i think on varactor applied the signal a signal vdc+Vms *sin(2*π*f) and the dc voltage must change from sin signal and change the capacity change ?
i sent also the ms file if anyone run this

 

Yes, when modulating, that is what happens. You can also use the varactor with DC to make a solid state tuning capacitor.

 

i can use a trimmer capacitor

 

it is strange why the multisim cant produce an fm signal

 

Please explain what you did to show that your multisim cannot produce an FM signal.

 

beacuse the collector voltage give this not a fm signal

 

The collector voltage does not show any signal!

Time to build an physical circuit. I have made 5 or ten of these and have not had any problems with them.

 

Hello,

There are no decoupling capacitors on the bas and power supply as in @DickCappels posted circuit.
In that circuit C4 decouples the bas and C6 the powersupply.

Bertus

 

thank .. i would to include a multsim file beacause a teach a lesson about simulation

 

Hello,

There are no decoupling capacitors on the bas and power supply as in @DickCappels posted circuit.
In that circuit C4 decouples the bas and C6 the powersupply.

Bertus

where put the capacitor in multism circuit ?

 

Hello,

Here is the schematic that Dick posted again:



In your schematic C4 would be between net 7 and net 0, accross R2.
C6 would be between net 8 and net 0, accross the battery.

Bertus

 

HI friends

i draw a circui in multiism for production of a FM signal but not working

than k you in advance

George

First, the varicap that you are using is not in a position to affect the frequency of the oscillator. The frequency is set by L1 and C1 and D1 has no effect. Tjhe circuit as shown is not able to be frequency modulated by a varicap diode with one end grounded. Look up "colpits oscillators" and see how it can be done.

 

@MisterBill2

You are right, the frequency is determined by the parallel resonant circuit in the oscillator's collector circuit. Because of the power supply bypass capacitor the capacitance of the transistor base-collector junction appears to be in parallel with the tank. The bypass capacitor does not appear in the SPICE simulation and may not be needed for the simulation but will be important if a physical circuit is made. In the case of the SPICE simulation we think that the power supply (V4) serves the same purpose as the bypass capacitor.

 

First, the varicap that you are using is not in a position to affect the frequency of the oscillator. The frequency is set by L1 and C1 and D1 has no effect. Tjhe circuit as shown is not able to be frequency modulated by a varicap diode with one end grounded. Look up "colpits oscillators" and see how it can be done.

hi thank for your reply i sant the ms file if you have multisim you can amke change

 

@MisterBill2

You are right, the frequency is determined by the parallel resonant circuit in the oscillator's collector circuit. Because of the power supply bypass capacitor the capacitance of the transistor base-collector junction appears to be in parallel with the tank. The bypass capacitor does not appear in the SPICE simulation and may not be needed for the simulation but will be important if a physical circuit is made. In the case of the SPICE simulation we think that the power supply (V4) serves the same purpose as the bypass capacitor.

Does this circuit even oscillate in the simulator? Do you know the values of C1 and of the varicap, D1? And what is the L1C1 resonant frequency?
I see no reasonable feedback path, aside from the voltage source impedance, which is zero in most simulators. At best this is a very poor oscillator circuit, and if the simulation is reasonably good it will not oscillate.
To produce FM the very first requirement is that the circuit be oscillating, which this circuit will not. The base to collector capacitance will produce feedback in the wrong phase relationship for oscillation.
In addition, for most bipolar transistor oscillators, the tuned circuit is associated with the base, not the collector, because the collector circuit is low impedance and broadens the tuning.
So for a reasonable oscillator put the tuned circuit between the base and ground, and then add a second coil to that, connected between the emitter and ground, but with a bypassed series resistor in line so that there can be some negative feedback to stabilize the operating point. Then D1 can be used to vary the resonant frequency of the L1C1 tuned circuit.

And I do not use simulators so far because they are not close enough to reality to serve my needs.

 

See

 

See
View attachment 190909

thank you sometimes the simulator gives reults very close to the reality the real fm system is in attach file

 

i set the FM system

 

your mutisim file produce a sin wave ? i can put a low frequncy signal (message)

 

I'll give you a tip. The C2 capacitor is too large. This increases the depth of modulation, but introduces great distortion. On the varicap, the voltage changes a lot and therefore the capacitance changes a lot. It is possible and necessary to reduce the carrier frequency level on the varicap. This is done by reducing the C2 value and connecting a capacitor parallel to the varicap. This will reduce the distortion.