stand-alone applications, coexistence with cellular or WiFi
transmissions in close proximity may require additional
filtering to prevent compressing the receiver front-end.
The mixer is configured for the desired band by the
MIXERMODE[1:0] bits. Refer to Table 4.
Synthesizer
The MAX2771 integrates a 20-bit, sigma-delta, fractional-N
synthesizer allowing the device to tune to a required
LO frequency with an accuracy of approximately ±30Hz
(when f
XTAL
≤ 32MHz). 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 through the RDIV bits in the PLL
Integer Division Ratio register, and can accommodate
reference frequencies from 8MHz to 44MHz. The
reference divider needs to be configured so the Phase
Frequency Detector comparison frequency falls between
0.05MHz and 32MHz. In Integer-N mode, if the integer
division ratio is divisible by 32, setting the PWRSAV bit of
the PLL Configuration Register to 1 will reduce the power
consumed by the PLL.
The PLL loop filter is the only external block of the
synthesizer. A typical PLL filter is the classic C-R-C
network at the charge-pump output. For example, see
the Typical Application Circuit for the recommended loop
filter component values for f
COMP
= 1.023MHz and loop
bandwidth = 56KHz, with charge pump current of 0.5mA
and L1 band VCO. To calculate the loop filter component
values for different LO frequencies, please refer to the
Design Resources section of the MAX2771 product page
on the Maxim Integrated website. 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
PLL reference division ratio, RDIV. For example, let the
TCXO frequency be 20MHz, R
DIV
be 1, and the nominal
LO frequency be 1575.42MHz. The following method can
be used when calculating divider ratios supporting various
reference and comparison frequencies:
TCXO
COMP
f 20MHz
= = =
LO
COMP
f
1575.42MHz
LO_Frequency_Divider 78.771
f 20MHz
= = =
Integer Divider = 78(d) = 000 0000 0100 1110 (binary)
Fractional Divider = 0.771 x 2
20
= 808452 (decimal) =
1100 0101 0110 0000 0100
In the fractional mode, the synthesizer should not be
operated with integer division ratios greater than 251.
There are two LO tuning bands provided. These are
referred to as the L1 band and L2/L5 band respectively.
The L1 band is designed for L1 and Commercial Mobile
Satellite Services (CMSS) such as Inmarsat used for
SBAS. The L2/L5 band is for L2 and L5 bands. The selection
of a band is done by programming the LOBAND bit in the
PLL Configuration register. For example, if the desired
LO frequency is 1227.6MHz, since this falls into the L2/
L5 band, set LOBAND = 1. Assuming the same comparison
TCXO frequencies as the previous example, the PLL
divider ratio would be set to 1227.6/20 = 61.38.
Integer Divider = 61(d) = 000 0000 0011 1101 (binary)
Fractional Divider = 0.38 x 2
20
= 398459 (decimal) = 0110
0001 0100 0111 1011
The LD output provides an indication of the PLL lock
state. Note that the lock detector requires a reference
clock in order to operate.
IF Filter
The IF filter of the receiver can be programmed to be a
low pass filter or a complex bandpass filter by setting the
bit FCENX bit in the Configuration 1 register to either 0
for low pass filter mode or 1 for bandpass filter mode.
See Table 5.
Table 4. Mixer Selection
MIXERMODE
(CONFIGURATION 1
REGISTER)
MODE
00 High-band mixer enabled
01 Low-band mixer enabled
10 Both mixers disabled
11 RESERVED
Table 5. IF Filter Mode Selection
FCENX
(CONFIGURATION 1
REGISTER)
FILTER MODE
0 Low Pass
1 Bandpass
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MAX2771 Multiband Universal GNSS Receiver
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