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FM Transmitter simulation in spice

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Junior Member level 2
Jun 21, 2015
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Hi, I am looking for some input on a FM transmitter I am trying to simulate in LT Spice. I am simply just trying to modulate an input sine wave with a carrier frequency. My circuit is attached with the simulation results, as well as the oscillator simulation with the input sine wave removed. I don't have any specific design requirements as of yet, I am just trying to simulate a simple modulation circuit. I am trying to achieve ASK with this, and the output waveform does not look correct (Also the simulation stops oscillating soon after I start the simulation).

Any suggestions for improvement are welcome, thank you.


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  • oscillator without Vin.png
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Your audio input level is WAY too high (1000 times too high). Instead of 20V p-p try it with 20mV p-p (0.02V p-p microphone level) but you will not see the deviation on a 'scope.
Also the audio input must use a series coupling capacitor so that the transistor is never turned off. The RF oscillation in your SIM stops when the input voltage goes negative which turns off the transistor.
I think your R1 value is much too low, try 47k.

Did you notice that your 'scope shows AM modulation which is normal for this very simple circuit that produces AM and FM at the same time.
Thank you for your help. Actually, my input signal is going to be a sine wave about +/- 9V with a variable frequency (not audio). I want to modulate and transmit this signal via RF. The receiving end will receive and correlate the signal to a specific frequency. I know there exists FM transmitter chips/micros that can do this, but I want to build it will discrete components. My initial thought was just a simple FM transmitter with ASK modulation. Is this something which can be achieved by the means I am proposing?

I will work on decoupling cap and the value of the discrete components.

Or any other simple modulation scheme would be welcomed. I am looking for a simple OOK circuit for this.

I don't understand your problem. OOK (another word for ASK) is achieved by turning the oscillator power supply (or a bias voltage) on and off. No FM involved.

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