C2 is only 100pF. It is a capacitor so it does not short the base to the emitter at DC, it stops the preamp transistor from amplifying the RF from the output of the transmitter. 100pF has a reactance of 80k ohms at 20kHz so it has no effect on the signal from the microphone. At 100MHz its reactance is only 16 ohms.

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Uncle $crooge

 

would not be the same idea if you use C2 a coupling capacitor before the base of the transistor, that way you certainly will block any DC from coming in.

 

C1 blocks DC from the microphone. As Audioguru said, C2 kills the gain of Q1 at high frequencies, to prevent the RF output signal from screwing up the mic preamp.

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Ron

 

Thanks for the reply, I got this. why there is no coupling capacitor before C2, and what does that LM IC doing to your transmitter with the capacitors in parallel?

 

C2 reduces the gain of the preamp transistor at radio frequencies. C1 is the coupling capacitor that couples the signal from the mic to the preamp transistor.

What does that LM IC doing to your transmitter with the capacitors in parallel?[/quote]
The LM2931A-5.0 is a low dropout 5V voltage regulator. Unlike an ordinary 7805 5V regulator that has a minimum input voltage of 7V, the LM2931 has a minimum input voltage of only 5.2V so the 9V battery will power it for a long time as it runs down. The 1000pF capacitors are for RF bypassing and the 100uF capacitors are for audio bypassing. The LM2931 must have a 100uF capacitor on its output for stability. The regulator keeps the oscillator from changing frequency as the battery voltage runs down and also keeps the preamp transistor biased correctly.

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Uncle $crooge

 

smart idea using the IC, but why did not you use a coupling capacitor before Q2?

 

He does, what do you think C3 is?.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

After reading Andrebc's posts, one wonders if he is looking at the same schematic we are.

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Ron

 

Chill out Mr smart! dont get bothered by other members questions, just ignore them if you cant respond.

 

I believe C12 is a coupling capacitor, how come it's not the same value as C1 and C2. Is that because the audio frequency will be different after it passes the the first RC circuit?

 

C12 is an RF coupling capacitor, and NOT an audio one.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

C1 and C3 are 330nF (330,000pF) and pass audio frequencies from 30Hz to 15kHz.
C12 is 30pF and passes radio frequencies from 88MHz to 108MHz. The reactance of C12 at 100MHz is only 53 ohms so it passes the radio frequency very well.

Q2 is the radio frequency oscillator and its output to C12 is near 100MHz.

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Uncle $crooge

 

Spam alert

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I have been duly chastised.

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Ron