Check out programmable delay lines from Maxim or Data Delay Devices. You have at least two issues. First, be careful of the minimum pulse width spec. If your 50 MHz signal has a 50-50 duty cycle, this gives a pulse width of 10 nsec which is short for a total of 100 nsec of delay. I think the Data Delay Devices parts are better in this regard. You may just make the minimum pulse width requirement. You can go shorter than the spec but be careful about timing distortion. Second, there is an inherent delay in addition to the first 10 nsec delay which you need. This may or may not be a problem depending on your system requirement.