Need help with a radio transmitter 27 MHz - AM

[Victor Petrescu]

Hello I have a request if anyone can help me with some answers about this project. I did a radio transmitter in 27 MHz with modulation AM ... and I have problems in maintaining oscillations and I think it is because quartz oscillator. And a second question do not understand why the current from the entry of the third transistor so I have a little ... and so I suspect that the transistor does not open as a resut i don't have enough gain. I would like if anyone can help me fix the problem. I attached 2 photos with the wiring diagram and simulation result. i used to simulate PSPICE .








Thank you very much!

 

[vfone]

Some Spice simulators have problems to start an oscillation when using a crystal model, or the equivalent circuit of a crystal.
Replace the entire equivalent crystal circuit with a simple shunt LC circuit (1uH and 33pF, and keep C9 in place) and you will get the 27MHz oscillation, just to see how the rest of the circuit works.

 

[Victor Petrescu]

I've done what you said and i obtained this



My concern is that i have quite a lot of amplification, but i'm thinking is because of the soft

 

[FvM]

The "amplification" is most likely provided by the output LC filter without an effective load.

The screenshot schematic is hard to read, I can't recognize the modulation source settings. Similarly the waveform doesn't give much information with the presently selected time scale.

 

[BigBoss]

You intend to AM but you're using C-Class amplifier for that..It's wrong because C-Class amplifiers need relatively high input driving levels.
You have to use A-Class amplifier to amplify the modulated signal.Simulators can sometimes not provide start-up conditions for oscillators as you can see that your oscilaltion is damped and it is dying down.( I guess the oscillator is heavily loaded by the amplifier).
Thake these points into account..

 

[E-design]

I reworked the bias values and relaxed the Q of the crystal equivalent circuit a bit. After that the circuit simulated quite quickly and reached a steady state under 250 uS. I agree with BigBoss that you need some buffer stage to prevent loading on the LC tank circuit.