MAX2741ETI Maxim Integrated Products, MAX2741ETI Datasheet - Page 5

• Manufacturer Part Number: MAX2741ETI
• Description: RF Receiver
• Manufacturer: Maxim Integrated Products

Specifications of MAX2741ETI

• Lead Free Status / Rohs Status: No

The MAX2741 GPS offers a high-performance super-
heterodyne receiver solution for low-power mobile
devices, with the benefit of using the system’s existing
clock reference. This receiver is ideal for integration into
mobile phone handsets using common reference fre-
quencies such as 10.0, 13.0, 14.4, 19.2, 20.0, and
26.0MHz. The only external components required are the
GPS RF filter, an IF filter (typically designed from inexpen-
sive discretes), a three-component PLL loop filter, and a
few other resistors and capacitors. The MAX2741 inte-
grates the reference oscillator core, the VCO and its tank,
the synthesizer, a 1- to 3-bit ADC, and all signal path
blocks except for the 1st IF filter. The typical application
area for the receiver is less than 2cm
The MAX2741 RF front-end LNA and mixer are the most
important in the signal path. This stage sets the noise
figure for the receiver, defining the sensitivity, and mixes
the 1575.42MHz L1-band GPS signal down to a 1st IF of
37.38MHz. The LNA itself has an NF of approximately
1.5dB; the cascaded NF of the front-end (including the
mixer) is approximately 4.7dB, and the cascaded gain
is typically 21dB.
The image-reject mixer is set up for a high-side injected
RFLO (1612.80MHz), and offers typically better than 30dB
rejection of the image noise (1650.18MHz). The -30dBm
input 3rd-order intercept (IIP3) of the RF strip, in conjunc-
tion with the GPS IF filter, provides excellent out-of-band
interferer immunity.
The 1st IF outputs (IFOUT±) are internally biased to
approximately 2V, and have a differential source
impedance of approximately 2.5kΩ. The IF filter can be
implemented as a discrete L/C filter, or as a monolithic
SAW or ceramic if one is available.
The 2nd conversion stage consists of an active mixer, a
variable-gain amplifier (VGA), and a tunable lowpass
filter. The IF mixer is configured for low-side LO injec-
tion for a 2nd IF of 3.78MHz. Total gain in this stage is
62dB, and the VGA offers 51dB of gain adjustment. The
VGA is typically controlled by the baseband IC through
the SPI interface to optimize the signal swing for digiti-
zation by the ADC.
The on-chip lowpass filter has an adjustable cutoff fre-
quency, programmable from 2.9MHz to 7.7MHz in 16
steps. This LPF further reduces out-of-band noise and
band-limits the signal to the ADC, ensuring that the
sampling process does not generate alias components.
RF/1st Conversion Stage (Front-End)
_______________________________________________________________________________________
Detailed Description
IF/2nd Conversion Stage
Integrated L1-Band GPS Receiver
2
.
DC offset compensation at the ADC input is performed by
an on-chip 4-bit DAC. This compensates for any DC error
introduced by transistor mismatch in the differential stage
driving the ADC input, allowing the downconverted GPS
signal’s DC level to be centered within the threshold volt-
ages of the ADC.
The on-chip ADC samples the down-converted GPS
signal at the 2nd IF (3.78MHz). Sampled output is pro-
vided in either 2-bit (1-bit magnitude, 1-bit sign) or 3-bit
(2-bit magnitude, 1-bit sign) formats, as determined by
the ADC mode configuration bit (CONFIG1:D15); see
Table 5 for details. The ADC sample clock (system GPS
clock) is derived either directly from the reference clock
(SYNTH:D9 = 1), or from an RFLO divide-by-96 block to
provide a 16.8MHz sample clock (SYNTH:D9 = 0). The
clock is available to the baseband processor at
GPSCLK (pin 15). The sampled ADC data bits are
available on pins 16, 17, and 18 (GPSIF2, GPSIF1, and
GPSIF0). The functionality of the pins is different in
each mode (2-bit vs. 3-bit)—see Table 5 in determining
the interface connection for the application circuit.
The MAX2741 integrates an integer-N synthesizer; all
blocks except the loop filter are on-chip. The reference
can be either a crystal (driven by the internal oscillator),
or a TCXO module. The oscillator provides a 5pF load
to the crystal. A TCXO module should provide a swing
in the 0.6V
The reference divider (/R) is programmable (SYNTH:
D7–D0), and can accommodate reference frequencies
up to 26MHz. The reference divider needs to be set so
the comparison frequency (f
phase detector is 200kHz. The VCO runs at twice the fre-
quency of the RFLO; the RFLO is therefore generated
from the VCO using a quadrature divide-by-2 block. The
RF LO is f
1st IF LO is f
IF LO division ratios are not adjustable. This configuration
allows for the use of reference frequencies common to
GSM, CDMA, TDMA, TD-SCDMA, and UMTS handsets:
9.6MHz (R = 48), 13.0MHz (R = 65), 14.4MHz (R = 72),
19.2MHz (R = 96), 26.0MHz (R = 130), etc.
COMP
P-P
COMP
to 2.2V
x 8064 (typically 1612.80MHz), and the
x 168 (typically 33.6MHz); the RF and
P-P
range.
COMP
) at the frequency/
Synthesizer
ADC
5