Simulation of one-Transistor FM transmitter and MicroCAP

fm transmitter simulation

Hii All,

Im trying one transistor FM modulator with demo version of MicroCAP.
The circuit makes two functions at the same time one is osc. The other is to amplify audio signal. This kind of a two function circuit had been named as reflex circuit. Amplified audio signal changes the voltage-dependent-capacitor between the collector and base of the BJT transistor. And this capacitor functions like a varactor.
But I can not see any modulation. What can the reason of this be? May not the simulator simulate this two function at the same time ?

Thanks

simulation of fm transmitter

Hi,

First, you need do simulate for a duration of about the period of the audio signal. But if the frequency of the audio signal is for example 1kHz and the frequency of the HF is 100MHz, the simulation time can be be very long due o the number of point to calculate. In this specific case, there will be HF 100 000 periods for 1 audio period.

Second, you need to measure the HF frequency when the audio signal is arround 0 volt and do a second measure of the HF frequency when audio signal is at its max amplitude. Otherwise you will not see any variation of the HF frequency directely by zooming on the display due to the ratio between the two periods 100 000 / 1).

One solution would be to decrease the HF frequency and increase the audio frequency to have a lower ratio.

fm transmitter simulator

Hi,
I think that you may have incorrect or non-accurate transistor model, which results a non-oscillating circuit to modulate.