Hig h -Fre q u e n c y Wa ve fo rm Ge n e ra t o r
When the MAX038’s frequency is controlled by a volt-
age source (V ) in series with a fixed resistor (R ), the
t = period when V
= 0V.
o
FADJ
IN
IN
Conversely, if V
by:
is known, the frequency is given
FADJ
output frequency is a direct function of V as shown in
IN
the above equations. Varying V modulates the oscilla-
IN
F = F x (1 - [0.2915 x V
])
[8]
x
o
FADJ
tor frequency. For example, using a 10kΩ resistor for
R
and sweeping V from 20mV to 7.5V produces
and the period (t ) is:
x
IN
IN
large frequency deviations (up to 375:1). Select R so
IN
t = t ÷ (1 - [0.2915 x V ])
FADJ
[9]
x
o
that I stays within the 2µA to 750µA range. The band-
IN
MAX038
width of the IIN control amplifier, which limits the modu-
lating signal’s highest frequency, is typically 2MHz.
Programming FADJ
FADJ has a 250µA constant current sink to V- that must
be furnished by the voltage source. The source is usu-
ally an op-amp output, and the temperature coefficient
of the current sink becomes unimportant. For manual
adjustment of the deviation, a variable resistor can be
IIN can be used as a summing point to add or subtract
currents from several sources. This allows the output
frequency to be a function of the sum of several vari-
ables. As V approaches 0V, the I error increases
IN
IN
used to set V
, but then the 250µA current sink’s
FADJ
due to the offset voltage of IIN.
temperature coefficient becomes significant. Since
external resistors cannot match the internal tempera -
ture-coefficient curve, using external resistors to pro-
Output frequency will be offset 1% from its final value
for 10 seconds after power-up.
gram V
is intended only for manual operation,
FADJ
FADJ Input
The outp ut fre q ue nc y c a n b e mod ula te d b y FADJ ,
which is intended principally for fine frequency control,
usually inside phase-locked loops. Once the funda -
whe n the op e ra tor c a n c orre c t for a ny e rrors . This
restriction does not apply when V is a true voltage
source.
FADJ
mental, or center frequency (F ) is set by I , it may be
A variable resistor, R , connected between REF (+2.5V)
F
IN
o
changed further by setting FADJ to a voltage other than
0V. This voltage can vary from -2.4V to +2.4V, causing
the output frequency to vary from 1.7 to 0.30 times the
and FADJ provides a convenient means of manually
setting the frequency deviation. The resistance value
(R ) is:
F
value when FADJ is 0V (F ±70%). Voltages beyond
±2.4V can cause instability or cause the frequency
change to reverse slope.
o
R = (V
- V ) ÷ 250µA
[10]
are signed numbers, so use correct
F
REF
FADJ
V
REF
and V
FADJ
algebraic convention. For example, if V
(+58.3% deviation), the formula becomes:
is -2.0V
FADJ
The voltage on FADJ required to cause the output to
deviate from F by D (expressed in %) is given by the
o
x
R = (+2.5V - (-2.0V)) ÷ 250µA
F
formula:
= (4.5V) ÷ 250µA
VFADJ = -0.0343 x D
[5]
, the volta g e on FADJ , is b e twe e n
x
= 18kΩ
whe re V
FADJ
-2.4V and +2.4V.
Disabling FADJ
Note: While I is directly proportional to the fundamen-
The FADJ circuit adds a small temperature coefficient
to the output frequency. For critical open-loop applica-
tions, it can be turned off by connecting FADJ to GND
(not REF) through a 12kΩ resistor (R1 in Figure 2). The
-250µA current sink at FADJ causes -3V to be devel-
oped across this resistor, producing two results. First,
the FADJ circuit remains in its linear region, but discon-
nects itself from the main oscillator, improving tempera-
ture stability. Second, the oscillator frequency doubles.
If FADJ is turned off in this manner, be sure to correct
equations 1-4 and 6-9 above, and 12 and 14 below by
IN
tal, or center frequency (F ), V
is linearly related to
FADJ
o
% deviation from F . V
goes to either side of 0V,
o
FADJ
corresponding to plus and minus deviation.
The voltage on FADJ for any frequency is given by the
formula:
V
FADJ
= (F - F ) ÷ (0.2915 x F ) [6]
x
o
o
where:
F = output frequency
x
F = frequency when V
= 0V.
o
FADJ
doubling F or halving t . Although this method doubles
o
o
Likewise, for period calculations:
the normal output frequency, it does not double the
upper frequency limit. Do not operate FADJ open cir-
cuit or with voltages more negative than -3.5V. Doing
so may cause transistor saturation inside the IC, lead-
ing to unwanted changes in frequency and duty cycle.
V
FADJ
= 3.43 x (t - t ) ÷ t
x
[7]
x
o
where:
t = output period
x
10 ______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
