25 kHz PWM from either Rpi or Arduino Uno?

[SnipeYa]

I'm fairly new to this whole hobby electronics thing, I'm looking for a way to output a 25kHz PWM signal to a fan but neither the Arduino Uno or Raspberry Pi 3B seem to output that kind of frequency. 

Is there some kind of trickery I can use to boost either of these upto the required speed?

[tikker]

I have no experience with that myself, but maybe this topic or this one can help for the Arduino.

[dany_boy]

You might want to read this article from the arduino website:

https://playground.arduino.cc/Code/PwmFrequency

TL:DR: No you cannot set a 25KHz PWM signal with hardware in the arduino. Bu either way, why do you need 25KHz for controlling a fan? The default 490Hz works very well for 99% of power control, specially if you add an LC filter on the output of your switching transistor

[SnipeYa]

13 hours ago, tikker said:

I have no experience with that myself, but maybe this topic or this one can help for the Arduino.

Thanks! I've already taken a look at those topics after Googling around the web. Turn's out there are some pitfalls to speeding up the Arduino output directly...But I've found people making their own stand alone fan controller circuits use "555 timer IC's" so I'm looking into them to see if I can control the 555 timer PWM output from the Arduino somehow.
 

57 minutes ago, dany_boy said:

You might want to read this article from the arduino website:

https://playground.arduino.cc/Code/PwmFrequency

TL:DR: No you cannot set a 25KHz PWM signal with hardware in the arduino. Bu either way, why do you need 25KHz for controlling a fan? The default 490Hz works very well for 99% of power control, specially if you add an LC filter on the output of your switching transistor

From what I've tried the fan doesn't seem to recognize the PWM output from the Arduino, Tried a ramping up and down between 0 and 255.

Apparently the desired signal is 25kHz but will work with anything between 21-28kHz thats why I was aiming for 25kHz. 
Referance: http://www.formfactors.org/developer/specs/rev1_2_public.pdf 

Sorry I still only have a very basic understanding of circutry (Think Minecraft redstone logic level of complexity), Could you explain how the LC filter would be used?