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The Projects Forum Working on an electronics project and would like some suggestions, help or critiques? If you would like to comment or assist others with their projects, this is the place to do it.

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Old 09-11-2012, 07:11 AM
Ag565 Ag565 is offline
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Default FM Transmitter Modulation

I am designing an FM transmitter as a learning experience and for fun, but I am having some difficulty with FM modulation. I am using a VCO with a Colpitts topology and common emitter configuration. For modulation, I am using a varactor biased at 5V with a 1kHz, 5V amplitude message signal, which should allow the varactor capacitance to vary from 9pf to 19pf. I have an RF choke and DC blocking capacitor to couple the input signal to the oscillator. When I simulate my circuit in Multisim, I am able to measure my center frequency to be around 77MHz on the spectrum analyzer, but I am not seeing any frequency deviation on the output signal. Is the deviation too small for the resolution of the analyzer, which is ~9.6kHz or is my message input too weak once it gets to the oscillator because of the values I have chosen for coupling to the oscillator? I have linked to pictures of my circuit and frequency spectrum output. Thanks for any help!


Frequency Spectrum Output
FM Transmitter Circuit
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Old 09-11-2012, 04:58 PM
BillO BillO is offline
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Could you post a schematic?
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Old 09-11-2012, 05:19 PM
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MikeML MikeML is offline
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You want ((D1 in series with C5) in parallel with C2). The choke goes on the modulation input side in series with R5.
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Last edited by MikeML; 09-11-2012 at 05:25 PM.
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