formula, with constant cell, trouble with fill down

[Municorn1977]

I have a cell with a fraction put to decimal. When I use this cell in a
function and I Fill Down the cell that is used moves.
Ex.
cell D1 has the fraction.
B1 has equation =A1*D1
and I Fill Down the equation ends up =A2*D2 and =A3*D3 and so on
How do I keep cell D1 when I use the Fill Down?

 

[Bob I]

$D$1

 

[Gary''s Student]

=A1*D$1

 

[Municorn1977]

thanks

 

[Municorn1977]

thanks

$D$1