User Guide

Technical Documentation
NHD–4
System Module
P.A.M.S
Page 4–24
Original 11/97
Receiver Functional Description
Introduction
NHD–4, being a dual mode phone, has essentially two receivers, the
analog AMPS and the digital CDMA. These two receivers share a
common front end and only become distinct in the IF stage after mixing
down to 45 MHz. A diode switch, V10, channels the received signal to the
appropriate receiver. It is important to keep in mind that the AMPS and
CDMA signals are significantly different. The AMPS signal is a FM
modulated carrier with a channel bandwidth of 30 kHz. A CDMA signal is
a 1.23 MHz wide spread spectrum carrier pedastal that appears
noise–like. This functional description is divided into three major sections,
the Front End, the AMPS Receiver and the CDMA Receiver.
Antenna and Coaxial Cable (W400)
The receiver chain begins at the antenna. The antenna is impedance
matched to the coax with L400 and C400. The coaxial cable, W400,
routes the signal down the length of the phone to the bottom connector.
When no external RF connection is made at the bottom connector (X701),
the received signal is directed back up to the top of the phone via the
second, shorter coax length. When the bottom connector is in place, i.e.
with the car kit, the coax leading to the antenna is taken out of the circuit
and the received RF signal launches in from the external connection. It
then proceeds up the shorter length of coax to the top of the phone and
into the duplexor.
Duplexor (Z102)
The Duplexor, Z102 serves to isolate the transmit signal from the receiver
path, and vice versa. The received signal proceeds from the coaxial cable
W400, through C150 into the Duplexor at the point labeled RFOUT. In the
case of the receiver RFOUT is actually the RF input. The duplexor is the
largest device on the PCB and can be found on the TX schematic. This
signal exits the Duplexor on the opposite side that it entered, at the port
labeled RX_IN. It then proceeds through C762 into the RF LNA Switch,
N702.
RF LNA Switches (N701, N702) and SWAGC/RX_CAL Control Lines
NHD–4 has a low noise amplifier, or LNA that can be switched in and out
of RX chain. The LNA is always on during AMPS RX operation. The
switching operation is accomplished by two RF GaAs switches N701 and
N702.