Hig h -Fre q u e n c y Wa ve fo rm Ge n e ra t o r
MAX038
–5V +5V
PRECISION DUTY-CYCLE ADJUSTMENT CIRCUIT
R4
100k
R3
100k
–2.5V
+2.5V
20 17
V- V+ A1
4
FREQUENCY
C2
1µF
1
7
REF
REF
3
C1
1µF
C3
1nF
AO
R
IN
20k
MAX038
DADJ
MAX038
R2
50Ω
19
10
8
SINE-WAVE
OUTPUT
IIN
OUT
R7
100k
R5
100k
FADJ
16
15
N.C.
DV+
R1
12k
R6
5k
DGND
14
13
12
SYNC
PDI
N.C.
2 x 2.5V
5
F =
o
COSC
DADJ
PDO
R
x C
F
IN
GND GND GND GND GND
11 18
C
F
6
2
9
ADJUST R6 FOR MINIMUM SINE-WAVE DISTORTION
Figure 2. Operating Circuit with Sine-Wave Output and 50% Duty Cycle; SYNC and FADJ Disabled
With FADJ disabled, the output frequency can still be
changed by modulating I .
DADJ can be used to reduce the sine-wave distortion.
The unadjusted duty cycle (V = 0V) is 50% ±2%;
IN
DADJ
any deviation from exactly 50% causes even order har-
monic s to b e g e ne ra te d . By a p p lying a s ma ll
a d jus ta b le volta g e (typ ic a lly le s s tha n ± 100mV) to
Swept Frequency Operation
The output frequency can be swept by applying a vary-
ing signal to IIN or FADJ. IIN has a wider range, slightly
slower response, lower temperature coefficient, and
requires a single polarity current source. FADJ may be
used when the swept range is less than ±70% of the
center frequency, and it is suitable for phase-locked
loops and other low-deviation, high-accuracy closed-
loop controls. It uses a sweeping voltage symmetrical
about ground.
V
DADJ
, exact symmetry can be attained and the distor-
tion can be minimized (see Figure 2).
The voltage on DADJ needed to produce a specific
duty cycle is given by the formula:
V
DADJ
= (50% - dc) x 0.0575
[11]
or:
V
DADJ
= (0.5 - [t
÷ t ]) x 5.75
[12]
ON
o
Connecting a resistive network between REF, the volt-
age source, and FADJ or IIN is a convenient means of
offsetting the sweep voltage.
where:
V
= DADJ voltage (observe the polarity)
DADJ
dc = duty cycle (in %)
= ON (positive) time
Duty Cycle
The voltage on DADJ controls the waveform duty cycle
(d e fine d a s the p e rc e nta g e of time tha t the outp ut
t
ON
t = waveform period.
o
waveform is positive). Normally, V
= 0V, and the
DADJ
Conversely, if V
is known, the duty cycle and ON
DADJ
duty cycle is 50% (Figure 2). Varying this voltage from
+2.3V to -2.3V causes the output duty cycle to vary
from 15% to 85%, a b out -15% p e r volt. Volta g e s
beyond ±2.3V can shift the output frequency and/or
cause instability.
time are given by:
dc = 50% - (V
x 17.4)
[13]
DADJ
t
= t x (0.5 - [V
o DADJ
x 0.174]) [14]
ON
______________________________________________________________________________________ 11
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
