50% duty cycle square wave from 1hz to 1Mhz, easiest way?

To AdamHI: have a look: Locked-Sync Sine Generator Covers Three Decades with Low Distortion | Analog Devices

You can't get 1kHz resolution in 200-300 kHz range using arduino Timers only.
16000000/80 = 200.000
16000000/79 = 202.531

Here is one way to build an arduino based PLL :
http://coolarduino.wordpress.com/2013/07/30/diy-am-radio-2/