Fm tx audio quality

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Re: Fm tx audio quality

Post by Admin on Sat May 25, 2013 8:45 am

Sorry, I did not understand the link.

BR Harry - SM0VPO
Nerja, Spain.

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Re: Fm tx audio quality

Post by joy22601 on Wed May 15, 2013 1:19 am

Yes Harry your explanation did help me understand. Thanks for explaining it to me. Here is the circuit. Everything looks ok? I used two 15 + 15p capacitors.
bit dot ly/180vb1i

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Re: Fm tx audio quality

Post by Admin on Tue May 14, 2013 6:37 pm

It is common practice to ALWAYS buffer an oscillator, especially one that is modulated. Loading an oscillator can cause the oscillators operating conditions to change, which in turn can affect audio symetry, stability and even harmonic content.

Remember that an oscillators amplifier gain should be operated close the point of oscillation, but strong enough to give reliable starting. If stage gain is too heavy then RF distortion can occur. The distortion can appear as harmonics but with modulation all manner of distortion can occur. Loading the oscillator will usually change the stage gain.

My "standard" method of buffering an oscillator is to connect an emitter-follower transistors directly to the oscillator emitter. Any load on the emitter-follower will usually result in less than 1% of the load being passed to the oscillator.

Does this help you any?
PS - We are back in Spain again :-)
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Re: Fm tx audio quality

Post by joy22601 on Sat May 11, 2013 7:36 am

i have to use a 100 ohms resistor in series with the output of the buffer transistor in order to get a good sound quality..

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Fm tx audio quality

Post by Joy22601 on Thu May 09, 2013 8:58 am

Hello Harry, does loading of a fm oscillator (88-108 mhz) affects the audio quality? I build the two transistor oscillator + buffer from your 150mw fm tx and the audio quality suffers if I drive a low value load.

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