28 October 2009

The EX-OR frequency doubler

After few years I've had this idea in mind, I could finally melt some solder and put together a frequency doubler using EX-OR digital gates.

The idea for my application is: feed a 2-IN XOR gate from a 74HC86 with properly phased signals and it generates a 2 * f_in (square) wave.

So, what are "properly phased signals"?

If the base frequency f_in, which is in form of a square wave, is delayed of T/4 and fed to the second input of XOR gate, then given the XOR truth table the gate will output a 2x frequency.

I leave the reader draw the two wave forms and XOR them graphically to see the effect.

How to delay the square wave then? Easy, use the intrinsic propagation delay of digital gates! Per datasheet a 74HC86 gate has a propagation delay of 11ns with a load of 50pF at 6V supply. According to another document of OnSemi the delay introduced by a gate varies linearly with the capacitive load, so I see a simple way to control delay between 50ns down to 5-6ns.

Why am I doing this? Besides for the scientific progress :-) I'm aiming at a simple TX/RX system for 50 MHz band.