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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?

Facts just don't care if you ignore them.
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Old 09-11-2012, 05:19 PM
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MikeML MikeML is online now
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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.
Mike ML.

I am an engineer and academic. When I talk about current it flows from positive to negative.

Last edited by MikeML; 09-11-2012 at 05:25 PM.
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