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Registered Member #3925
Joined: Fri Jun 03 2011, 10:50AM
Location:
Posts: 121
hi,
today i made a colpitts oscillator circuit, and modulated its output by putting it through a transistor, (i know my use of the correct terminology leaves a lot to be desired!), and controlling the transistor with an audio signal, and it seemed to work quite well but the power was very low, and the range was only about 6 metres,
am i doing this correctly or is there a better way? how would i increase its transmission power? and could i use an ecc81 dual triode, one triode for the oscillator and the other for the amplidude modulation?
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
wrote ... today i made a colpitts oscillator circuit, and modulated its output by putting it through a transistor, (i know my use of the correct terminology leaves a lot to be desired!), and controlling the transistor with an audio signal, and it seemed to work quite well but the power was very low, and the range was only about 6 metres,
By modulating the oscillator with the audio signal you are effectively building a single transistor mixer. The mixer itself can probably be optimized to get a higher gain, or you could design an amplifier to increase signal power.
One resource on the topic of mixers I found:
Note that with what you have the oscillator would be considered the LO (local oscillator) the audio signal is IF (intermediate frequency) or baseband, and the output in RF (radio frequency). the LO, IF, and RF are standard names for the three ports of a mixer. Depending on the type of mixer some or all of these ports might be interchangeable, but they are usually optimized for certain desirable characteristics like preventing LO signal from leaking into IF or RF port.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Here be a complete transmitter design using the famous double triode 6SN7, but you could easily make it work with your ECC81 with a few changes here and there, though its output will be less.
If you are modulating the oscillator supply, then besides having amplitude modulation you will have some frequency modulation, and perhaps other unwanted effects like sideband splatter too.
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colpitts oscillator am transmitter?
