Hig h -Fre q u e n c y Wa ve fo rm Ge n e ra t o r
tor compares the ÷N output with the MAX038 SYNC
La yo u t Co n s id e ra t io n s
Realizing the full performance of the MAX038 requires
careful attention to power-supply bypassing and board
layout. Use a low-impedance ground plane, and con-
nect all five GND pins directly to it. Bypass V+ and V-
directly to the ground plane with 1µF ceramic capaci-
tors or 1µF tantalum capacitors in parallel with 1nF
ceramics. Keep capacitor leads short (especially with
the 1nF ceramics) to minimize series inductance.
output and sends differential phase information to U5.
U5’s single-ended output is summed with an offset into
the FADJ input. (Using the DAC and the IIN pin for
coarse frequency control allows the FADJ pin to have
very fine control with reasonably fast response to switch
changes.)
A 50MHz, 50Ω lowpass filter in the output allows pas-
sage of 16MHz square waves and triangle waves with
reasonable fidelity, while stopping high-frequency noise
generated by the ÷N circuit.
MAX038
If SYNC is used, DV+ must be connected to V+, DGND
must be connected to the ground plane, and a second
1nF ceramic should be connected as close as possible
between DV+ and DGND (pins 16 and 15). It is not
necessary to use a separate supply or run separate
traces to DV+. If SYNC is disabled, leave DV+ open.
Do not open DGND.
___________________Ch ip To p o g ra p h y
Minimize the trace area around COSC (and the ground
plane area under COSC) to reduce parasitic capaci-
tance, and surround this trace with ground to prevent
coupling with other signals. Take similar precautions
V-
GND
REF
OUT
AO
GND
V+
with DADJ, FADJ, and IIN. Place C so its connection
F
to the ground plane is close to pin 6 (GND).
__________Ap p lic a t io n s In fo rm a t io n
A1
DV+
DGND
Fre q u e n c y S yn t h e s ize r
Figure 6 shows a frequency synthesizer that produces
accurate and stable sine, square, or triangle waves with
a frequency range of 8kHz to 16.383MHz in 1kHz incre-
ments. A Motorola MC145151 provides the crystal-con-
trolled oscillator, the ÷N circuit, and a high-speed phase
detector. The manual switches set the output frequency;
opening any switch increases the output frequency.
Ea c h s witc h c ontrols b oth the ÷ N outp ut a nd a n
MX7541 12-bit DAC, whose output is converted to a cur-
rent by using both halves of the MAX412 op amp. This
current goes to the MAX038 IIN pin, setting its coarse
frequency over a very wide range.
COSC
0. 118"
(2. 997mm)
SYNC
GND
DADJ
PDI
FADJ GND IIN
0. 106"
(2. 692mm)
PDO
GND
Fine frequency control (and phase lock) is achieved
from the MC145151 phase detector through the differ-
ential amplifier and lowpass filter, U5. The phase detec-
TRANSISTOR COUNT: 855
SUBSTRATE CONNECTED TO GND
16 ______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
