Question

Based on the circuit in the link below
http://circuit-zone.com/ediy_blog/724/simple-mp3-fm-transmitter.jpg

If I use a 7.3nF capacitor instead of the 10nF next to the 47k, use 10pF for the tuning, a 0.2uH inductor, 9nF capacitor for the 10nF next to the tuning, and 10pF instead of the 5pF under L1, what should be my output frequency in Hz?

Answer 1

the basic formula in determining frequency is \[f=\frac{1}{2\pi\sqrt{LC_{eq}}}\]

Answer 2

Which components do I take into account for using the formula?

Answer 3

the 7.3nF and the 10k\(\Omega\) varies the reactance at the input of the transmitter, this circuit of FM transmitter is of Reactance Modulator type of FM generation.

Answer 4

...the output tuning capacitor (labeled 1-30pF, you plan to replace with \(C_2=10pF\)) is in series with the input capacitor (10nF, you plan to replace with \(C_1=9nF\))... they form the series capacitor common to ground and parallel to series impedance consist of inductance \(L=0.2\mu H\) and capacitance \(C_3=10pF\) (originally 5pF in the circuit), so the equivalent capacitance \[C_{eq}=\frac{C_1C_2}{C_1+C_2}+C_3\]What frequency are you looking for this FM transmitter? This will put your transmitter in VHF band (30-300 MHz).

Answer 5

the LC network at the output is a Colpitt's feedback circuit...

Answer 6

So in this case, the output frequency would be 79.7 MHz?
I hope to get a frequency range between 88-107 MHz.

Answer 7

...that's what I think of... maybe you should reconsider other values for capacitor... increasing it further or the inductance...

Answer 8

the other option is you set the frequency you want, solve either the value for inductance L or equivalent capacitance Ceq...