REFERENCES
Garage Access Goes Digital by Jay Carter, N&V, February 2009
Counting Pulses With Timer0 by Chuck Hellebuyck, N&V, May 2009
www.analog.com/static/imported-files/Data_Sheets/AD8044.pdf
focus.ti.com/docs/prod/folders/print/cd74hc7046a.html
www.standardics.nxp.com/products/hc/datasheet/74hc4059.pdf
that frequency, but the relative amplitudes of the
harmonics in a square wave are different from
those in a triangle wave.
R4 through R6 are pull-up resistors for the
column scan lines on the keypad; these can be
omitted if the keypad you use has built-in pull-up
resistors.
I had hoped to include a high quality triangle wave
output from the 74HC7046 PLL in addition to the square
wave output, but design trade-offs prevented this. This PLL
has two different phase comparator circuits, but the lock
detector only works with phase comparator 2, and this
comparator does not produce triangle waves. Phase
comparator 1 produces a nice triangle wave in addition to
the square wave.
If you plan to use the synthesizer over a smaller
frequency range that is within the PLL’s lock range and
you need the triangle wave output more than the lock
detector output, you can re-route the 74HC4059 divider
output to phase comparator 1. The triangle wave output
will be available at the PLL’s VCOin pin.
If you intend to use the synthesizer only as a square
wave signal source for digital circuits, the AD8044 op-amp
isn’t absolutely necessary. However, I still highly
recommend a digital buffer between the PLL’s VCO
output and the outside world. This can be accomplished
by installing a 74HC14 hex Schmitt trigger in place of the
AD8044, with the following modifications:
• Clip off pin 4 of the 74HC14, then solder the 74HC14
to the board in the same orientation as the AD8044
(pin 1 is the square pad).
The buffered output of the PLL’s VCO is then available
at the trace on pin 8 of the 74HC14, and the buffered
output of the external frequency input to the PLL is
available at the trace on pin 6. NV
September 2010 35
