Oscillator with 2 transistors and 2 distortions !

[sina]

oscillation

I have a ocillator with 2 transistor but it has 2 distortionnce it is one other frquency on the sine wave (it is not harminic) I think my circuit has another resonance frequency but it is dammped like I mean we have a dammped sin wave in each period of main sine wave.another distortion is that sometimes my sine wave with frequency 500khz change to am modulated signal with push frequency 1khz I think it is because of city voltage network.
thank you

 

[E-design]

Re: oscillation

Attach your circuit drawing to this post for someone to help you.

 

[flatulent]

Re: oscillation

The bias network may be oscillating. One quick thing to try is to decrease the value of the coupling (in series with the signals) capacitors and increase the value of the shunt (power supply and bias filtering) capacitors.

 

[sina]

Re: oscillation

thank you
I have test the power supply I think it is beacuse of some other l or c in my ciruit
this is my circuit has been attach and 2 transistor of the first used for oscillation.this is the circuit of lc meter.

 

[artem]

oscillation

It could be power internal impedance is high that could explain why you have 1 kHz - most probably circuit used for osc gain control has timing which is close to 1 kH R3 C4
- put high capasity blocking capasitors to T1 and T2 power (around 200-1000 uF + high freq capasitor 0.1 -0.5 uF)
- separate detector/amplifier circuit (T3 T4 and T5) power from oscillator's via RC blocking circuit to filter out high freq
- optionally change C2 upper connection from + power to ground .

 

[E-design]

Re: oscillation

I will attack the problem as followsthis assume that you have a scope handy)

1/ Make sure that you have all the component values correct, like the 10mH for L0 (that it is not a 10uH etc.) A lot of time can be wasted fault-finding a circuit just to find out you mixed up a value!

2/ Break/lift the connection on LHS of C1. Inject a small external sinewave (1-10kHz) into this point and see what the signal (will be 180 deg out of phase with input) looks like on T2 collector with a scope. If it is distorted vary the external amplitude to see if it is possible to get a clean looking sinewave out. If you can't you will have to look at the bias around T2 and also that the transistor is not faulty or inserted the wrong way (this can happen)

3/ If this is working correctly, reconnect C1 again. Next look at the amplitude feedback control, R8,D1,D2. This will feedback a negative voltage onto the base of T1 to just sustain oscillations. This method helps to keep distortion low. You may have too much feedback (or too little causing the waveform to clip). You may try to alter R8 or remove R8-top and use an external adjustable -ve voltage of say 0 to -5v and see if you can find a setting that delivers a clean sinewave. Also pay attention to D1,D2 to make sure they are in the correct way.

4/ Lastly you can play with the feedback values R2, C3

You can also do an open-loop analyses by breaking the loop at this feedback to determine if you have enough gain and phase shift for reliable oscillations to occur.

 

[sina]

Re: oscillation

thank you it was so good with last method I get response I wonder that when I open the c between T1 collector and T2 base and input signal to it it works so good and it has equal amplitude for all frequencies and although the feedback network does not change the amplitude or although the phase of signal in the range of 1.5KHZ to 500KHZ