Maxim Integrated MAX2769 GPS Receiver - Manual (Page 2)

for any application. If automatic LNA selection is not desired, it can be disabled through the Config1 register <14:13>.
Internal Capacitive Load Trimming for Crystal References
When using the MAX2769 with a crystal reference, no tuning of external load capacitors is required to match devices—a bank
of internal crystal load capacitors can be programmed through the SPI interface to trim the load to yield the correct reference
frequency. The internal bank can be programmed over a range of about 11pF to 17pF (settings plus 9pF of parasitic
capacitance). A single series capacitor is placed between the crystal and the crystal/reference input. If the desired load value is
between 11pF and 17pF, the value of this coupling capacitor can be made large (e.g., 10nF), so as not to affect the
programmed value. For crystals with load capacitances below 11pF, the coupling capacitor can be made small to add in series
with the internal bank, reducing the load seen at the device. Either way, the final frequency trimming can be done internally
through the SPI interface.
Reference and IF Frequency Flexibility
The design accommodates a wide range of reference frequencies between 8MHz and 44MHz with a default setting of
16.328MHz. The IF frequency is adjustable in 63 steps between 0 and 12.5MHz, with a default setting of 4.092MHz. (It is
recommended that the IF frequency be kept at or below 4.092MHz, as additional steps may need to be taken to assure stability
at higher frequencies.) Because of a fractional-N synthesizer that permits small step size while maintaining excellent phase
noise, this flexibility does not compromise performance. No other product on the market has this degree of flexibility.
IF Filter Flexibility
Filtering at IF is important as it limits the noise bandwidth and improves sensitivity while eliminating interference. The
MAX2769's IF filtering is highly flexible. The design uses a complex polyphase Butterworth configuration that can be set to 3rd
or 5th order, and either bandpass or lowpass as the application dictates. The center frequency is also programmable to match
the selected IF. The 3dB bandwidths can be selected as 2.5MHz, 4.2MHz, 8.0MHz, or 18MHz. Users can choose a design that
optimizes performance for their application. (Note: These are two-sided 3dB bandwidths. When the lowpass option is selected,
the bandwidths are cut in half and become 1.25MHz, 2.1MHz, 4.0MHz, and 9MHz. In fact, the highest setting should only be
used in a lowpass configuration.) In the predecessor to this part, a 4.8MHz lowpass filter was required to pass the fixed-IF
frequency data. In this design, a 2.6MHz bandpass design could be employed, thus reducing the noise bandwidth by nearly
3dB and enhancing system sensitivity. Filters (in bandpass mode) are designed to have no more than 1dB droop at F
C
±1.023MHz.
High System Gain with a Wide Range of Level Control
To use the MAX2769 without an active antenna in a low-signal-strength environment, it is imperative that the receiver have
sufficient gain. This device typically has up to 110dB available gain (in analog mode) with 60dB to 65dB of gain adjustment.
Access to the Amplified RF Signal for Coexistence Filtering
While no external filtering is required for stand-alone applications, coexistence with cellular or WiLAN transmissions in close
proximity may require additional filtering to prevent overdriving the GPS receiver front-end. On the MAX2769, the RF signal
has been made accessible between the first LNA stage output and mixer input (pins 2 and 5 respectively). If filtering is not
desired, these ports can be connected through a coupling capacitor. However, filtering introduced at this point has minimal
effect on the excellent sensitivity of the receiver. (For example, for typical device parameters, a SAW filter with 1dB insertion
loss would degrade cascaded NF (and thus GPS sensitivity) by only about 0.15dB.
Flexibility of Output Modes
Most GPS devices provide only a single-output mode. The MAX2769's output can be programmed to be analog, CMOS, or
limited differential logic in unsigned or complimentary binary format with 1- to 3-bit output from the ADC.
Temperature Sensor and Status Monitoring
A temperature sensor is included, which can be calibrated externally if desired. While lock-detect status can be obtained at the
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