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[SOLVED] AM modulator qustion ?

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wiwi

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AM modulator question ?

Colpitts Oscillator Calculator - Ekswai

I tried to generate AM wave using that oscillator (fo is about 900 KHz) , I've applied 10kHz , 2v peak sinewave from a function generator connected between the transistor emitter and the ground (parallel to Re) , i got an AM signal but with very little envelope peak , which means very small percentage modulation , do u think it can transmit an audio signal to some distance?

plz tell me how can i improve the modulation index .

thnx
 

chuckey

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Without the circuit it is difficult. In general your are trying to change the applitude of the oscillations without actually stopping the oscillator. I would try to use "hi-level" modulation, insert you modulating signal in series with the collector line.
If you have a big carrier with low level modulation, the receiver's AGC will wind its gain down so your modulation appears to be even lower.
frank
 
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Apart from problems to achieve a sufficient modulation index, you'll also observe unwanted FM. For optimal results, the modulator should be separated from oscillator stage.

If we assume, that Vce saturation is the mechanism that sets the actual oscillator amplitude, then collector voltage modulation will in fact achieve the highest modulation index.
 
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wiwi

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here is the circuit diagram :

can the modulation index be improved by reducing the circuit gain or the bias value ?
 

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KerimF

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To get a reasonable linearity and a relatively high AM index while using a Colpitts Oscillator, the modulating signal could be inserted in series with the DC supply. I attached an example just as an added idea.
 

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    AM900K_01.png
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