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Fm modulator message signal amplitude control

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akhilpaulv

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Hi ,

I am designing a FM modulator. In the modulator the message signal is fed to VCO but the message signal has amplitude of 200 mV to 5 V vpp i want to reduce to 0 V to 0.025 mV .? Will somebody please suggest any circuit for that.

- Akhil
 

It depends to some extent on the frequency of the modulation and the impedances involved. If the frequency is low, all you need is a resistive divider with values such that 5V input gives 25uV output.

That is a 200,000:1 reduction in voltage which for most applications would seem excessive, are you sure you have the voltages correct?

Brian.
 

Using a 25uV (microvolt) modulation signal perhaps you will need a VCO with a gain of thousands of MHz per Volt, which will make it very unstable..
 

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