggestions how to simply reach the expected results? \$\endgroup\$ – B Happy Dec 14 '18 at 22:15
  • \$\begingroup\$ I've tried to simulate the circuit using simulink. Made some changes to the resonance and the input has capacitor in series with the signal generator. I had the output: raed.net/img?id=142 ... The left scope is the input signal, The right one is the one that should be modulated signal. I guess the output is an integrated signal.. Can you suggest .. Anything wrong I did ? \$\endgroup\$ – B Happy Dec 15 '18 at 20:10
  • \$\begingroup\$ That's the circuit used in matlab simulink: raed.net/img?id=143 \$\endgroup\$ – B Happy Dec 15 '18 at 20:13

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