MN1818 Design Guidelines
Micro Modular Technologies Pte. Ltd. Page 4 of 8
No. 3, Ubi Avenue 3, #05-01 Crocodile House 408857 Singapore
Tel: 65-6745-8832 Fax: 65-6293-0661 Email: sales@micro-modular.com www.micro-modular.com
RTC +3VBKUP must be between +2.5 volts and +3.6 volts for the MN1818 to recognize the device
and read the stored time correctly.
6 RF Interface
The MN1818 GPS Receiver Module accepts a standard L1 GPS C/A code signal (from a passive or
active antenna) on the RF Input pad of the module. In addition the RF Input provides +3V to power
an active antenna. If using a passive antenna, the antenna must appear as a DC open. If not, then
insert a good quality ceramic capacitor of approximately 27 pF to block the DC.
Using the MN1818 GPS Receiver with an active antenna requires special precautions. If the active
antenna has high gain and is placed close to the MN1818, then excessive RF gain could cause
oscillation between the antenna and the MN1818. This can be prevented by making sure the
MN1818 is well shielded from the active antenna. In addition, a DC block and simple Pi-pad
attenuator in the RF line would reduce the gain. Power could be restored to the active antenna by
using a 68 nH inductor from +3 to the antenna line. This inductor must be well bypassed on the DC
supply side with at least a 27 pF capacitor to ground.
7 Shielding and Filtering Requirements
The MN1818 is designed to receive a GPS signal that can be as low as -150 dBm. Any source of
interference near in frequency to the GPS signal could potentially jam the MN1818 and disrupt
reception of the signal.
For proper system design, the GPS antenna needs to be shielded from any potential jamming
source. For that reason, in most designs it makes more sense to shield the digital portion of the
product rather than the RF portion. This keeps the digital noise from radiating into the antenna and/or
antenna feed lines.
It is important to note the GPS signal level is well below any regulatory emissions requirement for
EMI and EMC. Thus while a product meets FCC class B or CISPR 22, it is possible the emissions
from the product will still seriously impact the MN1818 performance.
Excessive interference into the MN1818 via the antenna can result is low to very low reported C/Nos
of the satellite signals and subsequent excessive TTFF times. Assuming an 18mm square patch
antenna with good LNA, the reported C/Nos should be in the high 40s and low 50s. If the values are
below this, then interference needs to be considered as a problem and resolved. This can also be
checked by substituting an external active antenna and moving it closer to and away from the device
and noting the change in reported C/Nos. If any improvement in signal is noted as the external
antenna is moved away from the device, then additional shielding of the digital electronics is
required.
If the product contains an RF transmitter, then care must be taken to prevent overloading the front
end of the MN1818 if simultaneous operation is required. This overloading can come from several
sources.
First, the input LNA of the MN1818 does not have a preselect filter and is fairly broad band. If for
example a GSM transmitter was close by, then the GSM signal could overload the LNA. The output
of the LNA is going to be proportional to its input, and if the GSM signal so dominates, the GPS
