LISA-U1 series - System Integration Manual
3G.G2-HW-10002-3
Preliminary
Design-In
Page 96 of 125
A tank capacitor with low ESR is often used to smooth current spikes. This is most effective when placed as
close as possible to
VCC
. From main DC source, first connect the capacitor and then
VCC
. If the main DC
source is a switching DC-DC converter, place the large capacitor close to the DC-DC output and minimize
the
VCC
track length. Otherwise consider using separate capacitors for DC-DC converter and LISA-U1 series
module tank capacitor. Note that the capacitor voltage rating may be adequate to withstand the charger
over-voltage if battery-pack is used
VCC
is directly connected to the RF power amplifiers. Add capacitor in the pF range from
VCC
to GND along
the supply path
Since
VCC
is directly connected to RF Power Amplifiers, voltage ripple at high frequency may result in
unwanted spurious modulation of transmitter RF signal. This is more likely to happen with switching DC-DC
converters, in which case it is better to select the highest operating frequency for the switcher and add a
large L-C filter before connecting to the LISA-U1 series modules in the worst case
The large current generates a magnetic field that is not well isolated by PCB ground layers and which may
interact with other analog modules (e.g. VCO) even if placed on opposite side of PCB. In this case route
VCC
away from other sensitive functional units
The typical GSM burst has a periodic nature of approx. 217 Hz, which lies in the audible audio range. Avoid
coupling between
VCC
and audio lines (especially microphone inputs)
If
VCC
is protected by transient voltage suppressor / reverse polarity protection diode to ensure that the
voltage maximum ratings are not exceeded, place the protecting device along the path from the DC source
toward the LISA-U1 series module, preferably closer to the DC source (otherwise functionality may be
compromised)
VCC
line should be wide and short.
Route away from sensitive analog signals.
2.2.1.3
USB signal
The LISA-U1 series modules include a high-speed USB 2.0 compliant interface with a maximum throughput of
480 Mb/s (see Section 1.9.3). Signals
USB_D+
/
USB_D-
carry the USB serial data and signaling. The lines are
used in single ended mode for relatively low speed signaling handshake, as well as in differential mode for fast
signaling and data transfer. Characteristic impedance of
USB_D+
/
USB_D-
lines is specified by USB standard.
The most important parameter is the differential characteristic impedance applicable for odd-mode
electromagnetic field, which should be as close as possible to 90 differential: signal integrity may be degraded
if PCB layout is not optimal, especially when the USB signaling lines are very long.
Route
USB_D+
/
USB_D-
lines as a differential pair
Ensure the differential characteristic impedance is as close as possible to 90
Consider design rules for
USB_D+
/
USB_D-
similar to RF transmission lines, being them coupled differential
micro-strip or buried stripline: avoid any stubs, abrupt change of layout, and route on clear PCB area
2.2.1.4
Analog audio (LISA-U120 / LISA-U130 only)
Accurate analog audio design is very important to obtain clear and high quality audio. The GSM signal burst has
a repetition rate of 217 Hz that lies in the audible range. A careful layout is required to reduce the risk of noise
from audio lines due to both
VCC
burst noise coupling and RF detection.
Analog audio is separated in the two paths,
1.
Audio Input (uplink path):
MIC_P
/
MIC_N
2.
Audio Outputs (downlink path):
SPK_P
/
SPK_N
The most sensitive is the uplink path, since the analog input signals are in the microVolts range.
