Dollar deflation is directly linked to the amount of dollars printed above the amount of the GDP..... As long as the value in currency printed remains below or at the GDP figures than the dollar will be as strong as it can be abroad.
Now if you have a commodity based currency value like we used to have (Gold, Silver Certificates) then the amount of printed money is redeemable in that commodities amount at that particular trading value of the time requested. It also then limits the amount of printed money value, which would fluctuate on the world markets, and would at times be very volatile.
It's simple supply and demand, and when managed wisely, the value of money printed can be a very good tool for limiting many market factors. There is also the removal of some paper currencies, and some coins due to thier condition. This is the reason the fed doesn't want you to hoard currency. Too much that isn't accounted for can be a nightmare for them in certain situations.