# a capacitor in FM modulator

Discussion in 'Wireless & RF Design' started by baby_1, Mar 29, 2012.

1. ### baby_1 Thread Starter Member

Jun 3, 2011
39
0
what does the capacotir (120pf)do?

2. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
This is a common base configuration on the transistor RF design. The capacitor grounds any RF signals, but is low enough in value it is fundamentally invisible at audio frequencies.

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3. ### baby_1 Thread Starter Member

Jun 3, 2011
39
0
hello and thanks for your response
could you tell me about how can we calculate this capacitor value according this circuit?

4. ### panic mode Senior Member

Oct 10, 2011
1,328
305
|Xc| = 1/(2*pi*f*C)

since frequencies of carrier and audio signal differ greatly, you can use any value in quite wide range, for example 100pF - 2.2nF will do.

5. ### BlackCow Active Member

May 11, 2009
65
1
Just in case the OP doesn't understand what this is this formula gives you the reactance of a capacitor at a given frequency in Ohms. Reactance is like resistance but only AC signals.

6. ### BlackCow Active Member

May 11, 2009
65
1
Perhaps the OP doesn't know what reactance is?

This is this formula gives you the reactance of a capacitor at a given frequency in Ohms.

Reactance is like resistance but only for AC signals.