# How this simple rc circuit works

#### chenziqing

Joined Jan 16, 2012
2
Hi,all
Here is a brief description of my project, I would like to know the thoery behind how this simple RF circuit works mathematically.(especially the function of BJT)Thank you.

Following is the hyperlink of the schematic.
http://www.futurekit.com/2009/manual/future/eng/PDF_FK7/FK702E-1.pdf
The board is intended to be working at 94.9 Mhz, however the signal i measured at collector, base and emitter of the bjt are all 1khz in frequency. Is this supposed to be right?

#### crutschow

Joined Mar 14, 2008
27,008
No, it should be operating at the high frequency. The circuit is an oscillator (a Colpitts type I believe) with the BJT providing the amplification required for the oscillations to occur. A Google search will tell you more.

How are you measuring the frequency? Are you using a 10:1 probe?

#### chenziqing

Joined Jan 16, 2012
2
No, it should be operating at the high frequency. The circuit is an oscillator (a Colpitts type I believe) with the BJT providing the amplification required for the oscillations to occur. A Google search will tell you more.

How are you measuring the frequency? Are you using a 10:1 probe?
Hi, thank you so much for answering me! The 1KHz is not real frequency which means it is just used as a experimental purpose. The actual input is something like human voice. I just wanna know how does this BJT mix the 2 signal (human voice and carrier signal which is generated by the tank circuit)?
Thanks very much again!

#### crutschow

Joined Mar 14, 2008
27,008
The BJT's collector-base capacitance varies slightly with the change in bias caused by the input signal. This capacitance change slightly alters the oscillation frequency, giving FM modulation of the carrier.

#### Georacer

Joined Nov 25, 2009
5,182
@OP

I deleted your second post, as it was written in a language other than English, which is forbidden in the AAC forums.

If it contained crucial information, please repost it in English.

Thank you.