What does this capacitor do?

[Jellybean]

simplefm.png

In this super micro FM transmitter schematic, the electrolytic capacitor can be anything from 1-33uF. I used a 1uF cap when I built it but now I want to build this bigger one and should probably understand what it does because that has a 1-4.7uF cap.

simpleishfm.png

I'd be super grateful to know what happens if I change the capacitor and why it's there, shouldn't it just be making the voltage from the audio pulse?

[Jellybean]

The Images aren't showing how I want them to.

the smaller one is here:
https://anarchy.translocal.jp/radio/mic ... estTX.html

The bigger ones here:
https://anarchy.translocal.jp/radio/mic ... hanged.jpg

[Bton-FM]

The capacitor is AC coupling the audio onto the Varicap diode.

The greater the value, the more low frequencies will be let through.

[Albert H]

Go for the larger value. Also add a resistor (100k or so) from the negative end of the capacitor in question down to ground. This will prevent jumps in frequency when you plug in your audio. Also this circuit doesn't have any pre-emphasis, so the audio will sound muffled. Add a simple passive pre-emphasis network (use 51k in parallel with 1n fitted in series with the 4µ7 audio input capacitor)

You'll also need to add a resistor from the left-hand end of the 51Ω resistor to the "top" of the varicap diode to provide DC bias. Use 47kΩ or so.

Also note - you don't need to use the expensive varicap diode for modulation - a reverse-biased 1N4148 will give you enough deviation in that circuit!

You can also improve frequency stability by replacing the 51Ω resistor with a 78L09 regulator. This will stabilise the supply voltage to the oscillator. If you use the regulator, you'll have to filter it - use 100n capacitors to ground from the input and output pins.