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FM modulator with this oscillator

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julian403

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When I need to do an oscillator I always make this one:

Capture.JPG

Thats because its a simple circuit and when I implemented it always works.

So, to make an FM modulator I need to modify the frecuency so for that I can use a varicap. The circuit I designed is;

Capture1.JPG

as you can see in the next pictures, there is frecuency variation:

Capture2.JPG

Capture3.JPG

In the simulator it works but I dont know if it can works on real. I still need to make the inductors.

What do you think about this circuit?
 

It's not a frequency deviation, it's just voltage swing at somewhere..( collector,load or anywhere.. it's not visible )
You cannot predict a frequency deviation by doing transient simulation, it's not so easy.
Also, how that oscillator works ? I couldn't see its topology and feedback.. What type is it ??
 

I took a Pierce oscilador and I modified it to one transistor. I remember that I saw a paper from on semiconductor or texas which speaks about harmonic oscillator using this oscillator type.

You cannot predict a frequency deviation by doing transient simulation, it's not so easy.
Click to expand...

it reachs the permanent condition. It stabilizes obtaining a sinusoidal signal after the trasient. You can simulate it but remember to modify the simulator time step (to 10^-9 second)

Now, I need to figure out how to modify the frecuency with a voltage signal.
 

It should work. The FM deviation is dependent on the Q of the resonator (Q=R/L(f)) and delta f depends on the Ceq sensitivity with bias voltage as long as the bias inductor which shoud be a resistor much higher impedance than C(f). Notice that saturation effects occur when the Q * gain is too high and the sine wave flattens out towards Vce<1V.

I did it something like this using a crystal for a PLL which of course has a Q>10k so the pulling range is around 200ppm . But then it only had to cover temperature drift of the Tx Xtal over temp.
 
julian403 said:
I took a Pierce oscilador and I modified it to one transistor. I remember that I saw a paper from on semiconductor or texas which speaks about harmonic oscillator using this oscillator type.
it reachs the permanent condition. It stabilizes obtaining a sinusoidal signal after the trasient. You can simulate it but remember to modify the simulator time step (to 10^-9 second)
Now, I need to figure out how to modify the frecuency with a voltage signal.
Click to expand...
Interesting oscillator ..anyway..
Transient Simulation cannot give you frequency deviation because Transient Simulation works in time domain.You have to use Frequency Domain simulator such as Harmonic Balance but this simulation can be found in high end simulator packages only. ( Keysight ADS, AWR Microwave Office etc)
Or you can do FFT to obtain the frequency of Oscillation and you may repeat for each Vtune value then plot Frequency vs. Vtune in Excel.
But you should define the time point where the steady state oscillations occur.This is important..
 
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