AC530 Fastrax, AC530 Datasheet - Page 29

• Manufacturer Part Number: AC530
• Description: GPS Development Tools Application Board for UC530
• Manufacturer: Fastrax

Specifications of AC530

• Rohs: yes
• Product: Application Boards
• Tool Is For Evaluation Of: UC530
• Frequency: 1.575 GHz
• Operating Supply Voltage: 3.3 V
• Interface Type: UART
• Description/function: Fastrax Application Board AC530 provides the UC530 connectivity to the Fastrax Evaluation Kit or to other evaluation purposes
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Operating Supply Current: 25 mA

Page 29 of 39
Use a power plane layer dedicated solely for power nets. Use wide trace width or even copper plane areas to
achieve low impedance for power nets. Dedicate at least one layer as ground planes on adjacent layer above or
below power plane layer in order to maximize capacitance to ground plane.
7.3.2 Avoiding EMI
Any GPS receiver is vulnerable to external spurious EMI signals since GPS signals are very weak below thermal
noise floor. Any man made noise or spurious signals picked up by the nearby GPS antenna increases the noise floor
and reduces GPS signal levels. Carrier Wave (CW) type spurious signals like clock harmonics on GPS band may also
cause cross correlation products that may interfere with GPS signal tracking and cause position offsets.
The embedded GPS antenna may pick up local EMI signals and thus it is essential for good GPS performance that
the following measures against EMI are properly implemented:


The following picture gives a suggestion for e.g. a 6-layer PCB build up, which forms a Faraday shield together with
ground planes on PCB and with the shield over high speed electronics. Buried and blind via holes are used to keep
EMI signal inside ground planes. I/O signals that are routed outside the Faraday enclosure are filtered with a
suitable EMI filter. Power plane layer is used for supply nets with low impedance traces/planes.
7.4
The embedded GPS antenna provides optimal radiation efficiency 80% typ. with 80x40mm ground plane. The
antenna provides linear polarization with peak gain 1.1dBi and radiation pattern optimized for portable devices.
The antenna is insensitive to surroundings and has high tolerance against frequency shifts. However on small
ground plane widths the antenna gain and radiation efficiency reduces, e.g. the AC530 application board having
2012-05-03
UC530_Datasheet
High speed electronics like host CPU & memory bus are enclosed in a ‘Faraday shield’. The electrical enclosure
is formed by the ground planes on PCB + metal shield over components. Route signals at inner layers as
discussed previously. Use preferably a power plane(s) layer for supply nets.
Any signal that is routed outside the Faraday shield is protected against EMI noise on 1575MHz with a serial
RF filter like
Embedded antenna operation
o
o
o
a serial resistor (> 330ohm, suitable for I/O with low current)
with a dedicated EMI filter (or ferrite bead) suitable for higher current
with suitable by-pass capacitor e.g. 18pF (low impedance due to series resonance at 1575MHz).
CPU
Figure 15 Avoiding EMI with Faraday enclosure
Buried and blind
via holes for
signal traces
’Faraday enclosure’
Shield
EMI filter
GND via
holes
I/O connector,
indicator etc.
PCB 6-layer
Build up:
Top, GND plane
L2, signal
L3, signal
L4, GND plane
L5, Power plane
Bottom, GND plane