Micro Modular Technologies MN1818 Instruction Manual

MN1818 Design Guidelines
Micro Modular Technologies Pte. Ltd.
No. 3, Ubi Avenue 3, #05-01 Crocodile House 408857 Singapore
Tel: [phone] Fax: [phone] Email: [e-mail] www.micro-modular.com
1 Introduction
This document contains important technical information, design notes and helpful hints to assist the
designer in achieving first time success in bringing up a design using the MN1818 GPS Receiver
module. It contains design examples and suggestions on a wide variety of topics, including power
supply connections, special reset circuit requirements, RF interface, shielding and filtering
requirements, antenna considerations and other important subjects.
2 Power Supply
The MN1818 GPS Receiver Module is designed to operate from a single 3V power supply. Either a
switching power supply or a linear supply can be used to provide the 3V power. Sufficient decoupling
on the power supply is necessary to ensure good performance. It is recommended that the 3V power
supply be filtered by a 10uF tantalum capacitor. The decoupling capacitor should be placed as close
as possible to the module’s power supply pin.
If a switching power supply is used, it may be necessary to increase the value of the tantalum
capacitor to eliminate switching power supply ripple.
It should be noted that the DC supply must be clean. If there is any coupled AC noise (particularly
centered around L1), it is carried directly into the input of the MN1818 and could reduce the
sensitivity.
2.1 Ground Plane
To ensure optimal RF performance, the user should provide a solid ground plane underneath the
module. Sufficient via holes on this ground plane should be used to connect to the main board
ground plane.
The module does not use separate grounding for RF, power and digital signals. It is NOT
recommended that the user split the ground plane connecting the module to the power supply. This
is because if the user may introduce unwanted ground loops if separate RF ground and power
ground is used.
3 Reset Circuit Requirements
The RESET
¯¯¯¯¯¯ pin must be held low until 10 milliseconds after the 3-volt power supply has stabilized to
within proper tolerance. The RESET
¯¯¯¯¯¯ pin must be pulled low before power drops below specification
or on power down to prevent corruption of flash memory contents.

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Figure 1 – Suggested Reset Circuit
4 Sleep Mode
The standard software does support a software based sleep mode. The receiver is placed into sleep
by issuing the NMEA sleep command (please refer to the Orion NMEA manual). All navigation
processes are stopped, the RF put into a low power state and the baseband processor halted. The
current consumption of the MN18180 will drop to approximately 2 mA exclusive of any current being
drawn on the antenna port. The antenna supply voltage is not switched during sleep mode.
The MN1818 will awaken from sleep mode whenever a transition occurs on the RX0 or RX1 input
pin. Thus it is possible to send a NMEA command to waken the MN10818, but as the first character
is lost (the first transition is actually lost to the software), the receiver will awaken, but not respond to
the command.
For sleep mode to function properly, it is important to make sure the 32KHz RTC is present.
5 Real-Time Clock (RTC) Circuit
The MN1818 contains a built-in 32KHz RTC to maintain time whenever the MN1818 is in the Sleep
state. This allows the MN1818 to quickly reacquire satellites and enter navigation within the hot start
time period assuming the receiver had been in sleep mode for less than two hours.
The RTC circuit consists of a 12.5 pF 32KHz crystal and two capacitors (one on either side) returned
to the +3V supply (see Figure 2). Do not return the capacitors to ground as this will cause RTC
startup issues. Note that other non-RTC components are still required but have been omitted from
Figure 2 for clarity.
During customer product design, the user should test the startup characteristics of the RTC crystal
by inserting a series resistor between the crystal/capacitor and the RTC_XIN input on the MN1818.
The crystal would be suitable if the series resistor is 5 times the equivalent series resistance of the
RTC crystal and the RTC oscillator can be verified to start over the design voltage and temperature
margins.

MN1818 Design Guidelines
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Figure 2 – Internal RTC Circuit
The internal RTC does not maintain time when the RESET
¯¯¯¯¯¯ line is pulsed low and then high, nor does
it maintain time when power is removed to the MN1818. If either of these conditions is a requirement
to maintain time, the MN1818 also supports the addition of an external RTC circuit as shown in
Figure 3. Note the software only supports the Seiko S-35390A RTC IC when connected as shown.
Note that other non-RTC components are still required but have been omitted from Figure 2 for
clarity.
Figure 3 – External RTC Circuit
While the Seiko S-35390A can operate from 1.5 volts to 5.5 volts, it is important to realize the logic
levels expected by the external RTC are based upon the supply voltage. For this reason, the external

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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

MN1818 Design Guidelines
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signal would be attenuated and sensitivity of the receiver would be reduced. The OEM designer
would need to design suitable input filtering to the MN1818 to protect in this case.
A second case occurs in the collocated transmitter. The power amplifier has both a gain and a noise
figure. If we take an example of a power amp noise figure of 15 dB and 30 dB of gain, this would
mean that the power amp radiates broadband noise approximately 45 dB above thermal noise. This
means the power amp alone could present a noise source in the GPS band of -129 dBm. While this
would easily meet any regulatory emissions requirements, it would render the GPS receiver
inoperative. In this case, a suitable filter must be placed on the output of the power amplifier of the
collocated transmitter to avoid this case.
8 Layout Considerations
In order to get the optimal performance from the MN1818 GPS receiver module there are a few
points that the user should note when designing the layout.
8.1 Solid Ground Plane
It is necessary to provide a solid ground plane below the module. The liberal use of vias to connect
the top layer ground plane to the bottom layer and internal layers ground plane is necessary to
ensure a sold RF ground. The module uses the same ground for power supply, digital and RF
signals. Hence, the user should not attempt to split the ground plane.
8.2 Decoupling capacitors for power supply
Decoupling capacitors should be placed as close as possible to the VCC pin of the MN1818 GPS
receiver module. The recommended values are 10uF and 27pF in a parallel configuration.
8.3 Position of vias
As a word of caution, it is advisable to not place vias between pads or too near the pads as this
creates a possibility of a short circuit during soldering.
9 GPS Antenna Selection
Currently there are several types of GPS antennas available for the user to choose from. Each type
of antenna has both advantages and disadvantages which need to be carefully weighed in making a
selection. In addition, most antenna types are available in both an active (includes built in LNA) and
passive versions.
When selecting the antenna it is important both to consider the characteristics of the GPS signal
itself along with the characteristics of the antenna. The GPS signal is broadcast at 1.57542GHz and
is received from all visible GPS satellites. The receiver needs a minimum of four signals to compute
a 3D position. Ideally, the antenna should have an unrestricted view of the sky. Certain locations
may limit the visibility of the sky such as being close to a building, etc, so it is important that the
product in which the antenna is installed does not further limitation to satellite visibility.
The GPS signal is also right hand circularly polarized (RHCP) so best results are achieved under
most conditions with a right hand circularly polarized antenna. Under severe obscuration where
multipath signal reflections are present, a linearly polarized antenna my give better results under the
assumption that a reflected signal is better than no signal.

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Antennas are specified by antenna type, antenna gain, antenna pattern, polarization and axial ratio.
Antenna gain is typically considered to be the ratio of the signal level received by the antenna under
consideration at zenith as compared to a theoretical isotropic radiator (equal signal level in all
directions). The gain is measure in dBi (for a linearly polarized antenna) or dBic (for a circularly
polarized antenna). The gain of an antenna will vary depending upon elevation and azimuth of the
signal source with respect to the antenna. Graphically plotting this variation shows a visual
presentation of the antenna pattern. The axial ratio of an antenna is a measure of the quality of its
polarization. An axial ratio of 1 is perfect circular polarization, an infinite axial ratio in perfectly linear
polarization.
9.1 Patch Antennas
Patch antenna are typically square or round ceramic elements with metallic plating on both sides, the
top being the metallic antenna element and the bottom being the ground plane.
Figure 4 – Typical patch antenna
If a patch antenna is selected, it is important that it be oriented such that the top surface of the
antenna is horizontal with respect to the surface of the earth. Tilting the antenna away from the
horizontal will result in an artificial obscuration of potentially visible satellites.
While patch antenna are low cost and can provide good gain, it is important that the patch antenna
be used with a proper ground plane. The antenna vendor can provide assistance in this area. In
addition, patch antennas are detuned by the present of anything within its near field, such as a
plastic cover. The antenna vendor can tune the antenna to compensate for this detuning.

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9.2 Helix Antennas
Helix antennas are usually spirally wound onto a tubular ceramic piece (see Figure 5). For best
performance, the helix antenna needs to be vertical with respect to the surface of the earth. Helix
antennas do not require a ground plane, but may work better with one.
Figure 5 – Sarantel helix antenna (cover removed)
9.3 Chip Antennas
Chip antennas (Figure 6) are the smallest antennas available for GPS and are quite popular in small
handhelds. However, chip antennas are linearly polarized making them more receptive to multipath
signals which would degrade the computed position in some cases. Chip antennas also have very
specific ground plane requirements. The antenna vendor can provide assistance in this area and can
possibly tune the chip for a specific application.
Figure 6 – Chip Antenna

MN1818 Design Guidelines
Document no: MN1818_DG_080614
Micro Modular Technologies Pte. Ltd. Page 8 of 8
No. 3, Ubi Avenue 3, #05-01 Crocodile House 408857 Singapore
Tel: [phone] Fax: [phone] Email: [e-mail] www.micro-modular.com
10 Notices
All reference and informational documents (including marketing information, specifications, reference
designs, etc.) are provided for information only and are subject to change without notice. Reasonable
efforts have been made in the preparation of these document to assure their accuracy, however
Micro Modular Technologies Pte. Ltd. assumes no liability resulting from errors or omissions in
these, or any document, or from the use of the information contained herein. Micro Modular
Technologies Pte. Ltd. reserves the right to make changes in the product design and specifications
as needed and without notification to its users. Please check our website for the most current
documentation. All information contained herein is the property of Micro Modular Technologies Pte
Ltd. and may not be copied or reproduced, other than for your information, without prior written
consent.
11 Contact Information
Corporate Headquarters
Micro Modular Technologies Pte. Ltd.
No. 3, Ubi Avenue 3, #05-01
Crocodile House, Singapore 408857
Tel: [phone]
Fax: [phone]
Email: [e-mail]
Americas and Europe
Micro Modular Technologies Americas
14720 Creekside Lane
Longmont, CO 80503, U.S.A.
Tel: [phone]
Fax: [phone]
Email: [e-mail]
For a list of Regional Sales Representatives,
please see our web page:
www.micro-modular.com