SE4110L
GPS Receiver IC
DST-00002 ! Rev 6.4 ! May-26-2009 5 of 22
Functional Description
LNA
The internal LNA allows a high-performance, low-
power GPS receiver to be completed without using
any additional active components.
The GPS L1 input signal which is applied to LNA_IN
(pin 3), is a spread-spectrum signal centered on
1575.42 MHz with a 1.023 Mbps BPSK modulation.
The signal level at the antenna is typically -130 dBm
in open-sky conditions, dropping to below -150 dBm
in masked signal areas (e.g. indoors). The LNA noise
figure is the largest contributor to the sensitivity so it is
an important parameter; the lower, the better.
The LNA input requires a minimum of external
matching components to achieve good RF gain with
minimal noise figure: only a single series inductor and
single shunt capacitor are required. The input requires
a DC blocking capacitor if circuitry prior to the LNA
has a DC bias. Although attention should be paid to
track lengths and interference throughout the design,
the LNA input matching circuit is the only RF circuit
critically sensitive to layout.
The LNA output includes internal 50 "#matching for
connection to the mixer input, either directly or via an
optional external filter.
In applications where the internal LNA is not required,
the LNA can be disabled by connecting VCC_LNA
(pin 1) to GND. This will save approximately 1.9 mA
of active current.
Mixer RF Input
The mixer RF input, MIX_IN (pin 21), is a single-
ended 50 ! input designed to interface either to
LNA_OUT (pin 24) or to the output of an external
filter. An external active antenna can also be
connected to the mixer input.
The image reject mixer ensures that the receiver’s full
sensitivity is achieved without an external filter. For
applications where additional selectivity is required,
an external filter can be added between the
LNA_OUT and MIX_IN pins.
IF Filter
The SE4110L includes a fully integrated Intermediate
Frequency (IF) filter which provides excellent
interference rejection with no additional external
components. The filter has a 3rd order Butterworth
bandpass response.
The bandpass response has a nominal bandwidth of
2.2 MHz; the nominal center frequency is preset to
4.092 MHz. These parameters ensure very low
implementation loss in all frequency plan
configurations.
AGC and ADC
The SE4110L features a linear IF chain with 2-bit
SIGN / MAG ADC. SIGN is on pin 10, and MAG on
pin 11.
An Automatic Gain Control (AGC) system is included.
This provides over 40 dB of gain control range so that
the output signal level is held at an optimum level at
the input of the ADC.
The MAG data controls the AGC loop, such that the
MAG bit is active (HIGH) for approximately 33 % of
the time.
The SIGN and MAG signals are latched by the rising
edge of the sample clock, CLK_OUT (pin 9). The
SIGN and MAG signals are best sampled by the GPS
baseband IC on the rising edge of CLK_OUT, for
optimum sample and hold in the ADC.
The AGC time constant is determined by a single
external capacitor, connected between VAGC (pin 5),
and VSSN / GND. The settling-time of the AGC is
within 10ms with a 10nF capacitor.
The AGC system also features a control-inhibit
facility, via AGC_DIS (pin 6). By connecting AGC_DIS
to VDDN, the internal AGC controller is inhibited, and
the gain held at the level set prior to the inhibition.
While the AGC controller is inhibited, it is possible to
control the AGC gain from an external source by
applying a low-impedance voltage to VAGC (pin 5).
PLL and Loop Filter
The entire phase-locked loop (PLL) generating the
local oscillator for the mixer is contained on-chip, with
the exception of the PLL loop filter.
A PLL loop filter can be implemented by attaching a
series capacitor (220 pF) and a resistor (33 k!)
between VTUNE (pin 18) and GND / VSSN. The PLL
follows a classic 3
rd
-order response; this is achieved
in conjunction with an on-chip 10 pF capacitor
connected between VTUNE and GND / VSSN.
Typical PLL Loop Bandwidth is set to be 200 kHz.
The reference frequency for the PLL may be supplied
either externally or using the on-chip crystal oscillator.
