indeed freq.multiplication very important in modulation systems....bcoz it needs high frequency signals as a carrier....produce that high freq. will damage the stability of the oscillator ..ultimately the last resort is freq.multiplication by means of freq.multuiplier or harmonic generator
ok..
1)MODULATION needs high frequency signal as its carrier ie.after modulation output is nothing but high freq which contains carrier in it
2) some where filter will be needed to eliminate some high frequency components from modulated wave(nature of the method) .all filters will deviate from its characteristics once it's freq. of operation increases
as a solution ,perfrom modulation at lower level and than eliminate all wanted components at lower level after that go for frequency multiplication or translation to convert it in to high freq. (lower level---at low freq)
To add few more points..... Just check the following link which describes what a frequency multiplier is....
**broken link removed**
For the bus constraint......
Drive strength, type of logic (TTL, GTL, LVTTL), Pin lock (in case of PLDs) will be specified thro user constraint file in some standard format. That once againg depends on the tool vendor.
Frequency multiplication is differnt that frequency modulation!!
Multiplication is to get use of multiples of the input frequency of the circuit fin -- m*fin.
This is done by passing the inout frequecy through non linear element. This wil generate the multiples of the input frequency. After this we use a sharp BPF to select the wanted multiple.
Frequency modulation is to modulate the output frequency by another signal like we are doing inFM circuits.
Bus constrains is wide area. They may fan out limitation of the bus, dealy of the bus, max speed of the bus ......etc.