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发表于 2009-1-31 10:24:29
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JCOMM GPS Module
(rev 1.0)
GPS System Design Guide
Influences in GPS signal reception on GPS-enabled systems
1. Switching regulators
A. Frequencies used in high electrical current consumption ( over 1A ) are usually 300KHz, but
800KHz or 1.2MHz are used in medium electrical current consumption (100mA~300mA) to
give better current efficiency.
B. Under this current circumstance, switching signals generate 2nd, 3rd,…high degrees of
multiple high frequencies which sometimes generate 1310 times of frequency of 1.2MHz,
namely 1572MHz.
C. When it operates based on 1.2026107MHz, this frequency may be coupled with related
temperatures, R or C values, thus generating 1575.42MHz frequency. Consequently, this
kind of generated frequency surely lead to influence over the bandwidth reserved for GPS
signals since GPS frequency is also used at this frequency range.
D. The kinds of frequencies are normally radiated from power lines acting as an antenna line,
and in most cases, a high degree of radiation comes from coils built on the PCB.
E. Practically, radiations from power lines are usually removed by bypass capacitors or
separations of multi-level PCB lay-outs. However, since it is not enough for whole noise
reduction, a PCB designer should add a shield can over the coil in order to securely remove
much radiation coming from coils.
2. Low Drop Regulators
A. SGPS systems are electrically very sensitive with ripple voltages. In most cases, low drop
regulators are usually used for power supply to GPS subsystem, but it subsequently causes
feedback ripple noise internally.
B. For the reduction of overall ripple voltage, use of normal capacitors can only remove low
frequency ripple noises. For removal of high frequency ripple noises of 100K Hz ~ 1M Hz,
then you must use low-ESR type capacitors.
*** Attention for measurement of power ripple voltages
- In measurement of using oscilloscopes, it should be noted that a measured voltage through mutual
connection of both end of testing probes with short circuit is the limit of measurement by the
oscilloscope. For example, if 10mVpp is measured at this short circuit, then no ripple voltage can be
measurable under this limit level.
- The input voltage source for regulator should be battery. If a power supply from a test lab is used,
then a wrong measurement value is obtained due to hum noise generated
copyright
Copyright © 2007 JCOMM & MAXRISE Technology. All right reserved. 3
3. CPU clock noises
A. External clocks used for CPU clock supply are usually 10MHz~50MHz, but it’s usually
over-clocked internally to get 400~800MHz frequencies for internal ALU data/instruction
operation. Meanwhile, types of data streams are not regular, but quite random such that this
random data stream may become a harmonic radiated noise frequency to GPS frequency
band, leading to high noise radiation to GPS signal reception at a GPS antenna part..
B. Most noise sources for this kind of CPU operation-related noise radiation include
data/address lines connected among CPU, flash memory, DRAM, camera module, SD card
connector, etc.
C. In order to prevent from this various kinds of radiated noise frequencies, these parts and
data lines should be designed to form adjacent to each other and then place the required
separated shield cans to remove the radiated noises in part.
D. The radiated energies from a flexible cable on LCD module should be also shielded with
EMI tape or copper tape.
E. For a GPS antenna built in a system body, data cable should be also shielded with EMI tape
for prevention of radiated noise.
Copyright © 2007 JCOMM & MAXRISE Technology. All right reserved. 4
Methods for reduction of various external noises affecting GPS signal reception
1. Placement of electric components/parts
A. A GPS antenna should be placed as far as away from noise generating sources such as CPU,
memory, audio codec, power regulation, LCD module cable, SD socket, etc.
B. The opposite side of a PCB layer on which CPU and memory are mounted should be put on
with shield cans.
2. PCB lay-out
A. In normal cases, 6 layers of PCB should be designed as follows
i. 1st layer : Parts & GROUND
ii. 2nd layer : Signal line
iii. 3rd layer : GROUND
iv. 4th layer : GROUND & POWER
v. 5th layer : Signal line
vi. 6th layer : Parts & GROUND
1. GROUND layers should not include any signal line or power line.
2. Occasionally, grounds for analog and digital signals are separate, but this ground
separation may cause bad effect to radio frequency.
3. When connecting signal lines, signal hole should be as less as possible on ground layer.
4. Soldering area for a shield can built on its outer PCB layer should be treated with a lot
of ground holes to make a full connection between part layer and ground layer.
5. Long signal line coming out of a shield can area should be placed onto inner PCB layers.
6. Empty area on the part layer should be shielded with ground and also treated with a lot
of ground holes to make a full connection to ground layer.
3. Shield can
1. Every contact area between a shield can and a PCB layer should be fully soldmounted
to make a full conduction. If not, such shield can is not able to make a
good noise reduction effect.
2. As ways of easy repair services for system failures after sales or factory
productions, a shield can be a form of shield box with a cover in which the cover
can be easily removed and wrong parts can be easily replaced from PCB.
3. For much better shielding effect, only single shield can should not cover the whole
shielding area, but multiples of shield cans should cover multiples of separate
shielding areas, if possible.
4. If a shield can size is big, then high intensity of noise energy can be radiated from
the surface of the shield can going onto ground layer. Such kinds of noise radiation
will be another noise antenna source.
Copyright © 2007 JCOMM & MAXRISE Technology. All right reserved. 5
4. LCD display module
1. LCD cable should be shielded with EMI tape as much as possible, and connected to
ground.
2. LCD metal panel should be connected to ground via EMI gasket.
5. GPS data cable
1. In case of antenna built-in GPS system, a data cable between CPU and GPS unit
can be a noise source, so this data cable should be shielded with EMI tape.
6. SD socket
1. Since high speed data transfer between SD card and CPU induce a lot of noise, a
shield can should be also considered to remove the noise.
7. EMI spray
1. For noise reductions, sometimes EMI spray is treated in the GPS system. However,
noises are randomly radiated to various directions into the entire system, such that
the-above-described methods should be backed up with more secure noise
reduction method.
2. If more secure noise reduction methods are not implemented in product design
stage, then it would be very difficult to distinguish defected goods in mass
production stage, even a good product can be turned onto a defective product in
particular situations-in productions or on-field operations.
8. Improvement paths for noise reduction in mass production stage
1. Some improvements from sample tests with tens of product samples in test lab can
not guarantee an entire improvement. In an actual mass production stage, a lot of
new serious noises are found due to variously associated product/production
deviations.
2. All of these new noises should be analyzed thoroughly and find proper improving
ways through a lot of rigorous failure analysis procedure. After many cycles of
improvement procedures, then finally a manufacturer can have a highly reliable
GPS system which can secure highest production yield and lowest field user claims.
3. Especially, a manufacturer should perform the GPS signal reception test under full
CPU-resource utilizing operation since multiple simultaneous operations such as
audio/video play-back, TV watch can induce a lot of noises to GPS signal reception
during system operation.
4. If these kinds of multiple operation tests are not performed prior to field product
sales, a lot of end-users can claim GPS function errors after the sales or firmware
upgrade process...[em11] |
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