You want to use the Uno? You can set up a timer to output varying frequencies and output them on a pin. I have a page about timers which might help. The fastest you can get with a 16 MHz processor is 8 MHz output however you can get other frequencies by changing the timer parameters.
The ATtiny85 can output higher frequencies if you use the PLL clock option, I think you can then adjust the resulting frequency.
If not, is there a such thing as a variable oscillator?
Adafruit make a Clock Generator Breakout Board - 8KHz to 160MHz. You connect that via I2C to your Arduino and then program assorted frequencies as a clock output. That uses the Si5351A clock generator chip.
Example code to output 8 MHz on a Uno or Mega2560:
const byte CLOCKOUT = 11; // Mega 2560
const byte CLOCKOUT = 9; // Uno, Duemilanove, etc.
void setup ()
// set up 8 MHz timer on CLOCKOUT (OC1A)
pinMode (CLOCKOUT, OUTPUT);
// set up Timer 1
TCCR1A = bit (COM1A0); // toggle OC1A on Compare Match
TCCR1B = bit (WGM12) | bit (CS10); // CTC, no prescaling
OCR1A = 0; // output every cycle
} // end of setup
void loop ()
} // end of loop
To output 4 MHz change OCR1A to 1 rather than 0.