■ FIGURE 5. PCB for the frequency synthesizer mounted
in its enclosure.
the five volt voltage swing at the output of the synthesizer.
Any unused op-amp inputs should be tied to ground.
I designed a fair amount of flexibility into the schematic
and PCB to make this project as useful as possible. As
stated earlier, the third 74HC390 dual decade divider
which provides 10 Hz and 1 Hz signals is optional
and can be left out if desired. All of the decade
divider outputs are connected to the 74HC151
multiplexer so the PIC code can be modified to use
any of them as the PLL’s external frequency. This
could be helpful if you need lower frequencies or
smaller frequency steps than the “stock” design. For
example, you could create a 20-20,000 Hz audio
frequency generator with a 1 or 10 Hz step size by
increasing C1 to 1 µF or so, adjusting the loop filter
and lock detect filter component values, and adding
a few lines of code in the SELECT CASE argument to
accept frequency entries that are two, three, or four