MAX7058ATG+ Maxim Integrated Products, MAX7058ATG+ Datasheet - Page 10

• Manufacturer Part Number: MAX7058ATG+
• Description: RF Transmitter IC TX 315MHZ/390MHZ ASK
• Manufacturer: Maxim Integrated Products

Specifications of MAX7058ATG+

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

When the particular capacitance control input pin is
high, then the corresponding amount of capacitance is
added at PAOUT; this capacitance tuning works only at
315MHz. The 16 capacitor values are selected by set-
ting CAP1–CAP4; the capacitance resolution is 0.5pF.
The total capacitance varies from 0 to 7.5pF. For exam-
ple, if CAP1 and CAP3 are high and CAP4 and CAP2
are low when operating at 315MHz, then this circuit will
add 2.5pF at PAOUT.
The MAX7058 utilizes a fully integrated, programmable
PLL for its frequency synthesizer. All PLL components
including the loop filter are included on-chip. The divide
ratio is set at one of two fixed values: 21 (FSEL is set to
high) or 26 (FSEL is set to low).
315MHz/390MHz Dual-Frequency
ASK Transmitter
Table 2. Variable Capacitor Values and
Control Input Pins
10
CONTROL PIN STATE
______________________________________________________________________________________
(CAP4–CAP1)
CAPACITOR
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
315MHz (÷21)
ADDED SHUNT CAPACITANCE
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
Phase-Locked Loop
0
IN pF
390MHz (÷26)
0
The crystal (XTAL) oscillator in the MAX7058 is
designed to present a capacitance of approximately
6pF between XTAL1 and XTAL2. In most cases, this
corresponds to an 8pF load capacitance applied to the
external crystal when typical PCB parasitics are added.
The MAX7058 is designed to operate with a typical
10pF load capacitance crystal. It is very important to
use a crystal with a load capacitance equal to the
capacitance of the MAX7058 crystal oscillator plus
PCB parasitics. If a crystal designed to oscillate with a
different load capacitance is used, the crystal is pulled
away from its stated operating frequency, introducing
an error in the reference frequency. A crystal designed
to operate at a higher load capacitance than the value
specified for the oscillator will always be pulled higher
in frequency. Adding capacitance to increase the load
capacitance on the crystal will increase the startup time
and may prevent oscillation altogether.
In actuality, the oscillator pulls every crystal. The crys-
tal’s natural frequency is really below its specified fre-
quency, but when loaded with the specified load
capacitance, the crystal is pulled and oscillates at its
specified frequency. This pulling is already accounted
for in the specification of the load capacitance.
Additional pulling can be calculated if the electrical
parameters of the crystal are known. The frequency
pulling is given by:
where:
When the crystal is loaded as specified (i.e., C
C
spec
f
C
C
C
C
f
p
p
m
case
load
spec
is the amount the crystal frequency is pulled in ppm
=
), the frequency pulling equals zero.
is the motional capacitance of the crystal
C
2
is the actual load capacitance
m
is the case capacitance
is the specified load capacitance
⎛
⎜
⎝
C
case
1
+
C
load
Crystal (XTAL) Oscillator
−
C
case
+
1
C
spec
⎞
⎟ ×
⎠
10 0
load
6
=