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I'm student and I'm working on a school project. The project is a radio transmitter.

My question here is only about the frequency modulator part.

Here is my circuit :

enter image description here

Here is my output signal (blue) and input signal (brown) :

enter image description here

As you can see there is no modulation what so ever.

According to my teacher, my output should look something like this if my oscillating frequency is 93 MHz and my input signal is at 9.3 MHz:

enter image description here

My oscillating frequency is supposed to be 93 MHz. Here is how I did my calculations:

enter image description here

The *1.3 is just to make sure I respect the Barkhausen criterion

Do you guys have any suggestions to get an output at 93MHz when I input a signal at 9.3MHz?

Edit : I know the oscillating frequency can be calculated with the formula above but how can one theoretically calculate the amplitude of the oscillation and what affects it? I'm guessing RC and RL. Any suggestions?

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    \$\begingroup\$ Remove CC3. Does it oscillate now? If so you must find a way to supply the input (modulating) signal without presenting the low impedance to GND that CC3 and the AC source does. \$\endgroup\$
    – user16324
    Apr 21, 2020 at 14:36
  • \$\begingroup\$ Exactly my thought @BrianDrummond - it's not that there's no modulation - there's no oscillation. So remove 9open circuit) CC3 and get your circuit to oscillate. Then replace CC3 but have the modulation signal replaced by a short circuit and I bet it doesn't oscillate. \$\endgroup\$
    – Andy aka
    Apr 21, 2020 at 14:37
  • \$\begingroup\$ @Andyak & BrianDrummond thanks for the answers!! However if I open circuit (remove) CC3 then I just have nothing in simulation... Is it normal in simulation ? Any suggestions ? \$\endgroup\$
    – Yassine Elbouchaibi
    Apr 21, 2020 at 14:43
  • \$\begingroup\$ Then the oscillator isn't starting. What does the transient response look like if you connect R1 to a 9V step instead of +9V DC? That might be enough to kick the oscillator into life. (CC3 still open cct) \$\endgroup\$
    – user16324
    Apr 21, 2020 at 14:47
  • \$\begingroup\$ The 2N2222 is a tad shabby for 93 MHz - it has an fT of only 250 MHz and you are trying to make a 93 MHz oscillator - try an inductor with 100 times the value to see if it starts oscillating. Also try Re at ten times lower and ditch the Ce capacitor. \$\endgroup\$
    – Andy aka
    Apr 21, 2020 at 14:52

1 Answer 1

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First of all thanks to @BrianDrummond, @Andyak and @Leoman12 !!

This is my circuit now :

enter image description here

This is my transient :

enter image description here

As you can see it now oscillates. Theres is a DC component on the Vout but it goes away with time. Its hard to simulate because it would take a while with Orcad (my software) and a high resolution (low max step size).

So the key points were to :

  • Change the DC source to Step
  • Add RB3
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