fm

[sssaaa]

hi
can any one know what is the out put of first stage(oscillator) of this circuit on 9, 12, & 16 vots, if i want to use it alone. not with rf amplifier stage.
do i have to change L1 for this alone circuit or it will same. for 88-108 MHZ.

 

[audioguru2]

The transistor's resistors are selected for operation with one supply voltage. If you increase the supply voltage without changing the resistors to compensate for the resulting much higher current then the transistor will get hot and maybe smoke and melt.

The collector of the transistor is connected to the tuning coil and capacitor. Therefore if the antenna is also connected there then anything that comes near the antenna will change the RF frequency. The RF amplifier stage isolates the antenna from the RF oscillator.

 

[sssaaa]

means that the ossillator circuit not work fine alone.
i measure its output power (first stage) it is very good on 12 volt.
any idea to use it alone?

 

[audioguru2]

Some simple FM transmitters have the antenna connected to a tap on the coil. Then things getting near the antenna don't affect the tuned frequency as much.

The supply voltage also affects the tuned frequency. In my FM transmitter circuit the oscillator is supplied by a low dropout voltage regulator so its voltage stays the same as the battery runs down.

 

[walid1]

hi

Some simple FM transmitters have the antenna connected to a tap on the coil. Then things getting near the antenna don't affect the tuned frequency as much.

Are u mean by "things getting near the antenna " my hands, and why when connected to the tap of the coil the osc be more stable

 

[audioguru2]

Are u mean by "things getting near the antenna " my hands, and why when connected to the tap of the coil the osc be more stable

Yes. On FM transmitters that have the oscillator's collector connected to the antenna, your hands or anything metal that gets near the antenna changes the frequency of the oscillator. If you connect the antenna to a tap on the coil then the output power is less and there is less affect on the tuned frequency because only part of the coil is detuned.

 

[walid1]

are u mean that the tapped coil acts as a step down auto transformer!

 

[audioguru2]

The tap on the coil is at a lower impedance than at the end. Then a higher capacitance from your hand or metal object will have the same effect on detuning as a lower capacitance at the end which is a very high impedance.