Rainbow Electronics MAX2769B - Manual (Page 14)

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MAX2769B
Universal GPS Receiver
Synthesizer
The MAX2769B integrates a 20-bit sigma-delta fractional-
N synthesizer allowing the device to tune to a required
VCO frequency with an accuracy of approximately
Q
30Hz. The synthesizer includes a 10-bit reference
divider with a divisor range programmable from 1 to
1023, a 15-bit integer portion main divider with a divisor
range programmable from 36 to 32767, and also a 20-bit
fractional portion main divider. The reference divider is
programmable by bits RDIV in the PLL integer division
ratio register (see
Table 11 ), and can accommodate ref-
erence frequencies from 8MHz to 32MHz. The reference
divider needs to be set so the comparison frequency falls
between 0.05MHz to 32MHz.
The PLL loop filter is the only external block of the syn-
thesizer. A typical PLL filter is a classic C-R-C network
at the charge-pump output. The charge-pump output
sink and source current is 0.5mA by default, and the
LO tuning gain is 57MHz/V. As an example, see the
Typical Application Circuit for the recommended loop-
filter component values for f
COMP
= 1.023MHz and loop
bandwidth = 50kHz.
The desired integer and fractional divider ratios can be
calculated by dividing the LO frequency (f
LO
) by f
COMP
.
f COMP can be calculated by dividing the TCXO frequency
(f
TCXO
) by the reference division ratio (RDIV). For exam-
ple, let the TCXO frequency be 20MHz, RDIV be 1, and
the nominal LO frequency be 1575.42MHz. The following
method can be used when calculating divider ratios sup-
porting various reference and comparison frequencies:
TCXO
LO
COMP
f 20MHz
Comparison Frequency 20MHz
RDIV 1
f 1575.42MHz
LO Frequency Divider 78.771
f 20MHz
= = =
= = =
Integer Divider = 78(d) = 000 000 0100 1110 (binary)
Fractional Divider = 0.771 x 220 = 808452 (decimal) =
1100 0101 0110 0000 0100
In the fractional mode, the synthesizer should not be
operated with integer division ratios greater than 251.
Crystal Oscillator
The MAX2769B includes an on-chip crystal oscillator. A
parallel mode crystal is required when the crystal oscilla-
tor is being used. It is recommended that an AC-coupling
capacitor be used in series with the crystal and the XTAL
pin to optimize the desired load capacitance and to
center the crystal-oscillator frequency. Take the para-
sitic loss of interconnect traces on the PCB into account
when optimizing the load capacitance. For example, the
MAX2769B EV kit utilizes a 16.368MHz crystal that is
designed for a 12pF load capacitance. A series capaci-
tor of 23pF is used to center the crystal oscillator frequen-
cy, see
Figure 1 . In addition, the 5-bit serial-interface
word, XTALCAP in the PLL Configuration register, can
be used to vary the crystal-oscillator frequency electroni-
cally. The range of the electronic adjustment depends on
how much the chosen crystal frequency can be pulled
by the varying capacitor. The frequency of the crystal
oscillator used on the MAX2769B EV kit has a range of
approximately 200Hz.
The MAX2769B provides a reference clock output. The
frequency of the clock can be adjusted to crystal-oscilla-
tor frequency, a quarter of the oscillator frequency, a half
of the oscillator frequency (f
XTAL
P 16MHz), or twice the
oscillator frequency, by programming bits REFDIV in the
PLL Configuration register.
Table 2. Output Data Format
INTEGER
VALUE
SIGN/MAGNITUDE UNSIGNED BINARY TWO’S COMPLEMENT BINARY
1b 2b 3b 1b 2b 3b 1b 2b 3b
7 0 01 011 1 11 111 0 01 011
5 0 01 010 1 11 110 0 01 010
3 0 00 001 1 10 101 0 00 001
1 0 00 000 1 10 110 0 00 000
-1 1 10 100 0 01 011 1 11 111
-3 1 10 101 0 01 010 1 11 110
-5 1 11 110 0 00 001 1 10 101
-7 1 11 111 0 00 000 1 10 100