Question with the LC circuit

[vaineo]

I was wondering if, to figure out the frequency of this circuit, if you would add the value of C3 to what ever the value of C2 is and then do the equation for frequency. so if you have C2 set to 4pF then the C in the LC eq. would be 8.7pF and the L would still be 0.1uH.

 

[zachtheterrible]

Sorry I can't answer ur question, but can u answer mine? Where did u get that picture, cuz I have the same oscillator circuit, but w/ the whole transmitter.

 

[vaineo]

I got that one from here **broken link removed**, then just to save some space I cut the text out. I've seen the circuit used in more then one transmitter, it seems to be the bare essentials for the transmitter w/o any amplification stage.

 

[Russlk]

The value of C3 is not quite 4.7pF because the resistance of R2 and the Emitter of Q1 is in series. However, since the emitter resistance is low, the value would be close, and yes, the two caps add in parallel.

 

[Nigel Goodwin]

The value of C3 is not quite 4.7pF because the resistance of R2 and the Emitter of Q1 is in series. However, since the emitter resistance is low, the value would be close, and yes, the two caps add in parallel.

I wouldn't agree that the two capacitances are effectively in parallel, but it's not a point worth bothering about - stray capacitances (and particularly the aerial) will make it impossible to calculate with any meaningful accuracy. Simply build it, and search for the output on an FM radio - if you can't find it, adjust the capacitor (or move the coil windings) until you can - it's actually very easy to do!.

 

[vaineo]

thanks for the help, i designed a simple transmitter a while back but for some reason it only works when you tough the leds of the transistor lightly and with just the right moisture on your hand. I did forget to take into account the other capacitor. Still not sure why it is so sensative and unstable.

 

[Russlk]

Could be not enuf feedback, cold solder joint, maybe?

 

[vaineo]

i dought its a cold solder joint, checked them over a few times they seemed fine, i didn't know if when i was touching the pins it changes the capacitance and relocates the signal on the FM range.