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AM Transmitter using Colpitts Oscillator

Thread starter #1
I am experimenting with AM transmitters, I found this Colpitts oscillator circuit and have found the right inductor which produces a frequency of about
1,246,458 Herz which is what I want (in the middle of the AM frequency band).

I cannot figure out where to connect the audio input.

I tried connecting audio to the base of the transistor and also to the output, neither works.

Can anyone assist? I am an RF electronics newbie.

Colpitts-osc-01.gif
 

MikeMl

Well-Known Member
Most Helpful Member
#2
You need to vary +9V node in the circuit above from ~2Vmin to ~ 16Vmax in step with the audio waveform. That is called high-level Amplitude Modulation. It takes an audio amplifier to do that. When used that way, the amplifer is called a "modulator". That's what it takes to make an AM signal.

Playing with the bias on the base of the oscillator transistor might vary the frequency of the oscillator, and at best, produce a crude form of Frequency Modulation...
 
#4
I am experimenting with AM transmitters, I found this Colpitts oscillator circuit and have found the right inductor which produces a frequency of about
1,246,458 Herz which is what I want (in the middle of the AM frequency band).

I cannot figure out where to connect the audio input.

I tried connecting audio to the base of the transistor and also to the output, neither works.

Can anyone assist? I am an RF electronics newbie.

View attachment 109647
Any attempt to directly modulate that oscillator will result in frequency (rather than amplitude) modulation. You need to add an amplification stage to the output of your oscillator, and vary the supply voltage to the amplifier according to the audio that you want to impress upon your carrier. Another problem with your Colpitts oscillator is that it will wander in frequency as the battery supply drains, as the components warm or cool, or even according to the relative humidity! A stable, "free-running" oscillator isn't trivial to construct, so you're always better off using a crystal to determine your frequency. This guarantees frequency stability.

Look around the 'net for AM transmitter circuits to get an idea of the topology of a medium wave AM transmitter.

The simplest AM transmitter circuit that I've used successfully uses a crystal oscillator driving a modulated final stage. The AM is achieved by using the output of an LM386 audio amplifier IC as a "modulated supply" for the output transistor. If you want a circuit that works, let me know!
 

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