Arduino coding of producing frequency
The code below generates 38 kHz and modulates its duty cycle.

// Example of modulating a 38 kHz frequency duty cycle by reading a potentiometer
// Author: Nick Gammon
// Date: 24 September 2012

const byte POTENTIOMETER = A0;
const byte LED = 10;  // Timer 1 "B" output: OC1B

// Clock frequency divided by 38 kHz frequency desired
const long timer1_OCR1A_Setting = F_CPU / 38000L;

void setup() 
 {
  pinMode (LED, OUTPUT);

  // set up Timer 1 - gives us 38.005 kHz 
  // Fast PWM top at OCR1A
  TCCR1A = bit (WGM10) | bit (WGM11) | bit (COM1B1); // fast PWM, clear OC1B on compare
  TCCR1B = bit (WGM12) | bit (WGM13) | bit (CS10);   // fast PWM, no prescaler
  OCR1A =  timer1_OCR1A_Setting - 1;                 // zero relative  
  }  // end of setup

void loop()
  {
  // alter Timer 1 duty cycle in accordance with pot reading
  OCR1B = (((long) (analogRead (POTENTIOMETER) + 1) * timer1_OCR1A_Setting) / 1024L) - 1;

  // do other stuff here
  }