Motorola Timeport P8190, Service Manual

Page: 89 / 149

Motorola Confidential Proprietary
Theory of Operation
77
Two RF ports are designed in this trans-
ceiver; internal antenna and external an-
tenna. An RF switch controls which antenna
path is going to be used during operation.
The internal antenna will be used only when
there is no load at the EXT_ANT port.
RX Front End IC
The Front End IC is used to remove the RX
carrier frequency and produce the RX IF sig-
nal. It also has some internal buffers for the
receive signals and the VCO signals.
The RX signal is amplified within the Front
End IC and then routed to an exter-nal fil-
ter. The signal is then injected back into the
Front End IC and is mixed with the VCO and
the result is the IF signal which is filtered
prior to entering the Custom IC.
The Custom IC can be divided into two func-
tional subsystems: The ZIF (Zero-Interme-
diate-Frequency) which provides all of the
functions of the back-end of a receiver, and
the SYN (SYNthesizer) which contains
phase-locked loops and modulators to pro-
duce the Local Oscillator (LO) and modulated
transmit carrier.
The ZIF implements the back end of the re-
ceiver. The incoming signal is attenuated,
amplified and mixed down to an extremely
low frequency (Baseband)-not quite DC. The
first amplifier and an Automatic Gain Con-
trol (AGC) circuit adjusts the amplifier gain
to maintain a constant level in the baseband
filter.
The first mixing that occurs in the ZIF re-
quires a 2nd Local Oscillator that is running
at 221.184MHz. The 2nd LO is divided down
then phased shifted before being mixed with
the IF signal., producing the baseband sig-
nal
The baseband signal is low-pass filtered us-
ing a programmable low-pass filter. In
TDMA mode, the baseband signal is routed
to the DCI via the RX_I and RX_Q lines. In
analog mode, the baseband signal is up-con-
verted and then FM-demodulated, produc-
ing the DEMOD signal which is routed to
the DCI for filtering and de-emphasis.
The RSSI (Receive Signal Strength Indica-
tor) is a voltage that increases with respect
to the received signal strength. The RSSI sig-
nal has a filtering capacitance that changes
with respect to the operating mode(e.i. ana-
log, digital).
Antenna Circuit
Custom IC