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I am trying to build a simulation of Direct Modulation for Frequency modulation using a varactor diode (MV2201) and bjt npn 2n3904 using the circuit shown below. I cannot use RFC in my circuit right now.

Theoretical varactor diode FM generator
Image source: https://slideplayer.com/slide/5672291/ (Slide no.9)
(from: Principles of Electronic Communication Systems by Louis E Frenzel, Jr., Third Edition.)

Here is my LTSpice model, I am having trouble finding good set of values for these components to get it to work, currently i am getting a constant frequency sin wave as output when input is sin wave (10khz 1v). Just want to learn what is going wrong with my circuit and why is varactor diode not affecting the output and creating FM. I have adjusted the c1 and l1 to get 200khz center frequency for sin wave but dont understanding why isnt varactor working.output LTSpice Model spectrum

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The varactor is not going to have any bias in the first schematic because L1 is a DC short to ground. If you were to turn R4 to the top end it would probably burn up as Vcc would be pretty much shorted to ground.

In your LTspice version the varactor is not going to have any bias because L6 is a DC short to ground, and C12 blocks the DC bias voltage V8 from getting to it.

You need to feed the DC bias to the diode through an RF choke and maybe a resistor and connect the diode to the oscillator inductor through a capacitor.

Some of your component values will need to be tweaked, the capacitive divider has 10 pF and 100 nF, typically these should be closer to being the same value, maybe 100 pF each. And the voltage divider bias looks off, try something like 18k for the top resistor and 33k for the bottom, I think that will get it in the ballpark, you can experiment from there.

I'll play around with it a bit and if I come up with a good working version I'll add it.

Update: Have a look at this article which shows direct FM of an oscillator using an MV2201 diode. Note the way the varactor diode is DC biased and how the DC is blocked from the oscillator with a capacitor (C6) while the RF is blocked from the bias and signal supplies by an inductor (L1), this is what I was saying earlier.

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  • \$\begingroup\$ Thank you so much GodJihyo! Really appreciate your help, I don't understand RF choke since its my first circuit project, I dont know how to add it in LTspice, I tried applying your ideas and made this circuit but the output doesnt seem to change, if i increase the capacitance the output goes flat, mediafire.com/view/eozo202f7qj7ohc/… \$\endgroup\$
    – Rajat
    Commented Oct 29 at 16:21
  • \$\begingroup\$ @Rajat An RF choke is simply an inductor used to block RF. Instead of a capacitor from your signal source and bias to the varactor try a 100 uH inductor in series with a resistor, maybe 1k. In your updated schematic you've added another supply to the anode of the varactor, that is going to forward bias it which is not what you want, it should be reverse biased by a few volts. I'm going to add another link to my answer, have a look at that. \$\endgroup\$
    – GodJihyo
    Commented Oct 29 at 16:55
  • \$\begingroup\$ I have seen this article before, and it is the reason i used V8 with 4V in my circuit, I tried your idea and updated the circuit, Please look to it, if its better. I observe the amplitude going down with time, and it consists of other frequencies as well. mediafire.com/view/qbgz14k25k96t25/image.png/file Frequency Spectrum also has some other frequencies around a peak mediafire.com/view/hzi95c15yqssxjl/… \$\endgroup\$
    – Rajat
    Commented Oct 29 at 19:40
  • \$\begingroup\$ Hey! I feel like FM is generated after i put L = 1000u and C = 1p like you said, heres the result, That little Amplitude rise and all feels like FM... Still can't say its Fm conclusively. Need you to comment on it :D . mediafire.com/view/v2mzkp0vzvalbnl/… mediafire.com/view/cnszxnaipdo7q5t/… mediafire.com/view/6t5ebyl7ysdwp7b/… \$\endgroup\$
    – Rajat
    Commented Oct 29 at 19:58
  • \$\begingroup\$ @Rajat Yes, it looks like it should be generating some FM now. you might increase that 1p value, it limits how much the varactor can affect the frequency. The total capacitance will be the diode capacitance in series with that 1p, so if the diode changes from 5p to 10p the total changes from 0.833p to 0.909p. If you use 100p the total will go from 4.76p to 9.09p. Also I think the voltage divider bias resistors need to be different, 100 is rather low. Try maybe 100k and 47k. \$\endgroup\$
    – GodJihyo
    Commented Oct 29 at 20:33

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